Compare updated template evaluator (#4092)

src/Sdk/WorkflowParser/ObjectTemplating/ContextValueNode.cs

@@ -0,0 +1,21 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// This expression node retrieves a user-defined named-value. This is used during expression evaluation.
+    /// </summary>
+    internal sealed class ContextValueNode : NamedValue
+    {
+        protected override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            return (context.State as TemplateContext).ExpressionValues[Name];
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/EmptyTraceWriter.cs

@@ -0,0 +1,25 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    internal class EmptyTraceWriter : ITraceWriter
+    {
+        public void Error(
+            String format,
+            params Object[] args)
+        {
+        }
+
+        public void Info(
+            String format,
+            params Object[] args)
+        {
+        }
+
+        public void Verbose(
+            String format,
+            params Object[] args)
+        {
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/ExpressionTraceWriter.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Wraps an ITraceWriter so it can be passed for expression evaluation.
+    /// </summary>
+    internal sealed class ExpressionTraceWriter : GitHub.Actions.Expressions.ITraceWriter
+    {
+        public ExpressionTraceWriter(ITraceWriter trace)
+        {
+            m_trace = trace;
+        }
+
+        public void Info(String message)
+        {
+            m_trace.Info("{0}", message);
+        }
+
+        public void Verbose(String message)
+        {
+            m_trace.Verbose("{0}", message);
+        }
+
+        private readonly ITraceWriter m_trace;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/IObjectReader.cs

@@ -0,0 +1,26 @@
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Interface for reading a source object (or file).
+    /// This interface is used by TemplateReader to build a TemplateToken DOM.
+    /// </summary>
+    internal interface IObjectReader
+    {
+        Boolean AllowLiteral(out LiteralToken token);
+
+        Boolean AllowSequenceStart(out SequenceToken token);
+
+        Boolean AllowSequenceEnd();
+
+        Boolean AllowMappingStart(out MappingToken token);
+
+        Boolean AllowMappingEnd();
+
+        void ValidateStart();
+
+        void ValidateEnd();
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/IObjectWriter.cs

@@ -0,0 +1,31 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Interface for building an object. This interface is used by
+    /// TemplateWriter to convert a TemplateToken DOM to another format.
+    /// </summary>
+    internal interface IObjectWriter
+    {
+        void WriteNull();
+
+        void WriteBoolean(Boolean value);
+
+        void WriteNumber(Double value);
+
+        void WriteString(String value);
+
+        void WriteSequenceStart();
+
+        void WriteSequenceEnd();
+
+        void WriteMappingStart();
+
+        void WriteMappingEnd();
+
+        void WriteStart();
+
+        void WriteEnd();
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/ITraceWriter.cs

@@ -0,0 +1,19 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    public interface ITraceWriter
+    {
+        void Error(
+            String format,
+            params Object[] args);
+
+        void Info(
+            String format,
+            params Object[] args);
+
+        void Verbose(
+            String format,
+            params Object[] args);
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/ITraceWriterExtensions.cs

@@ -0,0 +1,10 @@
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    internal static class ITraceWriterExtensions
+    {
+        internal static GitHub.Actions.Expressions.ITraceWriter ToExpressionTraceWriter(this ITraceWriter trace)
+        {
+            return new ExpressionTraceWriter(trace);
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/BooleanDefinition.cs

@@ -0,0 +1,57 @@
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class BooleanDefinition : ScalarDefinition
+    {
+        internal BooleanDefinition()
+        {
+        }
+
+        internal BooleanDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.Boolean:
+                        var mapping = definitionPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.Boolean}");
+                        foreach (var mappingPair in mapping)
+                        {
+                            var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Boolean} key");
+                            switch (mappingKey.Value)
+                            {
+                                default:
+                                    mappingKey.AssertUnexpectedValue($"{TemplateConstants.Definition} {TemplateConstants.Boolean} key");
+                                    break;
+                            }
+                        }
+                        break;
+
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.Boolean;
+
+        internal override Boolean IsMatch(LiteralToken literal)
+        {
+            return literal is BooleanToken;
+        }
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/Definition.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    /// <summary>
+    /// Defines the allowable schema for a user defined type
+    /// </summary>
+    internal abstract class Definition
+    {
+        protected Definition()
+        {
+        }
+
+        protected Definition(MappingToken definition)
+        {
+            for (var i = 0; i < definition.Count; )
+            {
+                var definitionKey = definition[i].Key.AssertString($"{TemplateConstants.Definition} key");
+                if (String.Equals(definitionKey.Value, TemplateConstants.Context, StringComparison.Ordinal))
+                {
+                    var context = definition[i].Value.AssertSequence($"{TemplateConstants.Context}");
+                    definition.RemoveAt(i);
+                    var readerContext = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+                    var evaluatorContext = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+                    foreach (TemplateToken item in context)
+                    {
+                        var itemStr = item.AssertString($"{TemplateConstants.Context} item").Value;
+                        readerContext.Add(itemStr);
+
+                        // Remove min/max parameter info
+                        var paramIndex = itemStr.IndexOf('(');
+                        if (paramIndex > 0)
+                        {
+                            evaluatorContext.Add(String.Concat(itemStr.Substring(0, paramIndex + 1), ")"));
+                        }
+                        else
+                        {
+                            evaluatorContext.Add(itemStr);
+                        }
+                    }
+
+                    ReaderContext = readerContext.ToArray();
+                    EvaluatorContext = evaluatorContext.ToArray();
+                }
+                else if (String.Equals(definitionKey.Value, TemplateConstants.Description, StringComparison.Ordinal))
+                {
+                    definition.RemoveAt(i);
+                }
+                else
+                {
+                    i++;
+                }
+            }
+        }
+
+        internal abstract DefinitionType DefinitionType { get; }
+
+        /// <summary>
+        /// Used by the template reader to determine allowed expression values and functions.
+        /// Also used by the template reader to validate function min/max parameters.
+        /// </summary>
+        internal String[] ReaderContext { get; private set; } = new String[0];
+
+        /// <summary>
+        /// Used by the template evaluator to determine allowed expression values and functions.
+        /// The min/max parameter info is omitted.
+        /// </summary>
+        internal String[] EvaluatorContext { get; private set; } = new String[0];
+
+        internal abstract void Validate(
+            TemplateSchema schema,
+            String name);
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/DefinitionType.cs

@@ -0,0 +1,16 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal enum DefinitionType
+    {
+        Null,
+        Boolean,
+        Number,
+        String,
+        Sequence,
+        Mapping,
+        OneOf,
+        AllowedValues,
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/MappingDefinition.cs

@@ -0,0 +1,111 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class MappingDefinition : Definition
+    {
+        internal MappingDefinition()
+        {
+        }
+
+        internal MappingDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.Mapping:
+                        var mapping = definitionPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.Mapping}");
+                        foreach (var mappingPair in mapping)
+                        {
+                            var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Mapping} key");
+                            switch (mappingKey.Value)
+                            {
+                                case TemplateConstants.Properties:
+                                    var properties = mappingPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.Mapping} {TemplateConstants.Properties}");
+                                    foreach (var propertiesPair in properties)
+                                    {
+                                        var propertyName = propertiesPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Mapping} {TemplateConstants.Properties} key");
+                                        Properties.Add(propertyName.Value, new PropertyDefinition(propertiesPair.Value));
+                                    }
+                                    break;
+
+                                case TemplateConstants.LooseKeyType:
+                                    var looseKeyType = mappingPair.Value.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Mapping} {TemplateConstants.LooseKeyType}");
+                                    LooseKeyType = looseKeyType.Value;
+                                    break;
+
+                                case TemplateConstants.LooseValueType:
+                                    var looseValueType = mappingPair.Value.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Mapping} {TemplateConstants.LooseValueType}");
+                                    LooseValueType = looseValueType.Value;
+                                    break;
+
+                                default:
+                                    definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                                    break;
+                            }
+                        }
+                        break;
+
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.Mapping;
+
+        internal String LooseKeyType { get; set; }
+
+        internal String LooseValueType { get; set; }
+
+        internal Dictionary<String, PropertyDefinition> Properties { get; } = new Dictionary<String, PropertyDefinition>(StringComparer.Ordinal);
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+            // Lookup loose key type
+            if (!String.IsNullOrEmpty(LooseKeyType))
+            {
+                schema.GetDefinition(LooseKeyType);
+
+                // Lookup loose value type
+                if (!String.IsNullOrEmpty(LooseValueType))
+                {
+                    schema.GetDefinition(LooseValueType);
+                }
+                else
+                {
+                    throw new ArgumentException($"Property '{TemplateConstants.LooseKeyType}' is defined but '{TemplateConstants.LooseValueType}' is not defined on '{name}'");
+                }
+            }
+            // Otherwise validate loose value type not be defined
+            else if (!String.IsNullOrEmpty(LooseValueType))
+            {
+                throw new ArgumentException($"Property '{TemplateConstants.LooseValueType}' is defined but '{TemplateConstants.LooseKeyType}' is not defined");
+            }
+
+            // Lookup each property
+            foreach (var property in Properties)
+            {
+                if (String.IsNullOrEmpty(property.Value.Type))
+                {
+                    throw new ArgumentException($"Type not specified for the '{property.Key}' property on the '{name}' type");
+                }
+
+                schema.GetDefinition(property.Value.Type);
+            }
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/NullDefinition.cs

@@ -0,0 +1,58 @@
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class NullDefinition : ScalarDefinition
+    {
+        internal NullDefinition()
+        {
+        }
+
+        internal NullDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.Null:
+                        var mapping = definitionPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.Null}");
+                        foreach (var mappingPair in mapping)
+                        {
+                            var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Null} key");
+                            switch (mappingKey.Value)
+                            {
+                                default:
+                                    mappingKey.AssertUnexpectedValue($"{TemplateConstants.Definition} {TemplateConstants.Null} key");
+                                    break;
+                            }
+                        }
+                        break;
+                    
+                    case TemplateConstants.AllowedValues:
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.Null;
+
+        internal override Boolean IsMatch(LiteralToken literal)
+        {
+            return literal is NullToken;
+        }
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/NumberDefinition.cs

@@ -0,0 +1,57 @@
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class NumberDefinition : ScalarDefinition
+    {
+        internal NumberDefinition()
+        {
+        }
+
+        internal NumberDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.Number:
+                        var mapping = definitionPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.Number}");
+                        foreach (var mappingPair in mapping)
+                        {
+                            var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Number} key");
+                            switch (mappingKey.Value)
+                            {
+                                default:
+                                    mappingKey.AssertUnexpectedValue($"{TemplateConstants.Definition} {TemplateConstants.Number} key");
+                                    break;
+                            }
+                        }
+                        break;
+
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.Number;
+
+        internal override Boolean IsMatch(LiteralToken literal)
+        {
+            return literal is NumberToken;
+        }
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/OneOfDefinition.cs

@@ -0,0 +1,191 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    /// <summary>
+    /// Must resolve to exactly one of the referenced definitions
+    /// </summary>
+    internal sealed class OneOfDefinition : Definition
+    {
+        internal OneOfDefinition()
+        {
+        }
+
+        internal OneOfDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.OneOf:
+                        var oneOf = definitionPair.Value.AssertSequence(TemplateConstants.OneOf);
+                        foreach (var oneOfItem in oneOf)
+                        {
+                            var reference = oneOfItem.AssertString(TemplateConstants.OneOf);
+                            OneOf.Add(reference.Value);
+                        }
+                        break;
+
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.OneOf;
+
+        internal List<String> OneOf { get; } = new List<String>();
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+            if (OneOf.Count == 0)
+            {
+                throw new ArgumentException($"'{name}' does not contain any references");
+            }
+
+            var foundLooseKeyType = false;
+            var mappingDefinitions = default(List<MappingDefinition>);
+            var sequenceDefinition = default(SequenceDefinition);
+            var nullDefinition = default(NullDefinition);
+            var booleanDefinition = default(BooleanDefinition);
+            var numberDefinition = default(NumberDefinition);
+            var stringDefinitions = default(List<StringDefinition>);
+
+            foreach (var nestedType in OneOf)
+            {
+                var nestedDefinition = schema.GetDefinition(nestedType);
+
+                if (nestedDefinition.ReaderContext.Length > 0)
+                {
+                    throw new ArgumentException($"'{name}' is a one-of definition and references another definition that defines context. This is currently not supported.");
+                }
+
+                if (nestedDefinition is MappingDefinition mappingDefinition)
+                {
+                    if (mappingDefinitions == null)
+                    {
+                        mappingDefinitions = new List<MappingDefinition>();
+                    }
+
+                    mappingDefinitions.Add(mappingDefinition);
+
+                    if (!String.IsNullOrEmpty(mappingDefinition.LooseKeyType))
+                    {
+                        foundLooseKeyType = true;
+                    }
+                }
+                else if (nestedDefinition is SequenceDefinition s)
+                {
+                    // Multiple sequence definitions not allowed
+                    if (sequenceDefinition != null)
+                    {
+                        throw new ArgumentException($"'{name}' refers to more than one '{TemplateConstants.Sequence}'");
+                    }
+
+                    sequenceDefinition = s;
+                }
+                else if (nestedDefinition is NullDefinition n)
+                {
+                    // Multiple sequence definitions not allowed
+                    if (nullDefinition != null)
+                    {
+                        throw new ArgumentException($"'{name}' refers to more than one '{TemplateConstants.Null}'");
+                    }
+
+                    nullDefinition = n;
+                }
+                else if (nestedDefinition is BooleanDefinition b)
+                {
+                    // Multiple boolean definitions not allowed
+                    if (booleanDefinition != null)
+                    {
+                        throw new ArgumentException($"'{name}' refers to more than one '{TemplateConstants.Boolean}'");
+                    }
+
+                    booleanDefinition = b;
+                }
+                else if (nestedDefinition is NumberDefinition num)
+                {
+                    // Multiple number definitions not allowed
+                    if (numberDefinition != null)
+                    {
+                        throw new ArgumentException($"'{name}' refers to more than one '{TemplateConstants.Number}'");
+                    }
+
+                    numberDefinition = num;
+                }
+                else if (nestedDefinition is StringDefinition stringDefinition)
+                {
+                    // First string definition
+                    if (stringDefinitions == null)
+                    {
+                        stringDefinitions = new List<StringDefinition>();
+                    }
+                    // Multiple string definitions, all must be 'Constant'
+                    else if ((stringDefinitions.Count == 1 && String.IsNullOrEmpty(stringDefinitions[0].Constant))
+                        || String.IsNullOrEmpty(stringDefinition.Constant))
+                    {
+                        throw new ArgumentException($"'{name}' refers to more than one '{TemplateConstants.Scalar}', but some do not set '{TemplateConstants.Constant}'");
+                    }
+
+                    stringDefinitions.Add(stringDefinition);
+                }
+                else if (nestedDefinition is OneOfDefinition oneOfDefinition)
+                {
+                    // Allow one-of to reference another one-of
+                }
+                else
+                {
+                    throw new ArgumentException($"'{name}' refers to a '{nestedDefinition.DefinitionType}' definition");
+                }
+            }
+
+            if (mappingDefinitions?.Count > 1)
+            {
+                if (foundLooseKeyType)
+                {
+                    throw new ArgumentException($"'{name}' refers to two mappings that both set '{TemplateConstants.LooseKeyType}'");
+                }
+
+                var seenProperties = new Dictionary<String, PropertyDefinition>(StringComparer.Ordinal);
+
+                foreach (var mappingDefinition in mappingDefinitions)
+                {
+                    foreach (var newProperty in mappingDefinition.Properties)
+                    {
+                        // Already seen
+                        if (seenProperties.TryGetValue(newProperty.Key, out PropertyDefinition existingProperty))
+                        {
+                            // Types match
+                            if (String.Equals(existingProperty.Type, newProperty.Value.Type, StringComparison.Ordinal))
+                            {
+                                continue;
+                            }
+
+                            // Collision
+                            throw new ArgumentException($"'{name}' contains two mappings with the same property, but each refers to a different type. All matching properties must refer to the same type.");
+                        }
+                        // New
+                        else
+                        {
+                            seenProperties.Add(newProperty.Key, newProperty.Value);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/PropertyDefinition.cs

@@ -0,0 +1,47 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class PropertyDefinition
+    {
+        internal PropertyDefinition(TemplateToken token)
+        {
+            if (token is StringToken stringToken)
+            {
+                Type = stringToken.Value;
+            }
+            else
+            {
+                var mapping = token.AssertMapping($"{TemplateConstants.MappingPropertyValue}");
+                foreach (var mappingPair in mapping)
+                {
+                    var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.MappingPropertyValue} key");
+                    switch (mappingKey.Value)
+                    {
+                        case TemplateConstants.Type:
+                            Type = mappingPair.Value.AssertString($"{TemplateConstants.MappingPropertyValue} {TemplateConstants.Type}").Value;
+                            break;
+                        case TemplateConstants.Required:
+                            Required = mappingPair.Value.AssertBoolean($"{TemplateConstants.MappingPropertyValue} {TemplateConstants.Required}").Value;
+                            break;
+                        case TemplateConstants.Description:
+                            Description = mappingPair.Value.AssertString($"{TemplateConstants.MappingPropertyValue} {TemplateConstants.Description}").Value;
+                            break;
+                        default:
+                            mappingKey.AssertUnexpectedValue($"{TemplateConstants.MappingPropertyValue} key");
+                            break;
+                    }
+                }
+            }
+        }
+
+        internal String Type { get; set; }
+
+        internal Boolean Required { get; set; }
+
+        internal String Description { get; set; }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/ScalarDefinition.cs

@@ -0,0 +1,19 @@
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal abstract class ScalarDefinition : Definition
+    {
+        internal ScalarDefinition()
+        {
+        }
+
+        internal ScalarDefinition(MappingToken definition)
+            : base(definition)
+        {
+        }
+
+        internal abstract Boolean IsMatch(LiteralToken literal);
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/SequenceDefinition.cs

@@ -0,0 +1,69 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class SequenceDefinition : Definition
+    {
+        internal SequenceDefinition()
+        {
+        }
+
+        internal SequenceDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.Sequence:
+                        var mapping = definitionPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.Sequence}");
+                        foreach (var mappingPair in mapping)
+                        {
+                            var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Sequence} key");
+                            switch (mappingKey.Value)
+                            {
+                                case TemplateConstants.ItemType:
+                                    var itemType = mappingPair.Value.AssertString($"{TemplateConstants.Definition} {TemplateConstants.Sequence} {TemplateConstants.ItemType}");
+                                    ItemType = itemType.Value;
+                                    break;
+
+                                default:
+                                    mappingKey.AssertUnexpectedValue($"{TemplateConstants.Definition} {TemplateConstants.Sequence} key");
+                                    break;
+                            }
+                        }
+                        break;
+
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.Sequence;
+
+        internal String ItemType { get; set; }
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+            if (String.IsNullOrEmpty(ItemType))
+            {
+                throw new ArgumentException($"'{name}' does not define '{TemplateConstants.ItemType}'");
+            }
+
+            // Lookup item type
+            schema.GetDefinition(ItemType);
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/StringDefinition.cs

@@ -0,0 +1,116 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    internal sealed class StringDefinition : ScalarDefinition
+    {
+        internal StringDefinition()
+        {
+        }
+
+        internal StringDefinition(MappingToken definition)
+            : base(definition)
+        {
+            foreach (var definitionPair in definition)
+            {
+                var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                switch (definitionKey.Value)
+                {
+                    case TemplateConstants.String:
+                        var mapping = definitionPair.Value.AssertMapping($"{TemplateConstants.Definition} {TemplateConstants.String}");
+                        foreach (var mappingPair in mapping)
+                        {
+                            var mappingKey = mappingPair.Key.AssertString($"{TemplateConstants.Definition} {TemplateConstants.String} key");
+                            switch (mappingKey.Value)
+                            {
+                                case TemplateConstants.Constant:
+                                    var constantStringToken = mappingPair.Value.AssertString($"{TemplateConstants.Definition} {TemplateConstants.String} {TemplateConstants.Constant}");
+                                    Constant = constantStringToken.Value;
+                                    break;
+
+                                case TemplateConstants.IgnoreCase:
+                                    var ignoreCaseBooleanToken = mappingPair.Value.AssertBoolean($"{TemplateConstants.Definition} {TemplateConstants.String} {TemplateConstants.IgnoreCase}");
+                                    IgnoreCase = ignoreCaseBooleanToken.Value;
+                                    break;
+
+                                case TemplateConstants.RequireNonEmpty:
+                                    var requireNonEmptyBooleanToken = mappingPair.Value.AssertBoolean($"{TemplateConstants.Definition} {TemplateConstants.String} {TemplateConstants.RequireNonEmpty}");
+                                    RequireNonEmpty = requireNonEmptyBooleanToken.Value;
+                                    break;
+
+                                case TemplateConstants.IsExpression:
+                                    var isExpressionBooleanToken = mappingPair.Value.AssertBoolean($"{TemplateConstants.Definition} {TemplateConstants.String} {TemplateConstants.IsExpression}");
+                                    IsExpression = isExpressionBooleanToken.Value;
+                                    break;
+
+                                default:
+                                    mappingKey.AssertUnexpectedValue($"{TemplateConstants.Definition} {TemplateConstants.String} key");
+                                    break;
+                            }
+                        }
+                        break;
+
+                    case TemplateConstants.CoerceRaw:
+                        continue;
+
+                    default:
+                        definitionKey.AssertUnexpectedValue($"{TemplateConstants.Definition} key");
+                        break;
+                }
+            }
+        }
+
+        internal override DefinitionType DefinitionType => DefinitionType.String;
+
+        internal String Constant { get; set; }
+
+        internal Boolean IgnoreCase { get; set; }
+
+        internal Boolean RequireNonEmpty { get; set; }
+
+        internal Boolean IsExpression { get; set; }
+
+        internal override Boolean IsMatch(LiteralToken literal)
+        {
+            if (literal is StringToken str)
+            {
+                var value = str.Value;
+                if (!String.IsNullOrEmpty(Constant))
+                {
+                    var comparison = IgnoreCase ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal;
+
+                    if (String.Equals(Constant, value, comparison))
+                    {
+                        return true;
+                    }
+                }
+                else if (RequireNonEmpty)
+                {
+                    if (!String.IsNullOrEmpty(value))
+                    {
+                        return true;
+                    }
+                }
+                else
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        internal override void Validate(
+            TemplateSchema schema,
+            String name)
+        {
+            if (!String.IsNullOrEmpty(Constant) && RequireNonEmpty)
+            {
+                throw new ArgumentException($"Properties '{TemplateConstants.Constant}' and '{TemplateConstants.RequireNonEmpty}' cannot both be set");
+            }
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Schema/TemplateSchema.cs

@@ -0,0 +1,484 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text.RegularExpressions;
+using System.Threading;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Schema
+{
+    /// <summary>
+    /// This models the root schema object and contains definitions
+    /// </summary>
+    public sealed class TemplateSchema
+    {
+        internal TemplateSchema()
+            : this(null)
+        {
+        }
+
+        private TemplateSchema(MappingToken mapping)
+        {
+            // Add built-in type: null
+            var nullDefinition = new NullDefinition();
+            Definitions.Add(TemplateConstants.Null, nullDefinition);
+
+            // Add built-in type: boolean
+            var booleanDefinition = new BooleanDefinition();
+            Definitions.Add(TemplateConstants.Boolean, booleanDefinition);
+
+            // Add built-in type: number
+            var numberDefinition = new NumberDefinition();
+            Definitions.Add(TemplateConstants.Number, numberDefinition);
+
+            // Add built-in type: string
+            var stringDefinition = new StringDefinition();
+            Definitions.Add(TemplateConstants.String, stringDefinition);
+
+            // Add built-in type: sequence
+            var sequenceDefinition = new SequenceDefinition { ItemType = TemplateConstants.Any };
+            Definitions.Add(TemplateConstants.Sequence, sequenceDefinition);
+
+            // Add built-in type: mapping
+            var mappingDefinition = new MappingDefinition { LooseKeyType = TemplateConstants.String, LooseValueType = TemplateConstants.Any };
+            Definitions.Add(TemplateConstants.Mapping, mappingDefinition);
+
+            // Add built-in type: any
+            var anyDefinition = new OneOfDefinition();
+            anyDefinition.OneOf.Add(TemplateConstants.Null);
+            anyDefinition.OneOf.Add(TemplateConstants.Boolean);
+            anyDefinition.OneOf.Add(TemplateConstants.Number);
+            anyDefinition.OneOf.Add(TemplateConstants.String);
+            anyDefinition.OneOf.Add(TemplateConstants.Sequence);
+            anyDefinition.OneOf.Add(TemplateConstants.Mapping);
+            Definitions.Add(TemplateConstants.Any, anyDefinition);
+
+            if (mapping != null)
+            {
+                foreach (var pair in mapping)
+                {
+                    var key = pair.Key.AssertString($"{TemplateConstants.TemplateSchema} key");
+                    switch (key.Value)
+                    {
+                        case TemplateConstants.Version:
+                            var version = pair.Value.AssertString(TemplateConstants.Version);
+                            Version = version.Value;
+                            break;
+
+                        case TemplateConstants.Definitions:
+                            var definitions = pair.Value.AssertMapping(TemplateConstants.Definitions);
+                            foreach (var definitionsPair in definitions)
+                            {
+                                var definitionsKey = definitionsPair.Key.AssertString($"{TemplateConstants.Definitions} key");
+                                var definitionsValue = definitionsPair.Value.AssertMapping(TemplateConstants.Definition);
+                                var definition = default(Definition);
+                                foreach (var definitionPair in definitionsValue)
+                                {
+                                    var definitionKey = definitionPair.Key.AssertString($"{TemplateConstants.Definition} key");
+                                    switch (definitionKey.Value)
+                                    {
+                                        case TemplateConstants.Null:
+                                            definition = new NullDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.Boolean:
+                                            definition = new BooleanDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.Number:
+                                            definition = new NumberDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.String:
+                                            definition = new StringDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.Sequence:
+                                            definition = new SequenceDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.Mapping:
+                                            definition = new MappingDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.OneOf:
+                                            definition = new OneOfDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.AllowedValues:
+                                            // Ignore allowed-values in CSharp parser, we don't need to support events here
+                                            definition = new NullDefinition(definitionsValue);
+                                            break;
+
+                                        case TemplateConstants.CoerceRaw:
+                                        case TemplateConstants.Context:
+                                        case TemplateConstants.Description:
+                                            continue;
+
+                                        default:
+                                            definitionKey.AssertUnexpectedValue("definition mapping key"); // throws
+                                            break;
+                                    }
+
+                                    break;
+                                }
+
+                                if (definition == null)
+                                {
+                                    throw new ArgumentException($"Unable to determine definition details. Specify the '{TemplateConstants.Structure}' property");
+                                }
+
+                                Definitions.Add(definitionsKey.Value, definition);
+                            }
+                            break;
+
+                        default:
+                            key.AssertUnexpectedValue($"{TemplateConstants.TemplateSchema} key"); // throws
+                            break;
+                    }
+                }
+            }
+        }
+
+        internal Dictionary<String, Definition> Definitions { get; } = new Dictionary<String, Definition>(StringComparer.Ordinal);
+
+        internal String Version { get; }
+
+        /// <summary>
+        /// Loads a user's schema file
+        /// </summary>
+        internal static TemplateSchema Load(IObjectReader objectReader)
+        {
+            var context = new TemplateContext
+            {
+                CancellationToken = CancellationToken.None,
+                Errors = new TemplateValidationErrors(maxErrors: 10, maxMessageLength: 500),
+                Memory = new TemplateMemory(
+                    maxDepth: 50,
+                    maxEvents: 1000000, // 1 million
+                    maxBytes: 1024 * 1024), // 1 mb
+                TraceWriter = new EmptyTraceWriter(),
+            };
+
+            var value = TemplateReader.Read(context, TemplateConstants.TemplateSchema, objectReader, null, Schema, out _);
+
+            if (context.Errors.Count > 0)
+            {
+                throw new TemplateValidationException(context.Errors);
+            }
+
+            var mapping = value.AssertMapping(TemplateConstants.TemplateSchema);
+            var schema = new TemplateSchema(mapping);
+            schema.Validate();
+            return schema;
+        }
+
+        internal IEnumerable<T> Get<T>(Definition definition)
+            where T : Definition
+        {
+            if (definition is T match)
+            {
+                yield return match;
+            }
+            else if (definition is OneOfDefinition oneOf)
+            {
+                foreach (var reference in oneOf.OneOf)
+                {
+                    var nestedDefinition = GetDefinition(reference);
+                    if (nestedDefinition is T match2)
+                    {
+                        yield return match2;
+                    }
+                }
+            }
+        }
+
+        internal Definition GetDefinition(String type)
+        {
+            if (Definitions.TryGetValue(type, out Definition value))
+            {
+                return value;
+            }
+
+            throw new ArgumentException($"Schema definition '{type}' not found");
+        }
+
+        internal Boolean HasProperties(MappingDefinition definition)
+        {
+            return definition.Properties.Count > 0;
+        }
+
+        internal Boolean TryGetProperty(
+            MappingDefinition definition,
+            String name,
+            out String type)
+        {
+            if (definition.Properties.TryGetValue(name, out PropertyDefinition property))
+            {
+                type = property.Type;
+                return true;
+            }
+
+            type = null;
+            return false;
+        }
+
+        internal Boolean TryMatchKey(
+            List<MappingDefinition> definitions,
+            String key,
+            out String valueType)
+        {
+            valueType = null;
+
+            // Check for a matching well known property
+            var notFoundInSome = false;
+            for (var i = 0; i < definitions.Count; i++)
+            {
+                var definition = definitions[i];
+
+                if (TryGetProperty(definition, key, out String t))
+                {
+                    if (valueType == null)
+                    {
+                        valueType = t;
+                    }
+                }
+                else
+                {
+                    notFoundInSome = true;
+                }
+            }
+
+            // Check if found
+            if (valueType != null)
+            {
+                // Filter the matched definitions if needed
+                if (notFoundInSome)
+                {
+                    for (var i = 0; i < definitions.Count;)
+                    {
+                        if (TryGetProperty(definitions[i], key, out _))
+                        {
+                            i++;
+                        }
+                        else
+                        {
+                            definitions.RemoveAt(i);
+                        }
+                    }
+                }
+
+                return true;
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// The built-in schema for reading schema files
+        /// </summary>
+        private static TemplateSchema Schema
+        {
+            get
+            {
+                if (s_schema == null)
+                {
+                    var schema = new TemplateSchema();
+
+                    StringDefinition stringDefinition;
+                    SequenceDefinition sequenceDefinition;
+                    MappingDefinition mappingDefinition;
+                    OneOfDefinition oneOfDefinition;
+
+                    // template-schema
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Version, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Definitions, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Definitions)));
+                    schema.Definitions.Add(TemplateConstants.TemplateSchema, mappingDefinition);
+
+                    // definitions
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.LooseKeyType = TemplateConstants.NonEmptyString;
+                    mappingDefinition.LooseValueType = TemplateConstants.Definition;
+                    schema.Definitions.Add(TemplateConstants.Definitions, mappingDefinition);
+
+                    // definition
+                    oneOfDefinition = new OneOfDefinition();
+                    oneOfDefinition.OneOf.Add(TemplateConstants.NullDefinition);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.BooleanDefinition);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.NumberDefinition);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.StringDefinition);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.SequenceDefinition);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.MappingDefinition);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.OneOfDefinition);
+                    schema.Definitions.Add(TemplateConstants.Definition, oneOfDefinition);
+
+                    // null-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Null, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NullDefinitionProperties)));
+                    schema.Definitions.Add(TemplateConstants.NullDefinition, mappingDefinition);
+
+                    // null-definition-properties
+                    mappingDefinition = new MappingDefinition();
+                    schema.Definitions.Add(TemplateConstants.NullDefinitionProperties, mappingDefinition);
+
+                    // boolean-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Boolean, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.BooleanDefinitionProperties)));
+                    schema.Definitions.Add(TemplateConstants.BooleanDefinition, mappingDefinition);
+
+                    // boolean-definition-properties
+                    mappingDefinition = new MappingDefinition();
+                    schema.Definitions.Add(TemplateConstants.BooleanDefinitionProperties, mappingDefinition);
+
+                    // number-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Number, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NumberDefinitionProperties)));
+                    schema.Definitions.Add(TemplateConstants.NumberDefinition, mappingDefinition);
+
+                    // number-definition-properties
+                    mappingDefinition = new MappingDefinition();
+                    schema.Definitions.Add(TemplateConstants.NumberDefinitionProperties, mappingDefinition);
+
+                    // string-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.String, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.StringDefinitionProperties)));
+                    schema.Definitions.Add(TemplateConstants.StringDefinition, mappingDefinition);
+
+                    // string-definition-properties
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Constant, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.IgnoreCase, new PropertyDefinition(new StringToken(null, null, null,TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.RequireNonEmpty, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.IsExpression, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    schema.Definitions.Add(TemplateConstants.StringDefinitionProperties, mappingDefinition);
+
+                    // sequence-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Sequence, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceDefinitionProperties)));
+                    schema.Definitions.Add(TemplateConstants.SequenceDefinition, mappingDefinition);
+
+                    // sequence-definition-properties
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.ItemType, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NonEmptyString)));
+                    schema.Definitions.Add(TemplateConstants.SequenceDefinitionProperties, mappingDefinition);
+
+                    // mapping-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Mapping, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.MappingDefinitionProperties)));
+                    schema.Definitions.Add(TemplateConstants.MappingDefinition, mappingDefinition);
+
+                    // mapping-definition-properties
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Properties, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Properties)));
+                    mappingDefinition.Properties.Add(TemplateConstants.LooseKeyType, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.LooseValueType, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NonEmptyString)));
+                    schema.Definitions.Add(TemplateConstants.MappingDefinitionProperties, mappingDefinition);
+
+                    // properties
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.LooseKeyType = TemplateConstants.NonEmptyString;
+                    mappingDefinition.LooseValueType = TemplateConstants.PropertyValue;
+                    schema.Definitions.Add(TemplateConstants.Properties, mappingDefinition);
+
+                    // property-value
+                    oneOfDefinition = new OneOfDefinition();
+                    oneOfDefinition.OneOf.Add(TemplateConstants.NonEmptyString);
+                    oneOfDefinition.OneOf.Add(TemplateConstants.MappingPropertyValue);
+                    schema.Definitions.Add(TemplateConstants.PropertyValue, oneOfDefinition);
+
+                    // mapping-property-value
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Type, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.NonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Required, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    schema.Definitions.Add(TemplateConstants.MappingPropertyValue, mappingDefinition);
+
+                    // one-of-definition
+                    mappingDefinition = new MappingDefinition();
+                    mappingDefinition.Properties.Add(TemplateConstants.Description, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.String)));
+                    mappingDefinition.Properties.Add(TemplateConstants.CoerceRaw, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.Boolean)));
+                    mappingDefinition.Properties.Add(TemplateConstants.Context, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.OneOf, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    mappingDefinition.Properties.Add(TemplateConstants.AllowedValues, new PropertyDefinition(new StringToken(null, null, null, TemplateConstants.SequenceOfNonEmptyString)));
+                    schema.Definitions.Add(TemplateConstants.OneOfDefinition, mappingDefinition);
+
+                    // non-empty-string
+                    stringDefinition = new StringDefinition();
+                    stringDefinition.RequireNonEmpty = true;
+                    schema.Definitions.Add(TemplateConstants.NonEmptyString, stringDefinition);
+
+                    // sequence-of-non-empty-string
+                    sequenceDefinition = new SequenceDefinition();
+                    sequenceDefinition.ItemType = TemplateConstants.NonEmptyString;
+                    schema.Definitions.Add(TemplateConstants.SequenceOfNonEmptyString, sequenceDefinition);
+
+                    schema.Validate();
+
+                    Interlocked.CompareExchange(ref s_schema, schema, null);
+                }
+
+                return s_schema;
+            }
+        }
+
+        private void Validate()
+        {
+            var oneOfPairs = new List<KeyValuePair<String, OneOfDefinition>>();
+
+            foreach (var pair in Definitions)
+            {
+                var name = pair.Key;
+
+                if (!s_definitionNameRegex.IsMatch(name ?? String.Empty))
+                {
+                    throw new ArgumentException($"Invalid definition name '{name}'");
+                }
+
+                var definition = pair.Value;
+
+                // Delay validation for 'one-of' definitions
+                if (definition is OneOfDefinition oneOf)
+                {
+                    oneOfPairs.Add(new KeyValuePair<String, OneOfDefinition>(name, oneOf));
+                }
+                // Otherwise validate now
+                else
+                {
+                    definition.Validate(this, name);
+                }
+            }
+
+            // Validate 'one-of' definitions
+            foreach (var pair in oneOfPairs)
+            {
+                var name = pair.Key;
+                var oneOf = pair.Value;
+                oneOf.Validate(this, name);
+            }
+        }
+
+        private static readonly Regex s_definitionNameRegex = new Regex("^[a-zA-Z_][a-zA-Z0-9_-]*$", RegexOptions.Compiled);
+        private static TemplateSchema s_schema;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Telemetry.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Tracks telemetry data during workflow parsing.
+    /// </summary>
+    public sealed class Telemetry
+    {
+        /// <summary>
+        /// Gets or sets the count of YAML anchors encountered during parsing.
+        /// </summary>
+        public Int32 YamlAnchors { get; set; }
+
+        /// <summary>
+        /// Gets or sets the count of YAML aliases encountered during parsing.
+        /// </summary>
+        public Int32 YamlAliases { get; set; }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateConstants.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Linq;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    internal static class TemplateConstants
+    {
+        internal const String AllowedValues = "allowed-values";
+        internal const String Any = "any";
+        internal const String Boolean = "boolean";
+        internal const String BooleanDefinition = "boolean-definition";
+        internal const String BooleanDefinitionProperties = "boolean-definition-properties";
+        internal const String CloseExpression = "}}";
+        internal const String CoerceRaw = "coerce-raw";
+        internal const String Constant = "constant";
+        internal const String Context = "context";
+        internal const String Definition = "definition";
+        internal const String Definitions = "definitions";
+        internal const String Description = "description";
+        internal const String IgnoreCase = "ignore-case";
+        internal const String InsertDirective = "insert";
+        internal const String IsExpression = "is-expression";
+        internal const String ItemType = "item-type";
+        internal const String LooseKeyType = "loose-key-type";
+        internal const String LooseValueType = "loose-value-type";
+        internal const String MaxConstant = "MAX";
+        internal const String Mapping = "mapping";
+        internal const String MappingDefinition = "mapping-definition";
+        internal const String MappingDefinitionProperties = "mapping-definition-properties";
+        internal const String MappingPropertyValue = "mapping-property-value";
+        internal const String NonEmptyString = "non-empty-string";
+        internal const String Null = "null";
+        internal const String NullDefinition = "null-definition";
+        internal const String NullDefinitionProperties = "null-definition-properties";
+        internal const String Number = "number";
+        internal const String NumberDefinition = "number-definition";
+        internal const String NumberDefinitionProperties = "number-definition-properties";
+        internal const String OneOf = "one-of";
+        internal const String OneOfDefinition = "one-of-definition";
+        internal const String OpenExpression = "${{";
+        internal const String PropertyValue = "property-value";
+        internal const String Properties = "properties";
+        internal const String Required = "required";
+        internal const String RequireNonEmpty = "require-non-empty";
+        internal const String Scalar = "scalar";
+        internal const String Sequence = "sequence";
+        internal const String SequenceDefinition = "sequence-definition";
+        internal const String SequenceDefinitionProperties = "sequence-definition-properties";
+        internal const String Type = "type";
+        internal const String SequenceOfNonEmptyString = "sequence-of-non-empty-string";
+        internal const String String = "string";
+        internal const String StringDefinition = "string-definition";
+        internal const String StringDefinitionProperties = "string-definition-properties";
+        internal const String Structure = "structure";
+        internal const String TemplateSchema = "template-schema";
+        internal const String Version = "version";
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateContext.cs

@@ -0,0 +1,261 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using GitHub.Actions.Expressions;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Schema;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Context object that is flowed through while loading and evaluating object templates
+    /// </summary>
+    public sealed class TemplateContext
+    {
+        internal CancellationToken CancellationToken { get; set; }
+
+        internal TemplateValidationErrors Errors
+        {
+            get
+            {
+                if (m_errors == null)
+                {
+                    m_errors = new TemplateValidationErrors();
+                }
+
+                return m_errors;
+            }
+
+            set
+            {
+                m_errors = value;
+            }
+        }
+
+        /// <summary>
+        /// Available functions within expression contexts
+        /// </summary>
+        internal IList<IFunctionInfo> ExpressionFunctions
+        {
+            get
+            {
+                if (m_expressionFunctions == null)
+                {
+                    m_expressionFunctions = new List<IFunctionInfo>();
+                }
+
+                return m_expressionFunctions;
+            }
+        }
+
+        /// <summary>
+        /// Available values within expression contexts
+        /// </summary>
+        internal IDictionary<String, Object> ExpressionValues
+        {
+            get
+            {
+                if (m_expressionValues == null)
+                {
+                    m_expressionValues = new Dictionary<String, Object>(StringComparer.OrdinalIgnoreCase);
+                }
+
+                return m_expressionValues;
+            }
+        }
+
+        internal TemplateMemory Memory { get; set; }
+
+        internal TemplateSchema Schema { get; set; }
+
+        /// <summary>
+        /// Gets or sets the telemetry data collected during parsing.
+        /// </summary>
+        public Telemetry Telemetry
+        {
+            get
+            {
+                if (m_telemetry == null)
+                {
+                    m_telemetry = new Telemetry();
+                }
+
+                return m_telemetry;
+            }
+            set
+            {
+                m_telemetry = value;
+            }
+        }
+
+        /// <summary>
+        /// State data for the current evaluation
+        /// </summary>
+        public IDictionary<String, Object> State
+        {
+            get
+            {
+                if (m_state == null)
+                {
+                    m_state = new Dictionary<String, Object>(StringComparer.OrdinalIgnoreCase);
+                }
+
+                return m_state;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether the expression function fromJson performs strict JSON parsing.
+        /// Used during evaluation only.
+        /// </summary>
+        internal Boolean StrictJsonParsing { get; set; }
+
+        internal ITraceWriter TraceWriter { get; set; }
+
+        private IDictionary<String, Int32> FileIds
+        {
+            get
+            {
+                if (m_fileIds == null)
+                {
+                    m_fileIds = new Dictionary<String, Int32>(StringComparer.OrdinalIgnoreCase);
+                }
+
+                return m_fileIds;
+            }
+            set
+            {
+                m_fileIds = value;
+            }
+        }
+
+        private List<String> FileNames
+        {
+            get
+            {
+                if (m_fileNames == null)
+                {
+                    m_fileNames = new List<String>();
+                }
+
+                return m_fileNames;
+            }
+            set
+            {
+                m_fileNames = value;
+            }
+        }
+
+        internal void Error(
+            TemplateToken value,
+            Exception ex)
+        {
+            Error(value?.FileId, value?.Line, value?.Column, ex);
+        }
+
+        internal void Error(
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            Exception ex)
+        {
+            var prefix = GetErrorPrefix(fileId, line, column);
+            Errors.Add(prefix, ex);
+            TraceWriter.Error(prefix, ex);
+        }
+
+        internal void Error(
+            TemplateToken value,
+            String message)
+        {
+            Error(value?.FileId, value?.Line, value?.Column, message);
+        }
+
+        internal void Error(
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            String message)
+        {
+            var prefix = GetErrorPrefix(fileId, line, column);
+            if (!String.IsNullOrEmpty(prefix))
+            {
+                message = $"{prefix} {message}";
+            }
+
+            Errors.Add(message);
+            TraceWriter.Error(message);
+        }
+
+        internal INamedValueInfo[] GetExpressionNamedValues()
+        {
+            if (m_expressionValues?.Count > 0)
+            {
+                return m_expressionValues.Keys.Select(x => new NamedValueInfo<ContextValueNode>(x)).ToArray();
+            }
+
+            return null;
+        }
+
+        internal Int32 GetFileId(String file)
+        {
+            if (!FileIds.TryGetValue(file, out Int32 id))
+            {
+                id = FileIds.Count + 1;
+                FileIds.Add(file, id);
+                FileNames.Add(file);
+                Memory.AddBytes(file);
+            }
+
+            return id;
+        }
+
+        internal String GetFileName(Int32 fileId)
+        {
+            return FileNames.Count >= fileId ? FileNames[fileId - 1] : null;
+        }
+
+        internal IReadOnlyList<String> GetFileTable()
+        {
+            return FileNames.AsReadOnly();
+        }
+
+        private String GetErrorPrefix(
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+        {
+            var fileName = fileId.HasValue ? GetFileName(fileId.Value) : null;
+            if (!String.IsNullOrEmpty(fileName))
+            {
+                if (line != null && column != null)
+                {
+                    return $"{fileName} {TemplateStrings.LineColumn(line, column)}:";
+                }
+                else
+                {
+                    return $"{fileName}:";
+                }
+            }
+            else if (line != null && column != null)
+            {
+                return $"{TemplateStrings.LineColumn(line, column)}:";
+            }
+            else
+            {
+                return String.Empty;
+            }
+        }
+
+        private TemplateValidationErrors m_errors;
+        private IList<IFunctionInfo> m_expressionFunctions;
+        private IDictionary<String, Object> m_expressionValues;
+        private IDictionary<String, Int32> m_fileIds;
+        private List<String> m_fileNames;
+        private IDictionary<String, Object> m_state;
+        private Telemetry m_telemetry;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateEvaluator.cs

@@ -0,0 +1,428 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Schema;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Expands expression tokens where the allowed context is available now. The allowed context is defined
+    /// within the schema. The available context is based on the ExpressionValues registered in the TemplateContext.
+    /// </summary>
+    internal partial class TemplateEvaluator
+    {
+        private TemplateEvaluator(
+            TemplateContext context,
+            TemplateToken template,
+            Int32 removeBytes)
+        {
+            m_context = context;
+            m_schema = context.Schema;
+            m_unraveler = new TemplateUnraveler(context, template, removeBytes);
+        }
+
+        internal static TemplateToken Evaluate(
+            TemplateContext context,
+            String type,
+            TemplateToken template,
+            Int32 removeBytes,
+            Int32? fileId)
+        {
+            TemplateToken result;
+
+            var evaluator = new TemplateEvaluator(context, template, removeBytes);
+            try
+            {
+                var availableContext = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+                foreach (var key in context.ExpressionValues.Keys)
+                {
+                    availableContext.Add(key);
+                }
+                foreach (var function in context.ExpressionFunctions)
+                {
+                    availableContext.Add($"{function.Name}()");
+                }
+
+                var definitionInfo = new DefinitionInfo(context.Schema, type, availableContext);
+                result = evaluator.Evaluate(definitionInfo);
+
+                if (result != null)
+                {
+                    evaluator.m_unraveler.ReadEnd();
+                }
+            }
+            catch (Exception ex)
+            {
+                context.Error(fileId, null, null, ex);
+                result = null;
+            }
+
+            return result;
+        }
+
+        private TemplateToken Evaluate(DefinitionInfo definition)
+        {
+            // Scalar
+            if (m_unraveler.AllowScalar(definition.Expand, out ScalarToken scalar))
+            {
+                if (scalar is LiteralToken literal)
+                {
+                    Validate(ref literal, definition);
+                    return literal;
+                }
+                else
+                {
+                    return scalar;
+                }
+            }
+
+            // Sequence start
+            if (m_unraveler.AllowSequenceStart(definition.Expand, out SequenceToken sequence))
+            {
+                var sequenceDefinition = definition.Get<SequenceDefinition>().FirstOrDefault();
+
+                // Legal
+                if (sequenceDefinition != null)
+                {
+                    var itemDefinition = new DefinitionInfo(definition, sequenceDefinition.ItemType);
+
+                    // Add each item
+                    while (!m_unraveler.AllowSequenceEnd(definition.Expand))
+                    {
+                        var item = Evaluate(itemDefinition);
+                        sequence.Add(item);
+                    }
+                }
+                // Illegal
+                else
+                {
+                    // Error
+                    m_context.Error(sequence, TemplateStrings.UnexpectedSequenceStart());
+
+                    // Skip each item
+                    while (!m_unraveler.AllowSequenceEnd(expand: false))
+                    {
+                        m_unraveler.SkipSequenceItem();
+                    }
+                }
+
+                return sequence;
+            }
+
+            // Mapping
+            if (m_unraveler.AllowMappingStart(definition.Expand, out MappingToken mapping))
+            {
+                var mappingDefinitions = definition.Get<MappingDefinition>().ToList();
+
+                // Legal
+                if (mappingDefinitions.Count > 0)
+                {
+                    if (mappingDefinitions.Count > 1 ||
+                        m_schema.HasProperties(mappingDefinitions[0]) ||
+                        String.IsNullOrEmpty(mappingDefinitions[0].LooseKeyType))
+                    {
+                        HandleMappingWithWellKnownProperties(definition, mappingDefinitions, mapping);
+                    }
+                    else
+                    {
+                        var keyDefinition = new DefinitionInfo(definition, mappingDefinitions[0].LooseKeyType);
+                        var valueDefinition = new DefinitionInfo(definition, mappingDefinitions[0].LooseValueType);
+                        HandleMappingWithAllLooseProperties(definition, keyDefinition, valueDefinition, mapping);
+                    }
+                }
+                // Illegal
+                else
+                {
+                    m_context.Error(mapping, TemplateStrings.UnexpectedMappingStart());
+
+                    while (!m_unraveler.AllowMappingEnd(expand: false))
+                    {
+                        m_unraveler.SkipMappingKey();
+                        m_unraveler.SkipMappingValue();
+                    }
+                }
+
+                return mapping;
+            }
+
+            throw new ArgumentException(TemplateStrings.ExpectedScalarSequenceOrMapping());
+        }
+
+        private void HandleMappingWithWellKnownProperties(
+            DefinitionInfo definition,
+            List<MappingDefinition> mappingDefinitions,
+            MappingToken mapping)
+        {
+            // Check if loose properties are allowed
+            String looseKeyType = null;
+            String looseValueType = null;
+            DefinitionInfo? looseKeyDefinition = null;
+            DefinitionInfo? looseValueDefinition = null;
+            if (!String.IsNullOrEmpty(mappingDefinitions[0].LooseKeyType))
+            {
+                looseKeyType = mappingDefinitions[0].LooseKeyType;
+                looseValueType = mappingDefinitions[0].LooseValueType;
+            }
+
+            var keys = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+            var hasExpressionKey = false;
+
+            while (m_unraveler.AllowScalar(definition.Expand, out ScalarToken nextKeyScalar))
+            {
+                // Expression
+                if (nextKeyScalar is ExpressionToken)
+                {
+                    hasExpressionKey = true;
+                    var anyDefinition = new DefinitionInfo(definition, TemplateConstants.Any);
+                    mapping.Add(nextKeyScalar, Evaluate(anyDefinition));
+                    continue;
+                }
+
+                // Not a string, convert
+                if (!(nextKeyScalar is StringToken nextKey))
+                {
+                    nextKey = new StringToken(nextKeyScalar.FileId, nextKeyScalar.Line, nextKeyScalar.Column, nextKeyScalar.ToString());
+                }
+
+                // Duplicate
+                if (!keys.Add(nextKey.Value))
+                {
+                    m_context.Error(nextKey, TemplateStrings.ValueAlreadyDefined(nextKey.Value));
+                    m_unraveler.SkipMappingValue();
+                    continue;
+                }
+
+                // Well known
+                if (m_schema.TryMatchKey(mappingDefinitions, nextKey.Value, out String nextValueType))
+                {
+                    var nextValueDefinition = new DefinitionInfo(definition, nextValueType);
+                    var nextValue = Evaluate(nextValueDefinition);
+                    mapping.Add(nextKey, nextValue);
+                    continue;
+                }
+
+                // Loose
+                if (looseKeyType != null)
+                {
+                    if (looseKeyDefinition == null)
+                    {
+                        looseKeyDefinition = new DefinitionInfo(definition, looseKeyType);
+                        looseValueDefinition = new DefinitionInfo(definition, looseValueType);
+                    }
+
+                    Validate(nextKey, looseKeyDefinition.Value);
+                    var nextValue = Evaluate(looseValueDefinition.Value);
+                    mapping.Add(nextKey, nextValue);
+                    continue;
+                }
+
+                // Error
+                m_context.Error(nextKey, TemplateStrings.UnexpectedValue(nextKey.Value));
+                m_unraveler.SkipMappingValue();
+            }
+
+            // Only one
+            if (mappingDefinitions.Count > 1)
+            {
+                var hitCount = new Dictionary<String, Int32>();
+                foreach (MappingDefinition mapdef in mappingDefinitions)
+                {
+                    foreach (String key in mapdef.Properties.Keys)
+                    {
+                        if (!hitCount.TryGetValue(key, out Int32 value))
+                        {
+                            hitCount.Add(key, 1);
+                        }
+                        else
+                        {
+                            hitCount[key] = value + 1;
+                        }
+                    }
+                }
+
+                List<String> nonDuplicates = new List<String>();
+                foreach (String key in hitCount.Keys)
+                {
+                    if (hitCount[key] == 1)
+                    {
+                        nonDuplicates.Add(key);
+                    }
+                }
+                nonDuplicates.Sort();
+
+                String listToDeDuplicate = String.Join(", ", nonDuplicates);
+                m_context.Error(mapping, TemplateStrings.UnableToDetermineOneOf(listToDeDuplicate));
+            }
+            else if (mappingDefinitions.Count == 1 && !hasExpressionKey)
+            {
+                foreach (var property in mappingDefinitions[0].Properties)
+                {
+                    if (property.Value.Required)
+                    {
+                        if (!keys.Contains(property.Key))
+                        {
+                            m_context.Error(mapping, $"Required property is missing: {property.Key}");
+                        }
+                    }
+                }
+            }
+
+            m_unraveler.ReadMappingEnd();
+        }
+
+        private void HandleMappingWithAllLooseProperties(
+            DefinitionInfo mappingDefinition,
+            DefinitionInfo keyDefinition,
+            DefinitionInfo valueDefinition,
+            MappingToken mapping)
+        {
+            var keys = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+
+            while (m_unraveler.AllowScalar(mappingDefinition.Expand, out ScalarToken nextKeyScalar))
+            {
+                // Expression
+                if (nextKeyScalar is ExpressionToken)
+                {
+                    if (nextKeyScalar is BasicExpressionToken)
+                    {
+                        mapping.Add(nextKeyScalar, Evaluate(valueDefinition));
+                    }
+                    else
+                    {
+                        var anyDefinition = new DefinitionInfo(mappingDefinition, TemplateConstants.Any);
+                        mapping.Add(nextKeyScalar, Evaluate(anyDefinition));
+                    }
+
+                    continue;
+                }
+
+                // Not a string
+                if (!(nextKeyScalar is StringToken nextKey))
+                {
+                    nextKey = new StringToken(nextKeyScalar.FileId, nextKeyScalar.Line, nextKeyScalar.Column, nextKeyScalar.ToString());
+                }
+
+                // Duplicate
+                if (!keys.Add(nextKey.Value))
+                {
+                    m_context.Error(nextKey, TemplateStrings.ValueAlreadyDefined(nextKey.Value));
+                    m_unraveler.SkipMappingValue();
+                    continue;
+                }
+
+                // Validate
+                Validate(nextKey, keyDefinition);
+
+                // Add the pair
+                var nextValue = Evaluate(valueDefinition);
+                mapping.Add(nextKey, nextValue);
+            }
+
+            m_unraveler.ReadMappingEnd();
+        }
+
+        private void Validate(
+            StringToken stringToken,
+            DefinitionInfo definition)
+        {
+            var literal = stringToken as LiteralToken;
+            Validate(ref literal, definition);
+        }
+
+        private void Validate(
+            ref LiteralToken literal,
+            DefinitionInfo definition)
+        {
+            // Legal
+            var literal2 = literal;
+            if (definition.Get<ScalarDefinition>().Any(x => x.IsMatch(literal2)))
+            {
+                return;
+            }
+
+            // Not a string, convert
+            if (literal.Type != TokenType.String)
+            {
+                var stringToken = new StringToken(literal.FileId, literal.Line, literal.Column, literal.ToString());
+
+                // Legal
+                if (definition.Get<StringDefinition>().Any(x => x.IsMatch(stringToken)))
+                {
+                    literal = stringToken;
+                    return;
+                }
+            }
+
+            // Illegal
+            m_context.Error(literal, TemplateStrings.UnexpectedValue(literal));
+        }
+
+        private struct DefinitionInfo
+        {
+            public DefinitionInfo(
+                TemplateSchema schema,
+                String name,
+                HashSet<String> availableContext)
+            {
+                m_schema = schema;
+                m_availableContext = availableContext;
+
+                // Lookup the definition
+                Definition = m_schema.GetDefinition(name);
+
+                // Determine whether to expand
+                m_allowedContext = Definition.EvaluatorContext;
+                if (Definition.EvaluatorContext.Length > 0)
+                {
+                    Expand = m_availableContext.IsSupersetOf(m_allowedContext);
+                }
+                else
+                {
+                    Expand = false;
+                }
+            }
+
+            public DefinitionInfo(
+                DefinitionInfo parent,
+                String name)
+            {
+                m_schema = parent.m_schema;
+                m_availableContext = parent.m_availableContext;
+
+                // Lookup the definition
+                Definition = m_schema.GetDefinition(name);
+
+                // Determine whether to expand
+                if (Definition.EvaluatorContext.Length > 0)
+                {
+                    m_allowedContext = new HashSet<String>(parent.m_allowedContext.Concat(Definition.EvaluatorContext), StringComparer.OrdinalIgnoreCase).ToArray();
+                    Expand = m_availableContext.IsSupersetOf(m_allowedContext);
+                }
+                else
+                {
+                    m_allowedContext = parent.m_allowedContext;
+                    Expand = parent.Expand;
+                }
+            }
+
+            public IEnumerable<T> Get<T>()
+                where T : Definition
+            {
+                return m_schema.Get<T>(Definition);
+            }
+
+            private HashSet<String> m_availableContext;
+            private String[] m_allowedContext;
+            private TemplateSchema m_schema;
+            public Definition Definition;
+            public Boolean Expand;
+        }
+
+        private readonly TemplateContext m_context;
+        private readonly TemplateSchema m_schema;
+        private readonly TemplateUnraveler m_unraveler;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateMemory.cs

@@ -0,0 +1,385 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GitHub.Actions.Expressions.Data;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+using Newtonsoft.Json.Linq;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Tracks characteristics about the current memory usage (CPU, stack, size)
+    /// </summary>
+    public sealed class TemplateMemory
+    {
+        /// <summary>
+        /// Creates a new instance
+        /// </summary>
+        /// <param name="maxBytes">The maximum allowed bytes</param>
+        public TemplateMemory(Int32 maxBytes)
+            : this(0, 0, maxBytes: maxBytes, null)
+        {
+        }
+
+        /// <summary>
+        /// Creates a new instance
+        /// </summary>
+        /// <param name="maxDepth">The maximum allowed depth</param>
+        /// <param name="maxEvents">The maximum allowed events</param>
+        /// <param name="maxBytes">The maximum allowed bytes</param>
+        internal TemplateMemory(
+            Int32 maxDepth,
+            Int32 maxEvents,
+            Int32 maxBytes)
+            : this(maxDepth, maxEvents, maxBytes, null)
+        {
+        }
+
+        /// <summary>
+        /// Creates a new instance
+        /// </summary>
+        /// <param name="maxDepth">The maximum allowed depth</param>
+        /// <param name="maxEvents">The maximum allowed events</param>
+        /// <param name="maxBytes">The maximum allowed bytes</param>
+        /// <param name="parent">Optional parent instance, for byte tracking only. Any bytes added/subtracted to the current instance, will be also added/subtracted to the parent instance.</param>
+        internal TemplateMemory(
+            Int32 maxDepth,
+            Int32 maxEvents,
+            Int32 maxBytes,
+            TemplateMemory parent)
+        {
+            m_maxDepth = maxDepth;
+            m_maxEvents = maxEvents;
+            m_maxBytes = maxBytes;
+            m_parent = parent;
+        }
+
+        public Int32 CurrentBytes => m_currentBytes;
+
+        public Int32 MaxBytes => m_maxBytes;
+
+        public void AddBytes(Int32 bytes)
+        {
+            checked
+            {
+                m_currentBytes += bytes;
+            }
+
+            if (m_currentBytes > m_maxBytes)
+            {
+                throw new InvalidOperationException(TemplateStrings.MaxObjectSizeExceeded());
+            }
+
+            m_parent?.AddBytes(bytes);
+        }
+
+        public void AddBytes(String value)
+        {
+            var bytes = CalculateBytes(value);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(
+            ExpressionData value,
+            Boolean traverse)
+        {
+            var bytes = CalculateBytes(value, traverse);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(
+            JToken value,
+            Boolean traverse)
+        {
+            var bytes = CalculateBytes(value, traverse);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(
+            TemplateToken value,
+            Boolean traverse = false)
+        {
+            var bytes = CalculateBytes(value, traverse);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(LiteralToken literal)
+        {
+            var bytes = CalculateBytes(literal);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(SequenceToken sequence)
+        {
+            var bytes = CalculateBytes(sequence);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(MappingToken mapping)
+        {
+            var bytes = CalculateBytes(mapping);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(BasicExpressionToken basicExpression)
+        {
+            var bytes = CalculateBytes(basicExpression);
+            AddBytes(bytes);
+        }
+
+        internal void AddBytes(InsertExpressionToken insertExpression)
+        {
+            var bytes = CalculateBytes(insertExpression);
+            AddBytes(bytes);
+        }
+
+        internal Int32 CalculateBytes(String value)
+        {
+            // This measurement doesn't have to be perfect
+            // https://codeblog.jonskeet.uk/2011/04/05/of-memory-and-strings/
+
+            checked
+            {
+                return StringBaseOverhead + ((value?.Length ?? 0) * sizeof(Char));
+            }
+        }
+
+        internal static Int32 CalculateBytes(
+            ExpressionData value,
+            Boolean traverse)
+        {
+            var enumerable = traverse ? value.Traverse() : new[] { value } as IEnumerable<ExpressionData>;
+            var result = 0;
+            foreach (var item in enumerable)
+            {
+                // This measurement doesn't have to be perfect
+                // https://codeblog.jonskeet.uk/2011/04/05/of-memory-and-strings/
+                if (item is StringExpressionData str)
+                {
+                    checked
+                    {
+                        result += TemplateMemory.MinObjectSize + TemplateMemory.StringBaseOverhead + (str.Value.Length * sizeof(Char));
+                    }
+                }
+                else if (item is ArrayExpressionData || item is DictionaryExpressionData || item is BooleanExpressionData || item is NumberExpressionData)
+                {
+                    // Min object size is good enough. Allows for base + a few fields.
+                    checked
+                    {
+                        result += TemplateMemory.MinObjectSize;
+                    }
+                }
+                else if (item is null)
+                {
+                    checked
+                    {
+                        result += IntPtr.Size;
+                    }
+                }
+                else
+                {
+                    throw new NotSupportedException($"Unexpected workflow context data type '{item.GetType().Name}'");
+                }
+            }
+
+            return result;
+        }
+
+        internal Int32 CalculateBytes(
+            JToken value,
+            Boolean traverse)
+        {
+            // This measurement doesn't have to be perfect
+            // https://codeblog.jonskeet.uk/2011/04/05/of-memory-and-strings/
+
+            if (value is null)
+            {
+                return MinObjectSize;
+            }
+
+            if (!traverse)
+            {
+                switch (value.Type)
+                {
+                    case JTokenType.String:
+                        checked
+                        {
+                            return StringBaseOverhead + (value.ToObject<String>().Length * sizeof(Char));
+                        }
+
+                    case JTokenType.Property:
+                        var property = value as JProperty;
+                        checked
+                        {
+                            return StringBaseOverhead + ((property.Name?.Length ?? 0) * sizeof(Char));
+                        }
+
+                    case JTokenType.Array:
+                    case JTokenType.Boolean:
+                    case JTokenType.Float:
+                    case JTokenType.Integer:
+                    case JTokenType.Null:
+                    case JTokenType.Object:
+                        return MinObjectSize;
+
+                    default:
+                        throw new NotSupportedException($"Unexpected JToken type '{value.Type}' when traversing object");
+                }
+            }
+
+            var result = 0;
+            do
+            {
+                // Descend as much as possible
+                while (true)
+                {
+                    // Add bytes
+                    var bytes = CalculateBytes(value, false);
+                    checked
+                    {
+                        result += bytes;
+                    }
+
+                    // Descend
+                    if (value.HasValues)
+                    {
+                        value = value.First;
+                    }
+                    // No more descendants
+                    else
+                    {
+                        break;
+                    }
+                }
+
+                // Next sibling or ancestor sibling
+                do
+                {
+                    var sibling = value.Next;
+
+                    // Sibling found
+                    if (sibling != null)
+                    {
+                        value = sibling;
+                        break;
+                    }
+
+                    // Ascend
+                    value = value.Parent;
+
+                } while (value != null);
+
+            } while (value != null);
+
+            return result;
+        }
+
+        internal Int32 CalculateBytes(
+            TemplateToken value,
+            Boolean traverse = false)
+        {
+            var enumerable = traverse ? value.Traverse() : new[] { value };
+            var result = 0;
+            foreach (var item in enumerable)
+            {
+                // This measurement doesn't have to be perfect
+                // https://codeblog.jonskeet.uk/2011/04/05/of-memory-and-strings/
+                switch (item.Type)
+                {
+                    case TokenType.Null:
+                    case TokenType.Boolean:
+                    case TokenType.Number:
+                        checked
+                        {
+                            result += MinObjectSize;
+                        }
+                        break;
+
+                    case TokenType.String:
+                        var stringToken = item as StringToken;
+                        checked
+                        {
+                            result += MinObjectSize + StringBaseOverhead + ((stringToken.Value?.Length ?? 0) * sizeof(Char));
+                        }
+                        break;
+
+                    case TokenType.Sequence:
+                    case TokenType.Mapping:
+                    case TokenType.InsertExpression:
+                        // Min object size is good enough. Allows for base + a few fields.
+                        checked
+                        {
+                            result += MinObjectSize;
+                        }
+                        break;
+
+                    case TokenType.BasicExpression:
+                        var basicExpression = item as BasicExpressionToken;
+                        checked
+                        {
+                            result += MinObjectSize + StringBaseOverhead + ((basicExpression.Expression?.Length ?? 0) * sizeof(Char));
+                        }
+                        break;
+
+                    default:
+                        throw new NotSupportedException($"Unexpected template type '{item.Type}'");
+                }
+            }
+
+            return result;
+        }
+
+        internal void SubtractBytes(Int32 bytes)
+        {
+            if (bytes > m_currentBytes)
+            {
+                throw new InvalidOperationException("Bytes to subtract exceeds total bytes");
+            }
+
+            m_currentBytes -= bytes;
+
+            m_parent?.SubtractBytes(bytes);
+        }
+
+        internal void SubtractBytes(
+            TemplateToken value,
+            Boolean traverse = false)
+        {
+            var bytes = CalculateBytes(value, traverse);
+            SubtractBytes(bytes);
+        }
+
+        internal void IncrementDepth()
+        {
+            if (m_depth++ >= m_maxDepth)
+            {
+                throw new InvalidOperationException(TemplateStrings.MaxObjectDepthExceeded());
+            }
+        }
+
+        internal void DecrementDepth()
+        {
+            m_depth--;
+        }
+
+        internal void IncrementEvents()
+        {
+            if (m_events++ >= m_maxEvents)
+            {
+                throw new InvalidOperationException(TemplateStrings.MaxTemplateEventsExceeded());
+            }
+        }
+
+        internal const Int32 MinObjectSize = 24;
+        internal const Int32 StringBaseOverhead = 26;
+        private readonly Int32 m_maxDepth;
+        private readonly Int32 m_maxEvents;
+        private readonly Int32 m_maxBytes;
+        private Int32 m_depth;
+        private Int32 m_events;
+        private Int32 m_currentBytes;
+        private TemplateMemory m_parent;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateReader.cs

@@ -0,0 +1,832 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using GitHub.Actions.Expressions.Sdk;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Schema;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Converts a source object format into a TemplateToken
+    /// </summary>
+    internal sealed class TemplateReader
+    {
+        private TemplateReader(
+            TemplateContext context,
+            TemplateSchema schema,
+            IObjectReader objectReader,
+            Int32? fileId)
+        {
+            m_context = context;
+            m_schema = schema;
+            m_memory = context.Memory;
+            m_objectReader = objectReader;
+            m_fileId = fileId;
+        }
+
+        internal static TemplateToken Read(
+            TemplateContext context,
+            String type,
+            IObjectReader objectReader,
+            Int32? fileId,
+            out Int32 bytes)
+        {
+            return Read(context, type, objectReader, fileId, context.Schema, out bytes);
+        }
+
+        internal static TemplateToken Read(
+            TemplateContext context,
+            String type,
+            IObjectReader objectReader,
+            Int32? fileId,
+            TemplateSchema schema,
+            out Int32 bytes)
+        {
+            TemplateToken result = null;
+
+            var reader = new TemplateReader(context, schema, objectReader, fileId);
+            var originalBytes = context.Memory.CurrentBytes;
+            try
+            {
+                objectReader.ValidateStart();
+                var definition = new DefinitionInfo(schema, type);
+                result = reader.ReadValue(definition);
+                objectReader.ValidateEnd();
+            }
+            catch (Exception ex)
+            {
+                context.Error(fileId, null, null, ex);
+            }
+            finally
+            {
+                bytes = context.Memory.CurrentBytes - originalBytes;
+            }
+
+            return result;
+        }
+
+        private TemplateToken ReadValue(DefinitionInfo definition)
+        {
+            m_memory.IncrementEvents();
+
+            // Scalar
+            if (m_objectReader.AllowLiteral(out LiteralToken literal))
+            {
+                var scalar = ParseScalar(literal, definition.AllowedContext);
+                Validate(ref scalar, definition);
+                m_memory.AddBytes(scalar);
+                return scalar;
+            }
+
+            // Sequence
+            if (m_objectReader.AllowSequenceStart(out SequenceToken sequence))
+            {
+                m_memory.IncrementDepth();
+                m_memory.AddBytes(sequence);
+
+                var sequenceDefinition = definition.Get<SequenceDefinition>().FirstOrDefault();
+
+                // Legal
+                if (sequenceDefinition != null)
+                {
+                    var itemDefinition = new DefinitionInfo(definition, sequenceDefinition.ItemType);
+
+                    // Add each item
+                    while (!m_objectReader.AllowSequenceEnd())
+                    {
+                        var item = ReadValue(itemDefinition);
+                        sequence.Add(item);
+                    }
+                }
+                // Illegal
+                else
+                {
+                    // Error
+                    m_context.Error(sequence, TemplateStrings.UnexpectedSequenceStart());
+
+                    // Skip each item
+                    while (!m_objectReader.AllowSequenceEnd())
+                    {
+                        SkipValue();
+                    }
+                }
+
+                m_memory.DecrementDepth();
+                return sequence;
+            }
+
+            // Mapping
+            if (m_objectReader.AllowMappingStart(out MappingToken mapping))
+            {
+                m_memory.IncrementDepth();
+                m_memory.AddBytes(mapping);
+
+                var mappingDefinitions = definition.Get<MappingDefinition>().ToList();
+
+                // Legal
+                if (mappingDefinitions.Count > 0)
+                {
+                    if (mappingDefinitions.Count > 1 ||
+                        m_schema.HasProperties(mappingDefinitions[0]) ||
+                        String.IsNullOrEmpty(mappingDefinitions[0].LooseKeyType))
+                    {
+                        HandleMappingWithWellKnownProperties(definition, mappingDefinitions, mapping);
+                    }
+                    else
+                    {
+                        var keyDefinition = new DefinitionInfo(definition, mappingDefinitions[0].LooseKeyType);
+                        var valueDefinition = new DefinitionInfo(definition, mappingDefinitions[0].LooseValueType);
+                        HandleMappingWithAllLooseProperties(definition, keyDefinition, valueDefinition, mapping);
+                    }
+                }
+                // Illegal
+                else
+                {
+                    m_context.Error(mapping, TemplateStrings.UnexpectedMappingStart());
+
+                    while (!m_objectReader.AllowMappingEnd())
+                    {
+                        SkipValue();
+                        SkipValue();
+                    }
+                }
+
+                m_memory.DecrementDepth();
+                return mapping;
+            }
+
+            throw new InvalidOperationException(TemplateStrings.ExpectedScalarSequenceOrMapping());
+        }
+
+        private void HandleMappingWithWellKnownProperties(
+            DefinitionInfo definition,
+            List<MappingDefinition> mappingDefinitions,
+            MappingToken mapping)
+        {
+            // Check if loose properties are allowed
+            String looseKeyType = null;
+            String looseValueType = null;
+            DefinitionInfo? looseKeyDefinition = null;
+            DefinitionInfo? looseValueDefinition = null;
+            if (!String.IsNullOrEmpty(mappingDefinitions[0].LooseKeyType))
+            {
+                looseKeyType = mappingDefinitions[0].LooseKeyType;
+                looseValueType = mappingDefinitions[0].LooseValueType;
+            }
+
+            var keys = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+            var hasExpressionKey = false;
+
+            while (m_objectReader.AllowLiteral(out LiteralToken rawLiteral))
+            {
+                var nextKeyScalar = ParseScalar(rawLiteral, definition.AllowedContext);
+                // Expression
+                if (nextKeyScalar is ExpressionToken)
+                {
+                    hasExpressionKey = true;
+                    // Legal
+                    if (definition.AllowedContext.Length > 0)
+                    {
+                        m_memory.AddBytes(nextKeyScalar);
+                        var anyDefinition = new DefinitionInfo(definition, TemplateConstants.Any);
+                        mapping.Add(nextKeyScalar, ReadValue(anyDefinition));
+                    }
+                    // Illegal
+                    else
+                    {
+                        m_context.Error(nextKeyScalar, TemplateStrings.ExpressionNotAllowed());
+                        SkipValue();
+                    }
+
+                    continue;
+                }
+
+                // Not a string, convert
+                if (!(nextKeyScalar is StringToken nextKey))
+                {
+                    nextKey = new StringToken(nextKeyScalar.FileId, nextKeyScalar.Line, nextKeyScalar.Column, nextKeyScalar.ToString());
+                }
+
+                // Duplicate
+                if (!keys.Add(nextKey.Value))
+                {
+                    m_context.Error(nextKey, TemplateStrings.ValueAlreadyDefined(nextKey.Value));
+                    SkipValue();
+                    continue;
+                }
+
+                // Well known
+                if (m_schema.TryMatchKey(mappingDefinitions, nextKey.Value, out String nextValueType))
+                {
+                    m_memory.AddBytes(nextKey);
+                    var nextValueDefinition = new DefinitionInfo(definition, nextValueType);
+                    var nextValue = ReadValue(nextValueDefinition);
+                    mapping.Add(nextKey, nextValue);
+                    continue;
+                }
+
+                // Loose
+                if (looseKeyType != null)
+                {
+                    if (looseKeyDefinition == null)
+                    {
+                        looseKeyDefinition = new DefinitionInfo(definition, looseKeyType);
+                        looseValueDefinition = new DefinitionInfo(definition, looseValueType);
+                    }
+
+                    Validate(nextKey, looseKeyDefinition.Value);
+                    m_memory.AddBytes(nextKey);
+                    var nextValue = ReadValue(looseValueDefinition.Value);
+                    mapping.Add(nextKey, nextValue);
+                    continue;
+                }
+
+                // Error
+                m_context.Error(nextKey, TemplateStrings.UnexpectedValue(nextKey.Value));
+                SkipValue();
+            }
+
+            // Only one
+            if (mappingDefinitions.Count > 1)
+            {
+                var hitCount = new Dictionary<String, Int32>();
+                foreach (MappingDefinition mapdef in mappingDefinitions)
+                {
+                    foreach (String key in mapdef.Properties.Keys)
+                    {
+                        if (!hitCount.TryGetValue(key, out Int32 value))
+                        {
+                            hitCount.Add(key, 1);
+                        }
+                        else
+                        {
+                            hitCount[key] = value + 1;
+                        }
+                    }
+                }
+
+                List<String> nonDuplicates = new List<String>();
+                foreach (String key in hitCount.Keys)
+                {
+                    if(hitCount[key] == 1)
+                    {
+                        nonDuplicates.Add(key);
+                    }
+                }
+                nonDuplicates.Sort();
+
+                String listToDeDuplicate = String.Join(", ", nonDuplicates);
+                m_context.Error(mapping, TemplateStrings.UnableToDetermineOneOf(listToDeDuplicate));
+            }
+            else if (mappingDefinitions.Count == 1 && !hasExpressionKey)
+            {
+                foreach (var property in mappingDefinitions[0].Properties)
+                {
+                    if (property.Value.Required)
+                    {
+                        if (!keys.Contains(property.Key))
+                        {
+                            m_context.Error(mapping, $"Required property is missing: {property.Key}");
+                        }
+                    }
+                }
+            }
+            ExpectMappingEnd();
+        }
+
+        private void HandleMappingWithAllLooseProperties(
+            DefinitionInfo mappingDefinition,
+            DefinitionInfo keyDefinition,
+            DefinitionInfo valueDefinition,
+            MappingToken mapping)
+        {
+            TemplateToken nextValue;
+            var keys = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+
+            while (m_objectReader.AllowLiteral(out LiteralToken rawLiteral))
+            {
+                var nextKeyScalar = ParseScalar(rawLiteral, mappingDefinition.AllowedContext);
+
+                // Expression
+                if (nextKeyScalar is ExpressionToken)
+                {
+                    // Legal
+                    if (mappingDefinition.AllowedContext.Length > 0)
+                    {
+                        m_memory.AddBytes(nextKeyScalar);
+                        nextValue = ReadValue(valueDefinition);
+                        mapping.Add(nextKeyScalar, nextValue);
+                    }
+                    // Illegal
+                    else
+                    {
+                        m_context.Error(nextKeyScalar, TemplateStrings.ExpressionNotAllowed());
+                        SkipValue();
+                    }
+
+                    continue;
+                }
+
+                // Not a string, convert
+                if (!(nextKeyScalar is StringToken nextKey))
+                {
+                    nextKey = new StringToken(nextKeyScalar.FileId, nextKeyScalar.Line, nextKeyScalar.Column, nextKeyScalar.ToString());
+                }
+
+                // Duplicate
+                if (!keys.Add(nextKey.Value))
+                {
+                    m_context.Error(nextKey, TemplateStrings.ValueAlreadyDefined(nextKey.Value));
+                    SkipValue();
+                    continue;
+                }
+
+                // Validate
+                Validate(nextKey, keyDefinition);
+                m_memory.AddBytes(nextKey);
+
+                // Add the pair
+                nextValue = ReadValue(valueDefinition);
+                mapping.Add(nextKey, nextValue);
+            }
+
+            ExpectMappingEnd();
+        }
+
+        private void ExpectMappingEnd()
+        {
+            if (!m_objectReader.AllowMappingEnd())
+            {
+                throw new Exception("Expected mapping end"); // Should never happen
+            }
+        }
+
+        private void SkipValue(Boolean error = false)
+        {
+            m_memory.IncrementEvents();
+
+            // Scalar
+            if (m_objectReader.AllowLiteral(out LiteralToken literal))
+            {
+                if (error)
+                {
+                    m_context.Error(literal, TemplateStrings.UnexpectedValue(literal));
+                }
+
+                return;
+            }
+
+            // Sequence
+            if (m_objectReader.AllowSequenceStart(out SequenceToken sequence))
+            {
+                m_memory.IncrementDepth();
+
+                if (error)
+                {
+                    m_context.Error(sequence, TemplateStrings.UnexpectedSequenceStart());
+                }
+
+                while (!m_objectReader.AllowSequenceEnd())
+                {
+                    SkipValue();
+                }
+
+                m_memory.DecrementDepth();
+                return;
+            }
+
+            // Mapping
+            if (m_objectReader.AllowMappingStart(out MappingToken mapping))
+            {
+                m_memory.IncrementDepth();
+
+                if (error)
+                {
+                    m_context.Error(mapping, TemplateStrings.UnexpectedMappingStart());
+                }
+
+                while (!m_objectReader.AllowMappingEnd())
+                {
+                    SkipValue();
+                    SkipValue();
+                }
+
+                m_memory.DecrementDepth();
+                return;
+            }
+
+            // Unexpected
+            throw new InvalidOperationException(TemplateStrings.ExpectedScalarSequenceOrMapping());
+        }
+
+        private void Validate(
+            StringToken stringToken,
+            DefinitionInfo definition)
+        {
+            var scalar = stringToken as ScalarToken;
+            Validate(ref scalar, definition);
+        }
+
+        private void Validate(
+            ref ScalarToken scalar,
+            DefinitionInfo definition)
+        {
+            switch (scalar.Type)
+            {
+                case TokenType.Null:
+                case TokenType.Boolean:
+                case TokenType.Number:
+                case TokenType.String:
+                    var literal = scalar as LiteralToken;
+
+                    // Legal
+                    if (definition.Get<ScalarDefinition>().Any(x => x.IsMatch(literal)))
+                    {
+                        return;
+                    }
+
+                    // Not a string, convert
+                    if (literal.Type != TokenType.String)
+                    {
+                        literal = new StringToken(literal.FileId, literal.Line, literal.Column, literal.ToString());
+
+                        // Legal
+                        if (definition.Get<StringDefinition>().Any(x => x.IsMatch(literal)))
+                        {
+                            scalar = literal;
+                            return;
+                        }
+                    }
+
+                    // Illegal
+                    m_context.Error(literal, TemplateStrings.UnexpectedValue(literal));
+                    break;
+
+                case TokenType.BasicExpression:
+
+                    // Illegal
+                    if (definition.AllowedContext.Length == 0)
+                    {
+                        m_context.Error(scalar, TemplateStrings.ExpressionNotAllowed());
+                    }
+
+                    break;
+
+                default:
+                    m_context.Error(scalar, TemplateStrings.UnexpectedValue(scalar));
+                    break;
+            }
+        }
+
+        private ScalarToken ParseScalar(
+            LiteralToken token,
+            String[] allowedContext)
+        {
+            // Not a string
+            if (token.Type != TokenType.String)
+            {
+                return token;
+            }
+
+            // Check if the value is definitely a literal
+            var raw = token.ToString();
+            Int32 startExpression;
+            if (String.IsNullOrEmpty(raw) ||
+                (startExpression = raw.IndexOf(TemplateConstants.OpenExpression)) < 0) // Doesn't contain ${{
+            {
+                return token;
+            }
+
+            // Break the value into segments of LiteralToken and ExpressionToken
+            var segments = new List<ScalarToken>();
+            var i = 0;
+            while (i < raw.Length)
+            {
+                // An expression starts here:
+                if (i == startExpression)
+                {
+                    // Find the end of the expression - i.e. }}
+                    startExpression = i;
+                    var endExpression = -1;
+                    var inString = false;
+                    for (i += TemplateConstants.OpenExpression.Length; i < raw.Length; i++)
+                    {
+                        if (raw[i] == '\'')
+                        {
+                            inString = !inString; // Note, this handles escaped single quotes gracefully. Ex. 'foo''bar'
+                        }
+                        else if (!inString && raw[i] == '}' && raw[i - 1] == '}')
+                        {
+                            endExpression = i;
+                            i++;
+                            break;
+                        }
+                    }
+
+                    // Check if not closed
+                    if (endExpression < startExpression)
+                    {
+                        m_context.Error(token, TemplateStrings.ExpressionNotClosed());
+                        return token;
+                    }
+
+                    // Parse the expression
+                    var rawExpression = raw.Substring(
+                        startExpression + TemplateConstants.OpenExpression.Length,
+                        endExpression - startExpression + 1 - TemplateConstants.OpenExpression.Length - TemplateConstants.CloseExpression.Length);
+                    var expression = ParseExpression(token.Line, token.Column, rawExpression, allowedContext, out Exception ex);
+
+                    // Check for error
+                    if (ex != null)
+                    {
+                        m_context.Error(token, ex);
+                        return token;
+                    }
+
+                    // Check if a directive was used when not allowed
+                    if (!String.IsNullOrEmpty(expression.Directive) &&
+                        ((startExpression != 0) || (i < raw.Length)))
+                    {
+                        m_context.Error(token, TemplateStrings.DirectiveNotAllowedInline(expression.Directive));
+                        return token;
+                    }
+
+                    // Add the segment
+                    segments.Add(expression);
+
+                    // Look for the next expression
+                    startExpression = raw.IndexOf(TemplateConstants.OpenExpression, i);
+                }
+                // The next expression is further ahead:
+                else if (i < startExpression)
+                {
+                    // Append the segment
+                    AddString(segments, token.Line, token.Column, raw.Substring(i, startExpression - i));
+
+                    // Adjust the position
+                    i = startExpression;
+                }
+                // No remaining expressions:
+                else
+                {
+                    AddString(segments, token.Line, token.Column, raw.Substring(i));
+                    break;
+                }
+            }
+
+            // Check if can convert to a literal
+            // For example, the escaped expression: ${{ '{{ this is a literal }}' }}
+            if (segments.Count == 1 &&
+                segments[0] is BasicExpressionToken basicExpression &&
+                IsExpressionString(basicExpression.Expression, out String str))
+            {
+                return new StringToken(m_fileId, token.Line, token.Column, str);
+            }
+
+            // Check if only ony segment
+            if (segments.Count == 1)
+            {
+                return segments[0];
+            }
+
+            // Build the new expression, using the format function
+            var format = new StringBuilder();
+            var args = new StringBuilder();
+            var argIndex = 0;
+            foreach (var segment in segments)
+            {
+                if (segment is StringToken literal)
+                {
+                    var text = ExpressionUtility.StringEscape(literal.Value) // Escape quotes
+                        .Replace("{", "{{") // Escape braces
+                        .Replace("}", "}}");
+                    format.Append(text);
+                }
+                else
+                {
+                    format.Append("{" + argIndex.ToString(CultureInfo.InvariantCulture) + "}"); // Append formatter
+                    argIndex++;
+
+                    var expression = segment as BasicExpressionToken;
+                    args.Append(", ");
+                    args.Append(expression.Expression);
+                }
+            }
+
+            var finalExpression = $"format('{format}'{args})";
+            if (!ExpressionToken.IsValidExpression(finalExpression, allowedContext, out Exception ex2))
+            {
+                m_context.Error(token, ex2);
+                return token;
+            }
+            return new BasicExpressionToken(m_fileId, token.Line, token.Column, finalExpression);
+        }
+
+        private ExpressionToken ParseExpression(
+            Int32? line,
+            Int32? column,
+            String value,
+            String[] allowedContext,
+            out Exception ex)
+        {
+            var trimmed = value.Trim();
+
+            // Check if the value is empty
+            if (String.IsNullOrEmpty(trimmed))
+            {
+                ex = new ArgumentException(TemplateStrings.ExpectedExpression());
+                return null;
+            }
+
+            // Try to find a matching directive
+            List<String> parameters;
+            if (MatchesDirective(trimmed, TemplateConstants.InsertDirective, 0, out parameters, out ex))
+            {
+                return new InsertExpressionToken(m_fileId, line, column);
+            }
+            else if (ex != null)
+            {
+                return null;
+            }
+
+            // Check if the value is an expression
+            if (!ExpressionToken.IsValidExpression(trimmed, allowedContext, out ex))
+            {
+                return null;
+            }
+
+            // Return the expression
+            return new BasicExpressionToken(m_fileId, line, column, trimmed);
+        }
+
+        private void AddString(
+            List<ScalarToken> segments,
+            Int32? line,
+            Int32? column,
+            String value)
+        {
+            // If the last segment was a LiteralToken, then append to the last segment
+            if (segments.Count > 0 && segments[segments.Count - 1] is StringToken lastSegment)
+            {
+                segments[segments.Count - 1] = new StringToken(m_fileId, line, column, lastSegment.Value + value);
+            }
+            // Otherwise add a new LiteralToken
+            else
+            {
+                segments.Add(new StringToken(m_fileId, line, column, value));
+            }
+        }
+
+        private static Boolean MatchesDirective(
+            String trimmed,
+            String directive,
+            Int32 expectedParameters,
+            out List<String> parameters,
+            out Exception ex)
+        {
+            if (trimmed.StartsWith(directive, StringComparison.Ordinal) &&
+                (trimmed.Length == directive.Length || Char.IsWhiteSpace(trimmed[directive.Length])))
+            {
+                parameters = new List<String>();
+                var startIndex = directive.Length;
+                var inString = false;
+                var parens = 0;
+                for (var i = startIndex; i < trimmed.Length; i++)
+                {
+                    var c = trimmed[i];
+                    if (Char.IsWhiteSpace(c) && !inString && parens == 0)
+                    {
+                        if (startIndex < i)
+                        {
+                            parameters.Add(trimmed.Substring(startIndex, i - startIndex));
+                        }
+
+                        startIndex = i + 1;
+                    }
+                    else if (c == '\'')
+                    {
+                        inString = !inString;
+                    }
+                    else if (c == '(' && !inString)
+                    {
+                        parens++;
+                    }
+                    else if (c == ')' && !inString)
+                    {
+                        parens--;
+                    }
+                }
+
+                if (startIndex < trimmed.Length)
+                {
+                    parameters.Add(trimmed.Substring(startIndex));
+                }
+
+                if (expectedParameters != parameters.Count)
+                {
+                    ex = new ArgumentException(TemplateStrings.ExpectedNParametersFollowingDirective(expectedParameters, directive, parameters.Count));
+                    parameters = null;
+                    return false;
+                }
+
+                ex = null;
+                return true;
+            }
+
+            ex = null;
+            parameters = null;
+            return false;
+        }
+
+        private static Boolean IsExpressionString(
+            String trimmed,
+            out String str)
+        {
+            var builder = new StringBuilder();
+
+            var inString = false;
+            for (var i = 0; i < trimmed.Length; i++)
+            {
+                var c = trimmed[i];
+                if (c == '\'')
+                {
+                    inString = !inString;
+
+                    if (inString && i != 0)
+                    {
+                        builder.Append(c);
+                    }
+                }
+                else if (!inString)
+                {
+                    str = default;
+                    return false;
+                }
+                else
+                {
+                    builder.Append(c);
+                }
+            }
+
+            str = builder.ToString();
+            return true;
+        }
+
+        private struct DefinitionInfo
+        {
+            public DefinitionInfo(
+                TemplateSchema schema,
+                String name)
+            {
+                m_schema = schema;
+
+                // Lookup the definition
+                Definition = m_schema.GetDefinition(name);
+
+                // Record allowed context
+                AllowedContext = Definition.ReaderContext;
+            }
+
+            public DefinitionInfo(
+                DefinitionInfo parent,
+                String name)
+            {
+                m_schema = parent.m_schema;
+
+                // Lookup the definition
+                Definition = m_schema.GetDefinition(name);
+
+                // Record allowed context
+                if (Definition.ReaderContext.Length > 0)
+                {
+                    AllowedContext = new HashSet<String>(parent.AllowedContext.Concat(Definition.ReaderContext), StringComparer.OrdinalIgnoreCase).ToArray();
+                }
+                else
+                {
+                    AllowedContext = parent.AllowedContext;
+                }
+            }
+
+            public IEnumerable<T> Get<T>()
+                where T : Definition
+            {
+                return m_schema.Get<T>(Definition);
+            }
+
+            private TemplateSchema m_schema;
+            public Definition Definition;
+            public String[] AllowedContext;
+        }
+
+        private readonly TemplateContext m_context;
+        private readonly Int32? m_fileId;
+        private readonly TemplateMemory m_memory;
+        private readonly IObjectReader m_objectReader;
+        private readonly TemplateSchema m_schema;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateStrings.cs

@@ -0,0 +1,281 @@
+// <auto-generated/>
+// *** AUTOMATICALLY GENERATED BY GenResourceClass -- DO NOT EDIT!!! ***
+using System;
+using System.Diagnostics;
+using System.ComponentModel;
+using System.Globalization;
+using System.Reflection;
+using System.Resources;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating {
+
+
+internal static class TemplateStrings
+{
+
+
+    //********************************************************************************************
+    /// Creates the resource manager instance.
+    //********************************************************************************************
+    static TemplateStrings()
+    {
+        s_resMgr = new ResourceManager("GitHub.Actions.WorkflowParser.ObjectTemplating.TemplateStrings", typeof(TemplateStrings).GetTypeInfo().Assembly);
+    }
+
+    public static ResourceManager Manager
+    {
+        get
+        {
+            return s_resMgr;
+        }
+    }
+
+    //********************************************************************************************
+    /// Returns a localized string given a resource string name.
+    //********************************************************************************************
+    public static String Get(
+        String          resourceName)
+    {
+        return s_resMgr.GetString(resourceName, CultureInfo.CurrentUICulture);
+    }
+
+    //********************************************************************************************
+    /// Returns a localized integer given a resource string name.
+    //********************************************************************************************
+    public static int GetInt(
+        String          resourceName)
+    {
+        return (int)s_resMgr.GetObject(resourceName, CultureInfo.CurrentUICulture);
+    }
+
+    //********************************************************************************************
+    /// Returns a localized string given a resource string name.
+    //********************************************************************************************
+    public static bool GetBool(
+        String          resourceName)
+    {
+        return (bool)s_resMgr.GetObject(resourceName, CultureInfo.CurrentUICulture);
+    }
+
+
+    //********************************************************************************************
+    /// A little helper function to alleviate some typing associated with loading resources and
+    /// formatting the strings.  In DEBUG builds, it also asserts that the number of format
+    /// arguments and the length of args match.
+    //********************************************************************************************
+    private static String Format(         // The formatted resource string.
+        String          resourceName,     // The name of the resource.
+        params Object[] args)             // Arguments to format.
+    {
+        String resource = Get(resourceName);
+
+#if DEBUG
+        // Check to make sure that the number of format string arguments matches the number of
+        // arguments passed in.
+        int formatArgCount = 0;
+        bool[] argSeen = new bool[100];
+        for (int i = 0; i < resource.Length; i++)
+        {
+            if (resource[i] == '{')
+            {
+                if (i + 1 < resource.Length &&
+                    resource[i + 1] == '{')
+                {
+                    i++;               // Skip the escaped curly braces.
+                }
+                else
+                {
+                    // Move past the curly brace and leading whitespace.
+                    i++;
+                    while (Char.IsWhiteSpace(resource[i]))
+                    {
+                        i++;
+                    }
+
+                    // Get the argument number.
+                    int length = 0;
+                    while (i + length < resource.Length && Char.IsDigit(resource[i + length]))
+                    {
+                        length++;
+                    }
+
+                    // Record it if it hasn't already been seen.
+                    int argNumber = int.Parse(resource.Substring(i, length), CultureInfo.InvariantCulture);
+                    if (!argSeen[argNumber])
+                    {
+                        formatArgCount++;  // Count it as a formatting argument.
+                        argSeen[argNumber] = true;
+                    }
+                }
+            }
+        }
+
+        Debug.Assert(args != null || formatArgCount == 0, 
+                     String.Format(CultureInfo.InvariantCulture, "The number of format arguments is {0}, but the args parameter is null.", formatArgCount));
+        Debug.Assert(args == null || formatArgCount == args.Length, 
+                     String.Format(CultureInfo.InvariantCulture, "Coding error using resource \"{0}\": The number of format arguments {1} != number of args {2}",
+                     resourceName, formatArgCount, args != null ? args.Length : 0));
+#endif // DEBUG
+
+
+        if (args == null)
+        {
+            return resource;
+        }
+
+        // If there are any DateTime structs in the arguments, we need to bracket them 
+        // to make sure they are within the supported range of the current calendar.
+        for (int i = 0; i < args.Length; i++)
+        {
+            // DateTime is a struct, we cannot use the as operator and null check.
+            if (args[i] is DateTime)
+            {
+                DateTime dateTime = (DateTime)args[i];
+
+                // We need to fetch the calendar on each Format call since it may change.
+                // Since we don't have more than one DateTime for resource, do not
+                // bother to cache this for the duration of the for loop.
+                Calendar calendar = DateTimeFormatInfo.CurrentInfo.Calendar;
+                if (dateTime > calendar.MaxSupportedDateTime)
+                {
+                    args[i] = calendar.MaxSupportedDateTime;
+                }
+                else if (dateTime < calendar.MinSupportedDateTime)
+                {
+                    args[i] = calendar.MinSupportedDateTime;
+                }
+            }
+        }
+
+        return String.Format(CultureInfo.CurrentCulture, resource, args);
+    }
+
+    // According to the documentation for the ResourceManager class, it should be sufficient to
+    // create a single static instance.  The following is an excerpt from the 1.1 documentation.
+    //   Using the methods of ResourceManager, a caller can access the resources for a particular
+    //   culture using the GetObject and GetString methods. By default, these methods return the
+    //   resource for the culture determined by the current cultural settings of the thread that made
+    //   the call.
+    private static ResourceManager s_resMgr;
+
+    /// <summary>
+    /// The expression directive &apos;{0}&apos; is not supported in this context
+    /// </summary>
+    public static String DirectiveNotAllowed(object arg0) { return Format("DirectiveNotAllowed", arg0); }
+
+    /// <summary>
+    /// The directive &apos;{0}&apos; is not allowed in this context. Directives are not supported for expressions that are embedded within a string. Directives are only supported when the entire value is an expression.
+    /// </summary>
+    public static String DirectiveNotAllowedInline(object arg0) { return Format("DirectiveNotAllowedInline", arg0); }
+
+    /// <summary>
+    /// An expression was expected
+    /// </summary>
+    public static String ExpectedExpression() { return Get("ExpectedExpression"); }
+
+    /// <summary>
+    /// Expected a mapping
+    /// </summary>
+    public static String ExpectedMapping() { return Get("ExpectedMapping"); }
+
+    /// <summary>
+    /// Exactly {0} parameter(s) were expected following the directive &apos;{1}&apos;. Actual parameter count: {2}
+    /// </summary>
+    public static String ExpectedNParametersFollowingDirective(object arg0, object arg1, object arg2) { return Format("ExpectedNParametersFollowingDirective", arg0, arg1, arg2); }
+
+    /// <summary>
+    /// Expected a scalar value
+    /// </summary>
+    public static String ExpectedScalar() { return Get("ExpectedScalar"); }
+
+    /// <summary>
+    /// Expected a scalar value, a sequence, or a mapping
+    /// </summary>
+    public static String ExpectedScalarSequenceOrMapping() { return Get("ExpectedScalarSequenceOrMapping"); }
+
+    /// <summary>
+    /// Expected a sequence
+    /// </summary>
+    public static String ExpectedSequence() { return Get("ExpectedSequence"); }
+
+    /// <summary>
+    /// A template expression is not allowed in this context
+    /// </summary>
+    public static String ExpressionNotAllowed() { return Get("ExpressionNotAllowed"); }
+
+    /// <summary>
+    /// The expression is not closed. An unescaped ${{ sequence was found, but the closing }} sequence was not found.
+    /// </summary>
+    public static String ExpressionNotClosed() { return Get("ExpressionNotClosed"); }
+
+    /// <summary>
+    /// Error message prefix that indicates the line and column where the error occurred
+    ///
+    /// (Line: {0}, Col: {1})
+    /// </summary>
+    public static String LineColumn(object arg0, object arg1) { return Format("LineColumn", arg0, arg1); }
+
+    /// <summary>
+    /// Maximum object depth exceeded
+    /// </summary>
+    public static String MaxObjectDepthExceeded() { return Get("MaxObjectDepthExceeded"); }
+
+    /// <summary>
+    /// Maximum object size exceeded
+    /// </summary>
+    public static String MaxObjectSizeExceeded() { return Get("MaxObjectSizeExceeded"); }
+
+    /// <summary>
+    /// Maximum events exceeded while evaluating the template. This may indicate an infinite loop or too many nested loops.
+    /// </summary>
+    public static String MaxTemplateEventsExceeded() { return Get("MaxTemplateEventsExceeded"); }
+
+    /// <summary>
+    /// The template is not valid.
+    /// </summary>
+    public static String TemplateNotValid() { return Get("TemplateNotValid"); }
+
+    /// <summary>
+    /// The template is not valid. {0}
+    /// </summary>
+    public static String TemplateNotValidWithErrors(object arg0) { return Format("TemplateNotValidWithErrors", arg0); }
+
+    /// <summary>
+    /// In {0} {1}: Error from called workflow
+    /// </summary>
+    public static String CalledWorkflowNotValidWithErrors(object arg0, object arg1) { return Format("CalledWorkflowNotValidWithErrors", arg0, arg1); }
+
+    /// <summary>
+    /// Unable to convert the object to a template token. Actual type &apos;{0}&apos;
+    /// </summary>
+    public static String UnableToConvertToTemplateToken(object arg0) { return Format("UnableToConvertToTemplateToken", arg0); }
+
+    /// <summary>
+    /// There&apos;s not enough info to determine what you meant. Add one of these properties: {0}
+    /// </summary>
+    public static String UnableToDetermineOneOf(object arg0) { return Format("UnableToDetermineOneOf", arg0); }
+
+    /// <summary>
+    /// A mapping was not expected
+    /// </summary>
+    public static String UnexpectedMappingStart() { return Get("UnexpectedMappingStart"); }
+
+    /// <summary>
+    /// A sequence was not expected
+    /// </summary>
+    public static String UnexpectedSequenceStart() { return Get("UnexpectedSequenceStart"); }
+
+    /// <summary>
+    /// Unexpected value &apos;{0}&apos;
+    /// </summary>
+    public static String UnexpectedValue(object arg0) { return Format("UnexpectedValue", arg0); }
+
+    /// <summary>
+    /// &apos;{0}&apos; is already defined
+    /// </summary>
+    public static String ValueAlreadyDefined(object arg0) { return Format("ValueAlreadyDefined", arg0); }
+
+
+}
+
+} // namespace

src/Sdk/WorkflowParser/ObjectTemplating/TemplateStrings.resx

@@ -0,0 +1,190 @@
+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <!-- 
+    Microsoft ResX Schema 
+    
+    Version 2.0
+    
+    The primary goals of this format is to allow a simple XML format 
+    that is mostly human readable. The generation and parsing of the 
+    various data types are done through the TypeConverter classes 
+    associated with the data types.
+    
+    Example:
+    
+    ... ado.net/XML headers & schema ...
+    <resheader name="resmimetype">text/microsoft-resx</resheader>
+    <resheader name="version">2.0</resheader>
+    <resheader name="reader">System.Resources.ResXResourceReader, System.Windows.Forms, ...</resheader>
+    <resheader name="writer">System.Resources.ResXResourceWriter, System.Windows.Forms, ...</resheader>
+    <data name="Name1"><value>this is my long string</value><comment>this is a comment</comment></data>
+    <data name="Color1" type="System.Drawing.Color, System.Drawing">Blue</data>
+    <data name="Bitmap1" mimetype="application/x-microsoft.net.object.binary.base64">
+        <value>[base64 mime encoded serialized .NET Framework object]</value>
+    </data>
+    <data name="Icon1" type="System.Drawing.Icon, System.Drawing" mimetype="application/x-microsoft.net.object.bytearray.base64">
+        <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
+        <comment>This is a comment</comment>
+    </data>
+                
+    There are any number of "resheader" rows that contain simple 
+    name/value pairs.
+    
+    Each data row contains a name, and value. The row also contains a 
+    type or mimetype. Type corresponds to a .NET class that support 
+    text/value conversion through the TypeConverter architecture. 
+    Classes that don't support this are serialized and stored with the 
+    mimetype set.
+    
+    The mimetype is used for serialized objects, and tells the 
+    ResXResourceReader how to depersist the object. This is currently not 
+    extensible. For a given mimetype the value must be set accordingly:
+    
+    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    that the ResXResourceWriter will generate, however the reader can 
+    read any of the formats listed below.
+    
+    mimetype: application/x-microsoft.net.object.binary.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
+            : and then encoded with base64 encoding.
+    
+    mimetype: application/x-microsoft.net.object.soap.base64
+    value   : The object must be serialized with 
+            : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
+            : and then encoded with base64 encoding.
+
+    mimetype: application/x-microsoft.net.object.bytearray.base64
+    value   : The object must be serialized into a byte array 
+            : using a System.ComponentModel.TypeConverter
+            : and then encoded with base64 encoding.
+    -->
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+    <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
+    <xsd:element name="root" msdata:IsDataSet="true">
+      <xsd:complexType>
+        <xsd:choice maxOccurs="unbounded">
+          <xsd:element name="metadata">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" />
+              </xsd:sequence>
+              <xsd:attribute name="name" use="required" type="xsd:string" />
+              <xsd:attribute name="type" type="xsd:string" />
+              <xsd:attribute name="mimetype" type="xsd:string" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="assembly">
+            <xsd:complexType>
+              <xsd:attribute name="alias" type="xsd:string" />
+              <xsd:attribute name="name" type="xsd:string" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="data">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+                <xsd:element name="comment" type="xsd:string" minOccurs="0" msdata:Ordinal="2" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" msdata:Ordinal="1" />
+              <xsd:attribute name="type" type="xsd:string" msdata:Ordinal="3" />
+              <xsd:attribute name="mimetype" type="xsd:string" msdata:Ordinal="4" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="resheader">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" />
+            </xsd:complexType>
+          </xsd:element>
+        </xsd:choice>
+      </xsd:complexType>
+    </xsd:element>
+  </xsd:schema>
+  <resheader name="resmimetype">
+    <value>text/microsoft-resx</value>
+  </resheader>
+  <resheader name="version">
+    <value>2.0</value>
+  </resheader>
+  <resheader name="reader">
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <resheader name="writer">
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <data name="DirectiveNotAllowed" xml:space="preserve">
+    <value>The expression directive '{0}' is not supported in this context</value>
+  </data>
+  <data name="DirectiveNotAllowedInline" xml:space="preserve">
+    <value>The directive '{0}' is not allowed in this context. Directives are not supported for expressions that are embedded within a string. Directives are only supported when the entire value is an expression.</value>
+  </data>
+  <data name="ExpectedExpression" xml:space="preserve">
+    <value>An expression was expected</value>
+  </data>
+  <data name="ExpectedMapping" xml:space="preserve">
+    <value>Expected a mapping</value>
+  </data>
+  <data name="ExpectedNParametersFollowingDirective" xml:space="preserve">
+    <value>Exactly {0} parameter(s) were expected following the directive '{1}'. Actual parameter count: {2}</value>
+  </data>
+  <data name="ExpectedScalar" xml:space="preserve">
+    <value>Expected a scalar value</value>
+  </data>
+  <data name="ExpectedScalarSequenceOrMapping" xml:space="preserve">
+    <value>Expected a scalar value, a sequence, or a mapping</value>
+  </data>
+  <data name="ExpectedSequence" xml:space="preserve">
+    <value>Expected a sequence</value>
+  </data>
+  <data name="ExpressionNotAllowed" xml:space="preserve">
+    <value>A template expression is not allowed in this context</value>
+  </data>
+  <data name="ExpressionNotClosed" xml:space="preserve">
+    <value>The expression is not closed. An unescaped ${{ sequence was found, but the closing }} sequence was not found.</value>
+  </data>
+  <data name="LineColumn" xml:space="preserve">
+    <value>(Line: {0}, Col: {1})</value>
+    <comment>Error message prefix that indicates the line and column where the error occurred</comment>
+  </data>
+  <data name="MaxObjectDepthExceeded" xml:space="preserve">
+    <value>Maximum object depth exceeded</value>
+  </data>
+  <data name="MaxObjectSizeExceeded" xml:space="preserve">
+    <value>Maximum object size exceeded</value>
+  </data>
+  <data name="MaxTemplateEventsExceeded" xml:space="preserve">
+    <value>Maximum events exceeded while evaluating the template. This may indicate an infinite loop or too many nested loops.</value>
+  </data>
+  <data name="TemplateNotValid" xml:space="preserve">
+    <value>The template is not valid.</value>
+  </data>
+  <data name="TemplateNotValidWithErrors" xml:space="preserve">
+    <value>The template is not valid. {0}</value>
+  </data>
+  <data name="CalledWorkflowNotValidWithErrors" xml:space="preserve">
+    <value>In {0} {1}: Error from called workflow</value>
+  </data>
+  <data name="UnableToConvertToTemplateToken" xml:space="preserve">
+    <value>Unable to convert the object to a template token. Actual type '{0}'</value>
+  </data>
+  <data name="UnableToDetermineOneOf" xml:space="preserve">
+    <value>There's not enough info to determine what you meant. Add one of these properties: {0}</value>
+  </data>
+  <data name="UnexpectedMappingStart" xml:space="preserve">
+    <value>A mapping was not expected</value>
+  </data>
+  <data name="UnexpectedSequenceStart" xml:space="preserve">
+    <value>A sequence was not expected</value>
+  </data>
+  <data name="UnexpectedValue" xml:space="preserve">
+    <value>Unexpected value '{0}'</value>
+  </data>
+  <data name="ValueAlreadyDefined" xml:space="preserve">
+    <value>'{0}' is already defined</value>
+  </data>
+</root>

src/Sdk/WorkflowParser/ObjectTemplating/TemplateValidationError.cs

@@ -0,0 +1,60 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Runtime.Serialization;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Provides information about an error which occurred during validation.
+    /// </summary>
+    [DataContract]
+    public class TemplateValidationError
+    {
+        public TemplateValidationError()
+        {
+        }
+
+        public TemplateValidationError(String message)
+            : this(null, message)
+        {
+        }
+
+        public TemplateValidationError(
+            String code,
+            String message)
+        {
+            Code = code;
+            Message = message;
+        }
+
+        [DataMember(Name = "code", EmitDefaultValue = false)]
+        public String Code
+        {
+            get;
+            set;
+        }
+
+        [DataMember(Name = "Message", EmitDefaultValue = false)]
+        public String Message
+        {
+            get;
+            set;
+        }
+
+        public static IEnumerable<TemplateValidationError> Create(Exception exception)
+        {
+            for (int i = 0; i < 50; i++)
+            {
+                yield return new TemplateValidationError(exception.Message);
+                if (exception.InnerException == null)
+                {
+                    break;
+                }
+
+                exception = exception.InnerException;
+            }
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateValidationErrors.cs

@@ -0,0 +1,157 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.Serialization;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Provides information about an error which occurred during validation.
+    /// </summary>
+    [DataContract]
+    public sealed class TemplateValidationErrors : IEnumerable<TemplateValidationError>
+    {
+        public TemplateValidationErrors()
+        {
+        }
+
+        public TemplateValidationErrors(
+            Int32 maxErrors,
+            Int32 maxMessageLength)
+        {
+            m_maxErrors = maxErrors;
+            m_maxMessageLength = maxMessageLength;
+        }
+
+        public Int32 Count => m_errors.Count;
+
+        public void Add(String message)
+        {
+            Add(new TemplateValidationError(message));
+        }
+
+        public void Add(Exception ex)
+        {
+            Add(null, ex);
+        }
+
+        public void Add(String messagePrefix, Exception ex)
+        {
+            for (int i = 0; i < 50; i++)
+            {
+                String message = !String.IsNullOrEmpty(messagePrefix) ? $"{messagePrefix} {ex.Message}" : ex.Message;
+                Add(new TemplateValidationError(message));
+                if (ex.InnerException == null)
+                {
+                    break;
+                }
+
+                ex = ex.InnerException;
+            }
+        }
+
+        public void Add(IEnumerable<TemplateValidationError> errors)
+        {
+            foreach (var error in errors)
+            {
+                Add(error);
+            }
+        }
+
+        public void Add(TemplateValidationError error)
+        {
+            // Check max errors
+            if (m_maxErrors <= 0 ||
+                m_errors.Count < m_maxErrors)
+            {
+                // Check max message length
+                if (m_maxMessageLength > 0 &&
+                    error.Message?.Length > m_maxMessageLength)
+                {
+                    error = new TemplateValidationError(error.Code, error.Message.Substring(0, m_maxMessageLength) + "[...]");
+                }
+
+                m_errors.Add(error);
+            }
+        }
+
+        /// <summary>
+        /// Add a prefix in the error message of the given index.
+        /// </summary>
+        public void PrefixMessage(int index, String prefix) {
+            if (index < 0 || index >= m_errors.Count) {
+                return;
+            }
+
+            var message = GetMessage(index);
+            if (!String.IsNullOrEmpty(message))
+            {
+                m_errors[index].Message = $"{prefix} {message}";
+            }
+            else
+            {
+                m_errors[index].Message = $"{prefix}";
+            }
+        }
+
+        public string GetMessage(int index)
+        {
+            if (index < 0 || index >= m_errors.Count)
+            {
+                return null;
+            }
+
+            return m_errors[index].Message;
+        }
+
+        /// <summary>
+        /// Throws <c ref="TemplateValidationException" /> if any errors.
+        /// </summary>
+        public void Check()
+        {
+            if (m_errors.Count > 0)
+            {
+                throw new TemplateValidationException(m_errors);
+            }
+        }
+
+        /// <summary>
+        /// Throws <c ref="TemplateValidationException" /> if any errors.
+        /// <param name="prefix">The error message prefix</param>
+        /// </summary>
+        public void Check(String prefix)
+        {
+            if (String.IsNullOrEmpty(prefix))
+            {
+                this.Check();
+            }
+            else if (m_errors.Count > 0)
+            {
+                var message = $"{prefix.Trim()} {String.Join(",", m_errors.Select(e => e.Message))}";
+                throw new TemplateValidationException(message, m_errors);
+            }
+        }
+
+        public void Clear()
+        {
+            m_errors.Clear();
+        }
+
+        public IEnumerator<TemplateValidationError> GetEnumerator()
+        {
+            return (m_errors as IEnumerable<TemplateValidationError>).GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return (m_errors as IEnumerable).GetEnumerator();
+        }
+
+        private readonly List<TemplateValidationError> m_errors = new List<TemplateValidationError>();
+        private readonly Int32 m_maxErrors;
+        private readonly Int32 m_maxMessageLength;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateValidationException.cs

@@ -0,0 +1,57 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.Serialization;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    public class TemplateValidationException : Exception
+    {
+        public TemplateValidationException()
+            : this(TemplateStrings.TemplateNotValid())
+        {
+        }
+
+        public TemplateValidationException(IEnumerable<TemplateValidationError> errors)
+            : this(TemplateStrings.TemplateNotValidWithErrors(string.Join(",", (errors ?? Enumerable.Empty<TemplateValidationError>()).Select(e => e.Message))))
+        {
+            m_errors = new List<TemplateValidationError>(errors ?? Enumerable.Empty<TemplateValidationError>());
+        }
+
+        public TemplateValidationException(
+            String message,
+            IEnumerable<TemplateValidationError> errors)
+            : this(message)
+        {
+            m_errors = new List<TemplateValidationError>(errors ?? Enumerable.Empty<TemplateValidationError>());
+        }
+
+        public TemplateValidationException(String message)
+            : base(message)
+        {
+        }
+
+        public TemplateValidationException(
+            String message,
+            Exception innerException)
+            : base(message, innerException)
+        {
+        }
+
+        public IList<TemplateValidationError> Errors
+        {
+            get
+            {
+                if (m_errors == null)
+                {
+                    m_errors = new List<TemplateValidationError>();
+                }
+                return m_errors;
+            }
+        }
+
+        private List<TemplateValidationError> m_errors;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/TemplateWriter.cs

@@ -0,0 +1,74 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating
+{
+    /// <summary>
+    /// Converts from a TemplateToken into another object format
+    /// </summary>
+    internal sealed class TemplateWriter
+    {
+        internal static void Write(
+            IObjectWriter objectWriter,
+            TemplateToken value)
+        {
+            objectWriter.WriteStart();
+            WriteValue(objectWriter, value);
+            objectWriter.WriteEnd();
+        }
+
+        private static void WriteValue(
+            IObjectWriter objectWriter,
+            TemplateToken value)
+        {
+            switch (value?.Type ?? TokenType.Null)
+            {
+                case TokenType.Null:
+                    objectWriter.WriteNull();
+                    break;
+
+                case TokenType.Boolean:
+                    var booleanToken = value as BooleanToken;
+                    objectWriter.WriteBoolean(booleanToken.Value);
+                    break;
+
+                case TokenType.Number:
+                    var numberToken = value as NumberToken;
+                    objectWriter.WriteNumber(numberToken.Value);
+                    break;
+
+                case TokenType.String:
+                case TokenType.BasicExpression:
+                case TokenType.InsertExpression:
+                    objectWriter.WriteString(value.ToString());
+                    break;
+
+                case TokenType.Mapping:
+                    var mappingToken = value as MappingToken;
+                    objectWriter.WriteMappingStart();
+                    foreach (var pair in mappingToken)
+                    {
+                        WriteValue(objectWriter, pair.Key);
+                        WriteValue(objectWriter, pair.Value);
+                    }
+                    objectWriter.WriteMappingEnd();
+                    break;
+
+                case TokenType.Sequence:
+                    var sequenceToken = value as SequenceToken;
+                    objectWriter.WriteSequenceStart();
+                    foreach (var item in sequenceToken)
+                    {
+                        WriteValue(objectWriter, item);
+                    }
+                    objectWriter.WriteSequenceEnd();
+                    break;
+
+                default:
+                    throw new NotSupportedException($"Unexpected type '{value.GetType()}'");
+            }
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/BasicExpressionToken.cs

@@ -0,0 +1,142 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Globalization;
+using System.Linq;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions;
+using GitHub.Actions.Expressions.Sdk;
+using GitHub.Actions.Expressions.Sdk.Functions;
+using Container = GitHub.Actions.Expressions.Sdk.Container;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public sealed class BasicExpressionToken : ExpressionToken
+    {
+        public BasicExpressionToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            String expression)
+            : base(TokenType.BasicExpression, fileId, line, column, null)
+        {
+            m_expression = expression;
+        }
+
+        internal String Expression
+        {
+            get
+            {
+                if (m_expression == null)
+                {
+                    m_expression = String.Empty;
+                }
+
+                return m_expression;
+            }
+        }
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+            return omitSource ? new BasicExpressionToken(null, null, null, m_expression) : new BasicExpressionToken(FileId, Line, Column, m_expression);
+        }
+
+        public override String ToString()
+        {
+            return $"{TemplateConstants.OpenExpression} {m_expression} {TemplateConstants.CloseExpression}";
+        }
+
+        public override String ToDisplayString()
+        {
+            var expressionParser = new ExpressionParser();
+            var expressionNode = expressionParser.ValidateSyntax(Expression, null);
+            if (expressionNode is Format formatNode)
+            {
+                // Make sure our first item is indeed a literal string so we can format it.
+                if (formatNode.Parameters.Count > 1 &&
+                    formatNode.Parameters.First() is Literal literalValueNode &&
+                    literalValueNode.Kind == ValueKind.String)
+                {
+                    // Get all other Parameters san the formatted string to pass into the formatter
+                    var formatParameters = formatNode.Parameters.Skip(1).Select(x => this.ConvertFormatParameterToExpression(x)).ToArray();
+                    if (formatParameters.Length > 0)
+                    {
+                        String formattedString = String.Empty;
+                        try
+                        {
+                            formattedString = String.Format(CultureInfo.InvariantCulture, (formatNode.Parameters[0] as Literal).Value as String, formatParameters);
+                        }
+                        catch (FormatException) { }
+                        catch (ArgumentNullException) { } // If this operation fails, revert to default display name
+                        if (!String.IsNullOrEmpty(formattedString))
+                        {
+                            return TrimDisplayString(formattedString);
+                        }
+                    }
+                }
+            }
+            return base.ToDisplayString();
+        }
+
+        internal StringToken EvaluateStringToken(
+            TemplateContext context,
+            out Int32 bytes)
+        {
+            return EvaluateStringToken(context, Expression, out bytes);
+        }
+
+        internal MappingToken EvaluateMappingToken(
+            TemplateContext context,
+            out Int32 bytes)
+        {
+            return EvaluateMappingToken(context, Expression, out bytes);
+        }
+
+        internal SequenceToken EvaluateSequenceToken(
+            TemplateContext context,
+            out Int32 bytes)
+        {
+            return EvaluateSequenceToken(context, Expression, out bytes);
+        }
+
+        internal TemplateToken EvaluateTemplateToken(
+            TemplateContext context,
+            out Int32 bytes)
+        {
+            return EvaluateTemplateToken(context, Expression, out bytes);
+        }
+
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            if (m_expression?.Length == 0)
+            {
+                m_expression = null;
+            }
+        }
+
+        private String ConvertFormatParameterToExpression(ExpressionNode node)
+        {
+            var nodeString = node.ConvertToExpression();
+
+            // If the node is a container, see if it starts with '(' and ends with ')' so we can simplify the string
+            // Should only simplify if only one '(' or ')' exists in the string
+            // We are trying to simplify the case (a || b) to a || b
+            // But we should avoid simplifying ( a && b
+            if (node is Container &&
+                nodeString.Length > 2 &&
+                nodeString[0] == ExpressionConstants.StartParameter &&
+                nodeString[nodeString.Length - 1] == ExpressionConstants.EndParameter &&
+                nodeString.Count(character => character == ExpressionConstants.StartParameter) == 1 &&
+                nodeString.Count(character => character == ExpressionConstants.EndParameter) == 1)
+            {
+                nodeString = nodeString = nodeString.Substring(1, nodeString.Length - 2);
+            }
+            return String.Concat(TemplateConstants.OpenExpression, " ", nodeString, " ", TemplateConstants.CloseExpression);
+        }
+
+        [DataMember(Name = "expr", EmitDefaultValue = false)]
+        private String m_expression;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/BooleanToken.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public sealed class BooleanToken : LiteralToken, IBoolean
+    {
+        public BooleanToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            Boolean value)
+            : base(TokenType.Boolean, fileId, line, column)
+        {
+            m_value = value;
+        }
+
+        public Boolean Value => m_value;
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+           return omitSource ? new BooleanToken(null, null, null, m_value) : new BooleanToken(FileId, Line, Column, m_value);
+        }
+
+        public override String ToString()
+        {
+           return m_value ? "true" : "false";
+        }
+
+        Boolean IBoolean.GetBoolean()
+        {
+            return Value;
+        }
+
+        [DataMember(Name = "bool", EmitDefaultValue = false)]
+        private Boolean m_value;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/ExpressionToken.cs

@@ -0,0 +1,95 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Runtime.Serialization;
+using System.Text.RegularExpressions;
+using GitHub.Actions.Expressions;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    /// <summary>
+    /// Base class for all template expression tokens
+    /// </summary>
+    [DataContract]
+    public abstract class ExpressionToken : ScalarToken
+    {
+        internal ExpressionToken(
+            Int32 templateType,
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            String directive)
+            : base(templateType, fileId, line, column)
+        {
+            Directive = directive;
+        }
+
+        [DataMember(Name = "directive", EmitDefaultValue = false)]
+        internal String Directive { get; }
+
+        internal static Boolean IsValidExpression(
+            String expression,
+            String[] allowedContext,
+            out Exception ex)
+        {
+            // Create dummy named values and functions
+            var namedValues = new List<INamedValueInfo>();
+            var functions = new List<IFunctionInfo>();
+            if (allowedContext?.Length > 0)
+            {
+                foreach (var contextItem in allowedContext)
+                {
+                    var match = s_function.Match(contextItem);
+                    if (match.Success)
+                    {
+                        var functionName = match.Groups[1].Value;
+                        var minParameters = Int32.Parse(match.Groups[2].Value, NumberStyles.None, CultureInfo.InvariantCulture);
+                        var maxParametersRaw = match.Groups[3].Value;
+                        var maxParameters = String.Equals(maxParametersRaw, TemplateConstants.MaxConstant, StringComparison.Ordinal)
+                            ? Int32.MaxValue
+                            : Int32.Parse(maxParametersRaw, NumberStyles.None, CultureInfo.InvariantCulture);
+                        functions.Add(new FunctionInfo<DummyFunction>(functionName, minParameters, maxParameters));
+                    }
+                    else
+                    {
+                        namedValues.Add(new NamedValueInfo<ContextValueNode>(contextItem));
+                    }
+                }
+            }
+
+            // Parse
+            Boolean result;
+            ExpressionNode root = null;
+            try
+            {
+                root = new ExpressionParser().CreateTree(expression, null, namedValues, functions) as ExpressionNode;
+
+                result = true;
+                ex = null;
+            }
+            catch (Exception exception)
+            {
+                result = false;
+                ex = exception;
+            }
+
+            return result;
+        }
+
+        private sealed class DummyFunction : Function
+        {
+            protected override Object EvaluateCore(
+                EvaluationContext context,
+                out ResultMemory resultMemory)
+            {
+                resultMemory = null;
+                return null;
+            }
+        }
+
+        private static readonly Regex s_function = new Regex(@"^([a-zA-Z0-9_]+)\(([0-9]+),([0-9]+|MAX)\)$", RegexOptions.Compiled);
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/InsertExpressionToken.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public sealed class InsertExpressionToken : ExpressionToken
+    {
+        public InsertExpressionToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+            : base(TokenType.InsertExpression, fileId, line, column, TemplateConstants.InsertDirective)
+        {
+        }
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+            return omitSource ? new InsertExpressionToken(null, null, null) : new InsertExpressionToken(FileId, Line, Column);
+        }
+
+        public override String ToString()
+        {
+            return $"{TemplateConstants.OpenExpression} insert {TemplateConstants.CloseExpression}";
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/LiteralToken.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public abstract class LiteralToken : ScalarToken
+    {
+        public LiteralToken(
+            Int32 tokenType,
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+            : base(tokenType, fileId, line, column)
+        {
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/MappingToken.cs

@@ -0,0 +1,245 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.Runtime.Serialization;
+using System.Threading;
+using GitHub.Actions.Expressions.Sdk;
+using Newtonsoft.Json;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    [JsonObject]
+    public sealed class MappingToken : TemplateToken, IEnumerable<KeyValuePair<ScalarToken, TemplateToken>>, IReadOnlyObject
+    {
+        public MappingToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+            : base(TokenType.Mapping, fileId, line, column)
+        {
+        }
+
+        internal Int32 Count => m_items?.Count ?? 0;
+
+        // IReadOnlyObject (for expressions)
+        Int32 IReadOnlyObject.Count
+        {
+            get
+            {
+                InitializeDictionary();
+                return m_dictionary.Count;
+            }
+        }
+
+        // IReadOnlyObject (for expressions)
+        IEnumerable<String> IReadOnlyObject.Keys
+        {
+            get
+            {
+                InitializeDictionary();
+                foreach (var key in m_dictionary.Keys)
+                {
+                    yield return key as String;
+                }
+            }
+        }
+
+        // IReadOnlyObject (for expressions)
+        IEnumerable<Object> IReadOnlyObject.Values
+        {
+            get
+            {
+                InitializeDictionary();
+                foreach (var value in m_dictionary.Values)
+                {
+                    yield return value;
+                }
+            }
+        }
+
+        public KeyValuePair<ScalarToken, TemplateToken> this[Int32 index]
+        {
+            get
+            {
+                return m_items[index];
+            }
+
+            set
+            {
+                m_items[index] = value;
+                m_dictionary = null;
+            }
+        }
+
+        // IReadOnlyObject (for expressions)
+        Object IReadOnlyObject.this[String key]
+        {
+            get
+            {
+                InitializeDictionary();
+                return m_dictionary[key];
+            }
+        }
+
+        public void Add(IEnumerable<KeyValuePair<ScalarToken, TemplateToken>> items)
+        {
+            foreach (var item in items)
+            {
+                Add(item);
+            }
+        }
+
+        public void Add(KeyValuePair<ScalarToken, TemplateToken> item)
+        {
+            if (m_items == null)
+            {
+                m_items = new List<KeyValuePair<ScalarToken, TemplateToken>>();
+            }
+
+            m_items.Add(item);
+            m_dictionary = null;
+        }
+
+        public void Add(
+            ScalarToken key,
+            TemplateToken value)
+        {
+            Add(new KeyValuePair<ScalarToken, TemplateToken>(key, value));
+        }
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+            var result = omitSource ? new MappingToken(null, null, null) : new MappingToken(FileId, Line, Column);
+            if (m_items?.Count > 0)
+            {
+                foreach (var pair in m_items)
+                {
+                    result.Add(pair.Key?.Clone(omitSource) as ScalarToken, pair.Value?.Clone(omitSource));
+                }
+            }
+            return result;
+        }
+
+        public IEnumerator<KeyValuePair<ScalarToken, TemplateToken>> GetEnumerator()
+        {
+            if (m_items?.Count > 0)
+            {
+                return m_items.GetEnumerator();
+            }
+            else
+            {
+                return (new List<KeyValuePair<ScalarToken, TemplateToken>>(0)).GetEnumerator();
+            }
+        }
+
+        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
+        {
+            if (m_items?.Count > 0)
+            {
+                return m_items.GetEnumerator();
+            }
+            else
+            {
+                return (new KeyValuePair<ScalarToken, TemplateToken>[0]).GetEnumerator();
+            }
+        }
+
+        public void Insert(
+            Int32 index,
+            KeyValuePair<ScalarToken, TemplateToken> item)
+        {
+            if (m_items == null)
+            {
+                m_items = new List<KeyValuePair<ScalarToken, TemplateToken>>();
+            }
+
+            m_items.Insert(index, item);
+            m_dictionary = null;
+        }
+
+        public void Insert(
+            Int32 index,
+            ScalarToken key,
+            TemplateToken value)
+        {
+            Insert(index, new KeyValuePair<ScalarToken, TemplateToken>(key, value));
+        }
+
+        public void RemoveAt(Int32 index)
+        {
+            m_items.RemoveAt(index);
+            m_dictionary = null;
+        }
+
+        // IReadOnlyObject (for expressions)
+        Boolean IReadOnlyObject.ContainsKey(String key)
+        {
+            InitializeDictionary();
+            return m_dictionary.Contains(key);
+        }
+
+        // IReadOnlyObject (for expressions)
+        IEnumerator IReadOnlyObject.GetEnumerator()
+        {
+            InitializeDictionary();
+            return m_dictionary.GetEnumerator();
+        }
+
+        // IReadOnlyObject (for expressions)
+        Boolean IReadOnlyObject.TryGetValue(
+            String key,
+            out Object value)
+        {
+            InitializeDictionary();
+            if (!m_dictionary.Contains(key))
+            {
+                value = null;
+                return false;
+            }
+
+            value = m_dictionary[key];
+            return true;
+        }
+
+        /// <summary>
+        /// Initializes the dictionary used for the expressions IReadOnlyObject interface
+        /// </summary>
+        private void InitializeDictionary()
+        {
+            if (m_dictionary == null)
+            {
+                var dictionary = new OrderedDictionary(StringComparer.OrdinalIgnoreCase);
+                if (m_items?.Count > 0)
+                {
+                    foreach (var pair in m_items)
+                    {
+                        if (pair.Key is StringToken stringToken &&
+                            !dictionary.Contains(stringToken.Value))
+                        {
+                            dictionary.Add(stringToken.Value, pair.Value);
+                        }
+                    }
+                }
+                Interlocked.CompareExchange(ref m_dictionary, dictionary, null);
+            }
+        }
+
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            if (m_items?.Count == 0)
+            {
+                m_items = null;
+            }
+        }
+
+        [DataMember(Name = "map", EmitDefaultValue = false)]
+        private List<KeyValuePair<ScalarToken, TemplateToken>> m_items;
+
+        private IDictionary m_dictionary;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/NullToken.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public sealed class NullToken : LiteralToken, INull
+    {
+        public NullToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+            : base(TokenType.Null, fileId, line, column)
+        {
+        }
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+           return omitSource ? new NullToken(null, null, null) : new NullToken(FileId, Line, Column);
+        }
+
+        public override String ToString()
+        {
+           return String.Empty;
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/NumberToken.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Globalization;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public sealed class NumberToken : LiteralToken, INumber
+    {
+        public NumberToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            Double value)
+            : base(TokenType.Number, fileId, line, column)
+        {
+            m_value = value;
+        }
+
+        public Double Value => m_value;
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+           return omitSource ? new NumberToken(null, null, null, m_value) : new NumberToken(FileId, Line, Column, m_value);
+        }
+
+        public override String ToString()
+        {
+            return m_value.ToString("G15", CultureInfo.InvariantCulture);
+        }
+
+        Double INumber.GetNumber()
+        {
+            return Value;
+        }
+
+        [DataMember(Name = "num", EmitDefaultValue = false)]
+        private Double m_value;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/ScalarToken.cs

@@ -0,0 +1,34 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    public abstract class ScalarToken : TemplateToken
+    {
+        protected ScalarToken(
+            Int32 type,
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+            : base(type, fileId, line, column)
+        {
+        }
+
+        public virtual String ToDisplayString()
+        {
+            return TrimDisplayString(ToString());
+        }
+
+        protected String TrimDisplayString(String displayString)
+        {
+            var firstLine = displayString.TrimStart(' ', '\t', '\r', '\n');
+            var firstNewLine = firstLine.IndexOfAny(new[] { '\r', '\n' });
+            if (firstNewLine >= 0)
+            {
+                firstLine = firstLine.Substring(0, firstNewLine);
+            }
+            return firstLine;
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/SequenceToken.cs

@@ -0,0 +1,149 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions.Sdk;
+using Newtonsoft.Json;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    [JsonObject]
+    public sealed class SequenceToken : TemplateToken, IEnumerable<TemplateToken>, IReadOnlyArray
+    {
+        public SequenceToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+            : base(TokenType.Sequence, fileId, line, column)
+        {
+        }
+
+        public Int32 Count => m_items?.Count ?? 0;
+
+        public TemplateToken this[Int32 index]
+        {
+            get
+            {
+                return m_items[index];
+            }
+
+            set
+            {
+                m_items[index] = value;
+            }
+        }
+
+        // IReadOnlyArray (for expressions)
+        Object IReadOnlyArray.this[Int32 index]
+        {
+            get
+            {
+                return m_items[index];
+            }
+        }
+
+        public void Add(TemplateToken value)
+        {
+            if (m_items == null)
+            {
+                m_items = new List<TemplateToken>();
+            }
+
+            m_items.Add(value);
+        }
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+            var result = omitSource ? new SequenceToken(null, null, null) : new SequenceToken(FileId, Line, Column);
+            if (m_items?.Count > 0)
+            {
+                foreach (var item in m_items)
+                {
+                    result.Add(item?.Clone(omitSource));
+                }
+            }
+            return result;
+        }
+
+        public IEnumerator<TemplateToken> GetEnumerator()
+        {
+            if (m_items?.Count > 0)
+            {
+                return m_items.GetEnumerator();
+            }
+            else
+            {
+                return (new TemplateToken[0] as IEnumerable<TemplateToken>).GetEnumerator();
+            }
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            if (m_items?.Count > 0)
+            {
+                return m_items.GetEnumerator();
+            }
+            else
+            {
+                return (new TemplateToken[0] as IEnumerable<TemplateToken>).GetEnumerator();
+            }
+        }
+
+        // IReadOnlyArray (for expressions)
+        IEnumerator IReadOnlyArray.GetEnumerator()
+        {
+            if (m_items?.Count > 0)
+            {
+                return m_items.GetEnumerator();
+            }
+            else
+            {
+                return (new TemplateToken[0] as IEnumerable<TemplateToken>).GetEnumerator();
+            }
+        }
+
+        public void Insert(
+            Int32 index,
+            TemplateToken item)
+        {
+            if (m_items == null)
+            {
+                m_items = new List<TemplateToken>();
+            }
+
+            m_items.Insert(index, item);
+        }
+
+        public void InsertRange(
+            Int32 index,
+            IEnumerable<TemplateToken> items)
+        {
+            if (m_items == null)
+            {
+                m_items = new List<TemplateToken>();
+            }
+
+            m_items.InsertRange(index, items);
+        }
+
+        public void RemoveAt(Int32 index)
+        {
+            m_items.RemoveAt(index);
+        }
+
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            if (m_items?.Count == 0)
+            {
+                m_items = null;
+            }
+        }
+
+        [DataMember(Name = "seq", EmitDefaultValue = false)]
+        private List<TemplateToken> m_items;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/StringToken.cs

@@ -0,0 +1,62 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    [DataContract]
+    public sealed class StringToken : LiteralToken, IString
+    {
+        public StringToken(
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            String value)
+            : base(TokenType.String, fileId, line, column)
+        {
+            m_value = value;
+        }
+
+        public String Value
+        {
+           get
+           {
+               if (m_value == null)
+               {
+                   m_value = String.Empty;
+               }
+
+               return m_value;
+           }
+        }
+
+        public override TemplateToken Clone(Boolean omitSource)
+        {
+           return omitSource ? new StringToken(null, null, null, m_value) : new StringToken(FileId, Line, Column, m_value);
+        }
+
+        public override String ToString()
+        {
+           return m_value ?? String.Empty;
+        }
+
+        String IString.GetString()
+        {
+            return Value;
+        }
+
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            if (m_value?.Length == 0)
+            {
+                m_value = null;
+            }
+        }
+
+        [DataMember(Name = "lit", EmitDefaultValue = false)]
+        private String m_value;
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/TemplateToken.cs

@@ -0,0 +1,292 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Runtime.Serialization;
+using GitHub.Actions.Expressions;
+using GitHub.Actions.Expressions.Sdk;
+using Newtonsoft.Json;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    /// <summary>
+    /// Base class for all template tokens
+    /// </summary>
+    [DataContract]
+    [JsonConverter(typeof(TemplateTokenJsonConverter))]
+    public abstract class TemplateToken
+    {
+        protected TemplateToken(
+            Int32 type,
+            Int32? fileId,
+            Int32? line,
+            Int32? column)
+        {
+            Type = type;
+            FileId = fileId;
+            Line = line;
+            Column = column;
+        }
+
+        [DataMember(Name = "file", EmitDefaultValue = false)]
+        internal Int32? FileId { get; private set; }
+
+        [DataMember(Name = "line", EmitDefaultValue = false)]
+        internal Int32? Line { get; private set; }
+
+        [DataMember(Name = "col", EmitDefaultValue = false)]
+        internal Int32? Column { get; private set; }
+
+        [DataMember(Name = "type", EmitDefaultValue = false)]
+        internal Int32 Type { get; }
+
+        public TemplateToken Clone()
+        {
+            return Clone(false);
+        }
+
+        public abstract TemplateToken Clone(Boolean omitSource);
+
+        protected StringToken EvaluateStringToken(
+            TemplateContext context,
+            String expression,
+            out Int32 bytes)
+        {
+            var originalBytes = context.Memory.CurrentBytes;
+            try
+            {
+                var tree = new ExpressionParser().CreateTree(expression, null, context.GetExpressionNamedValues(), context.ExpressionFunctions);
+                var options = new EvaluationOptions
+                {
+                    MaxMemory = context.Memory.MaxBytes,
+                    StrictJsonParsing = context.StrictJsonParsing,
+                };
+                var result = tree.Evaluate(context.TraceWriter.ToExpressionTraceWriter(), null, context, options);
+
+                if (result.Raw is LiteralToken literalToken)
+                {
+                    var stringToken = new StringToken(FileId, Line, Column, literalToken.ToString());
+                    context.Memory.AddBytes(stringToken);
+                    return stringToken;
+                }
+
+                if (!result.IsPrimitive)
+                {
+                    context.Error(this, "Expected a string");
+                    return CreateStringToken(context, expression);
+                }
+
+                var stringValue = result.Kind == ValueKind.Null ? String.Empty : result.ConvertToString();
+                return CreateStringToken(context, stringValue);
+            }
+            finally
+            {
+                bytes = context.Memory.CurrentBytes - originalBytes;
+            }
+        }
+
+        protected SequenceToken EvaluateSequenceToken(
+            TemplateContext context,
+            String expression,
+            out Int32 bytes)
+        {
+            var originalBytes = context.Memory.CurrentBytes;
+            try
+            {
+                var tree = new ExpressionParser().CreateTree(expression, null, context.GetExpressionNamedValues(), context.ExpressionFunctions);
+                var options = new EvaluationOptions
+                {
+                    MaxMemory = context.Memory.MaxBytes,
+                    StrictJsonParsing = context.StrictJsonParsing,
+                };
+                var result = tree.Evaluate(context.TraceWriter.ToExpressionTraceWriter(), null, context, options);
+                var templateToken = ConvertToTemplateToken(context, result);
+                if (templateToken is SequenceToken sequence)
+                {
+                    return sequence;
+                }
+
+                context.Error(this, TemplateStrings.ExpectedSequence());
+                return CreateSequenceToken(context);
+            }
+            finally
+            {
+                bytes = context.Memory.CurrentBytes - originalBytes;
+            }
+        }
+
+        protected MappingToken EvaluateMappingToken(
+            TemplateContext context,
+            String expression,
+            out Int32 bytes)
+        {
+            var originalBytes = context.Memory.CurrentBytes;
+            try
+            {
+                var tree = new ExpressionParser().CreateTree(expression, null, context.GetExpressionNamedValues(), context.ExpressionFunctions);
+                var options = new EvaluationOptions
+                {
+                    MaxMemory = context.Memory.MaxBytes,
+                    StrictJsonParsing = context.StrictJsonParsing,
+                };
+                var result = tree.Evaluate(context.TraceWriter.ToExpressionTraceWriter(), null, context, options);
+                var templateToken = ConvertToTemplateToken(context, result);
+                if (templateToken is MappingToken mapping)
+                {
+                    return mapping;
+                }
+
+                context.Error(this, TemplateStrings.ExpectedMapping());
+                return CreateMappingToken(context);
+            }
+            finally
+            {
+                bytes = context.Memory.CurrentBytes - originalBytes;
+            }
+        }
+
+        protected TemplateToken EvaluateTemplateToken(
+            TemplateContext context,
+            String expression,
+            out Int32 bytes)
+        {
+            var originalBytes = context.Memory.CurrentBytes;
+            try
+            {
+                var tree = new ExpressionParser().CreateTree(expression, null, context.GetExpressionNamedValues(), context.ExpressionFunctions);
+                var options = new EvaluationOptions
+                {
+                    MaxMemory = context.Memory.MaxBytes,
+                    StrictJsonParsing = context.StrictJsonParsing,
+                };
+                var result = tree.Evaluate(context.TraceWriter.ToExpressionTraceWriter(), null, context, options);
+                return ConvertToTemplateToken(context, result);
+            }
+            finally
+            {
+                bytes = context.Memory.CurrentBytes - originalBytes;
+            }
+        }
+
+        private TemplateToken ConvertToTemplateToken(
+            TemplateContext context,
+            EvaluationResult result)
+        {
+            // Literal
+            if (TryConvertToLiteralToken(context, result, out LiteralToken literal))
+            {
+                return literal;
+            }
+            // Known raw types
+            else if (!Object.ReferenceEquals(result.Raw, null))
+            {
+                if (result.Raw is SequenceToken sequence)
+                {
+                    context.Memory.AddBytes(sequence, true);
+                    return sequence;
+                }
+                else if (result.Raw is MappingToken mapping)
+                {
+                    context.Memory.AddBytes(mapping, true);
+                    return mapping;
+                }
+            }
+
+            // Leverage the expression SDK to traverse the object
+            if (result.TryGetCollectionInterface(out Object collection))
+            {
+                if (collection is IReadOnlyObject dictionary)
+                {
+                    var mapping = CreateMappingToken(context);
+
+                    foreach (KeyValuePair<String, Object> pair in dictionary)
+                    {
+                        var keyToken = CreateStringToken(context, pair.Key);
+                        var valueResult = EvaluationResult.CreateIntermediateResult(null, pair.Value);
+                        var valueToken = ConvertToTemplateToken(context, valueResult);
+                        mapping.Add(keyToken, valueToken);
+                    }
+
+                    return mapping;
+                }
+                else if (collection is IReadOnlyArray list)
+                {
+                    var sequence = CreateSequenceToken(context);
+
+                    foreach (var item in list)
+                    {
+                        var itemResult = EvaluationResult.CreateIntermediateResult(null, item);
+                        var itemToken = ConvertToTemplateToken(context, itemResult);
+                        sequence.Add(itemToken);
+                    }
+
+                    return sequence;
+                }
+            }
+
+            throw new ArgumentException(TemplateStrings.UnableToConvertToTemplateToken(result.Value?.GetType().FullName));
+        }
+
+        private Boolean TryConvertToLiteralToken(
+            TemplateContext context,
+            EvaluationResult result,
+            out LiteralToken literal)
+        {
+            if (result.Raw is LiteralToken literal2)
+            {
+                context.Memory.AddBytes(literal2);
+                literal = literal2;
+                return true;
+            }
+
+            switch (result.Kind)
+            {
+                case ValueKind.Null:
+                    literal = new NullToken(FileId, Line, Column);
+                    break;
+
+                case ValueKind.Boolean:
+                    literal = new BooleanToken(FileId, Line, Column, (Boolean)result.Value);
+                    break;
+
+                case ValueKind.Number:
+                    literal = new NumberToken(FileId, Line, Column, (Double)result.Value);
+                    break;
+
+                case ValueKind.String:
+                    literal = new StringToken(FileId, Line, Column, (String)result.Value);
+                    break;
+
+                default:
+                    literal = null;
+                    return false;
+            }
+
+            context.Memory.AddBytes(literal);
+            return true;
+        }
+
+        private StringToken CreateStringToken(
+            TemplateContext context,
+            String value)
+        {
+            var result = new StringToken(FileId, Line, Column, value);
+            context.Memory.AddBytes(result);
+            return result;
+        }
+
+        private SequenceToken CreateSequenceToken(TemplateContext context)
+        {
+            var result = new SequenceToken(FileId, Line, Column);
+            context.Memory.AddBytes(result);
+            return result;
+        }
+
+        private MappingToken CreateMappingToken(TemplateContext context)
+        {
+            var result = new MappingToken(FileId, Line, Column);
+            context.Memory.AddBytes(result);
+            return result;
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/TemplateTokenExtensions.cs

@@ -0,0 +1,291 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GitHub.Actions.Expressions;
+using GitHub.Actions.Expressions.Sdk;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    internal static class TemplateTokenExtensions
+    {
+        internal static BooleanToken AssertBoolean(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is BooleanToken booleanToken)
+            {
+                return booleanToken;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(BooleanToken)}' was expected.");
+        }
+
+        internal static NullToken AssertNull(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is NullToken nullToken)
+            {
+                return nullToken;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(NullToken)}' was expected.");
+        }
+
+        internal static NumberToken AssertNumber(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is NumberToken numberToken)
+            {
+                return numberToken;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(NumberToken)}' was expected.");
+        }
+
+        internal static StringToken AssertString(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is StringToken stringToken)
+            {
+                return stringToken;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(StringToken)}' was expected.");
+        }
+
+        internal static MappingToken AssertMapping(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is MappingToken mapping)
+            {
+                return mapping;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(MappingToken)}' was expected.");
+        }
+
+        internal static ScalarToken AssertScalar(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is ScalarToken scalar)
+            {
+                return scalar;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(ScalarToken)}' was expected.");
+        }
+
+        internal static SequenceToken AssertSequence(
+            this TemplateToken value,
+            string objectDescription)
+        {
+            if (value is SequenceToken sequence)
+            {
+                return sequence;
+            }
+
+            throw new ArgumentException($"Unexpected type '{value?.GetType().Name}' encountered while reading '{objectDescription}'. The type '{nameof(SequenceToken)}' was expected.");
+        }
+
+        internal static void AssertUnexpectedValue(
+            this LiteralToken literal,
+            string objectDescription)
+        {
+            throw new ArgumentException($"Error while reading '{objectDescription}'. Unexpected value '{literal.ToString()}'");
+        }
+
+        /// <summary>
+        /// Traverses the token and checks whether all required expression values
+        /// and functions are provided.
+        /// </summary>
+        public static bool CheckHasRequiredContext(
+            this TemplateToken token,
+            IReadOnlyObject expressionValues,
+            IList<IFunctionInfo> expressionFunctions)
+        {
+            var expressionTokens = token.Traverse()
+                .OfType<BasicExpressionToken>()
+                .ToArray();
+            var parser = new ExpressionParser();
+            foreach (var expressionToken in expressionTokens)
+            {
+                var tree = parser.ValidateSyntax(expressionToken.Expression, null);
+                foreach (var node in tree.Traverse())
+                {
+                    if (node is NamedValue namedValue)
+                    {
+                        if (expressionValues?.Keys.Any(x => string.Equals(x, namedValue.Name, StringComparison.OrdinalIgnoreCase)) != true)
+                        {
+                            return false;
+                        }
+                    }
+                    else if (node is Function function &&
+                        !ExpressionConstants.WellKnownFunctions.ContainsKey(function.Name) &&
+                        expressionFunctions?.Any(x => string.Equals(x.Name, function.Name, StringComparison.OrdinalIgnoreCase)) != true)
+                    {
+                        return false;
+                    }
+                }
+            }
+
+            return true;
+        }
+
+        /// <summary>
+        /// Traverses each token that is provided (including descendants) and
+        /// checks whether specific contexts or sub-properties of contexts are referenced.
+        /// If a conclusive determination cannot be made, then the pattern is considered matched.
+        /// For example, the expression "toJson(github)" matches the pattern "github.event" because
+        /// the value is passed to a function. Not enough information is known to determine whether
+        /// the function requires the sub-property. Therefore, it is assumed that it may.
+        ///
+        /// Wildcards are supported in the pattern, and are treated as matching any literal.
+        /// For example, the expression "needs.my-job.outputs.my-output" matches the pattern "needs.*.outputs".
+        /// </summary>
+        public static bool[] CheckReferencesContext(
+            this IList<TemplateToken> tokens,
+            params string[] patterns)
+        {
+            var result = new bool[patterns.Length];
+
+            var expressionTokens = tokens
+                .SelectMany(x => x.Traverse())
+                .OfType<BasicExpressionToken>()
+                .ToArray();
+            var parser = new ExpressionParser();
+            foreach (var expressionToken in expressionTokens)
+            {
+                var tree = parser.ValidateSyntax(expressionToken.Expression, null);
+                var isReferenced = tree.CheckReferencesContext(patterns);
+                for (var i = 0; i < patterns.Length; i++)
+                {
+                    if (isReferenced[i])
+                    {
+                        result[i] = true;
+                    }
+                }
+            }
+
+            return result;
+        }
+
+        /// <summary>
+        /// Returns all tokens (depth first)
+        /// </summary>
+        public static IEnumerable<TemplateToken> Traverse(this TemplateToken token)
+        {
+            return Traverse(token, omitKeys: false);
+        }
+
+        /// <summary>
+        /// Returns all tokens (depth first)
+        /// </summary>
+        public static IEnumerable<TemplateToken> Traverse(
+            this TemplateToken token,
+            bool omitKeys)
+        {
+            if (token != null)
+            {
+                yield return token;
+
+                if (token is SequenceToken || token is MappingToken)
+                {
+                    var state = new TraversalState(null, token);
+                    while (state != null)
+                    {
+                        if (state.MoveNext(omitKeys))
+                        {
+                            token = state.Current;
+                            yield return token;
+
+                            if (token is SequenceToken || token is MappingToken)
+                            {
+                                state = new TraversalState(state, token);
+                            }
+                        }
+                        else
+                        {
+                            state = state.Parent;
+                        }
+                    }
+                }
+            }
+        }
+
+        private sealed class TraversalState
+        {
+            public TraversalState(
+                TraversalState parent,
+                TemplateToken token)
+            {
+                Parent = parent;
+                m_token = token;
+            }
+
+            public bool MoveNext(bool omitKeys)
+            {
+                switch (m_token.Type)
+                {
+                    case TokenType.Sequence:
+                        var sequence = m_token as SequenceToken;
+                        if (++m_index < sequence.Count)
+                        {
+                            Current = sequence[m_index];
+                            return true;
+                        }
+                        else
+                        {
+                            Current = null;
+                            return false;
+                        }
+
+                    case TokenType.Mapping:
+                        var mapping = m_token as MappingToken;
+
+                        // Return the value
+                        if (m_isKey)
+                        {
+                            m_isKey = false;
+                            Current = mapping[m_index].Value;
+                            return true;
+                        }
+
+                        if (++m_index < mapping.Count)
+                        {
+                            // Skip the key, return the value
+                            if (omitKeys)
+                            {
+                                m_isKey = false;
+                                Current = mapping[m_index].Value;
+                                return true;
+                            }
+
+                            // Return the key
+                            m_isKey = true;
+                            Current = mapping[m_index].Key;
+                            return true;
+                        }
+
+                        Current = null;
+                        return false;
+
+                    default:
+                        throw new NotSupportedException($"Unexpected token type '{m_token.Type}'");
+                }
+            }
+
+            private TemplateToken m_token;
+            private int m_index = -1;
+            private bool m_isKey;
+            public TemplateToken Current;
+            public TraversalState Parent;
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/TemplateTokenJsonConverter.cs

@@ -0,0 +1,370 @@
+#nullable disable // Consider removing in the future to minimize likelihood of NullReferenceException; refer https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
+
+using System;
+using System.Reflection;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    /// <summary>
+    /// JSON serializer for TemplateToken objects
+    /// </summary>
+    internal sealed class TemplateTokenJsonConverter : JsonConverter
+    {
+        public override Boolean CanWrite
+        {
+            get
+            {
+                return true;
+            }
+        }
+
+        public override Boolean CanConvert(Type objectType)
+        {
+            return typeof(TemplateToken).GetTypeInfo().IsAssignableFrom(objectType.GetTypeInfo());
+        }
+
+        public override Object ReadJson(
+            JsonReader reader,
+            Type objectType,
+            Object existingValue,
+            JsonSerializer serializer)
+        {
+            switch (reader.TokenType)
+            {
+                case JsonToken.String:
+                    return new StringToken(null, null, null, reader.Value.ToString());
+                case JsonToken.Boolean:
+                    return new BooleanToken(null, null, null, (Boolean)reader.Value);
+                case JsonToken.Float:
+                    return new NumberToken(null, null, null, (Double)reader.Value);
+                case JsonToken.Integer:
+                    return new NumberToken(null, null, null, (Double)(Int64)reader.Value);
+                case JsonToken.Null:
+                    return new NullToken(null, null, null);
+                case JsonToken.StartObject:
+                    break;
+                default:
+                    return null;
+            }
+
+            Int32? type = null;
+            JObject value = JObject.Load(reader);
+            if (!value.TryGetValue("type", StringComparison.OrdinalIgnoreCase, out JToken typeValue))
+            {
+                type = TokenType.String;
+            }
+            else if (typeValue.Type == JTokenType.Integer)
+            {
+                type = (Int32)typeValue;
+            }
+            else
+            {
+                return existingValue;
+            }
+
+            Object newValue = null;
+            switch (type)
+            {
+                case TokenType.Null:
+                    newValue = new NullToken(null, null, null);
+                    break;
+
+                case TokenType.Boolean:
+                    newValue = new BooleanToken(null, null, null, default(Boolean));
+                    break;
+
+                case TokenType.Number:
+                    newValue = new NumberToken(null, null, null, default(Double));
+                    break;
+
+                case TokenType.String:
+                    newValue = new StringToken(null, null, null, null);
+                    break;
+
+                case TokenType.BasicExpression:
+                    newValue = new BasicExpressionToken(null, null, null, null);
+                    break;
+
+                case TokenType.InsertExpression:
+                    newValue = new InsertExpressionToken(null, null, null);
+                    break;
+
+                case TokenType.Sequence:
+                    newValue = new SequenceToken(null, null, null);
+                    break;
+
+                case TokenType.Mapping:
+                    newValue = new MappingToken(null, null, null);
+                    break;
+            }
+
+            if (value != null)
+            {
+                using JsonReader objectReader = value.CreateReader();
+                serializer.Populate(objectReader, newValue);
+            }
+
+            return newValue;
+        }
+
+        public override void WriteJson(
+            JsonWriter writer,
+            Object value,
+            JsonSerializer serializer)
+        {
+            if (value is TemplateToken token)
+            {
+                switch (token.Type)
+                {
+                    case TokenType.Null:
+                        if (token.FileId == null && token.Line == null && token.Column == null)
+                        {
+                            writer.WriteNull();
+                        }
+                        else
+                        {
+                            writer.WriteStartObject();
+                            writer.WritePropertyName("type");
+                            writer.WriteValue(token.Type);
+                            if (token.FileId != null)
+                            {
+                                writer.WritePropertyName("file");
+                                writer.WriteValue(token.FileId);
+                            }
+                            if (token.Line != null)
+                            {
+                                writer.WritePropertyName("line");
+                                writer.WriteValue(token.Line);
+                            }
+                            if (token.Column != null)
+                            {
+                                writer.WritePropertyName("col");
+                                writer.WriteValue(token.Column);
+                            }
+                            writer.WriteEndObject();
+                        }
+                        return;
+
+                    case TokenType.Boolean:
+                        var booleanToken = token as BooleanToken;
+                        if (token.FileId == null && token.Line == null && token.Column == null)
+                        {
+                            writer.WriteValue(booleanToken.Value);
+                        }
+                        else
+                        {
+                            writer.WriteStartObject();
+                            writer.WritePropertyName("type");
+                            writer.WriteValue(token.Type);
+                            if (token.FileId != null)
+                            {
+                                writer.WritePropertyName("file");
+                                writer.WriteValue(token.FileId);
+                            }
+                            if (token.Line != null)
+                            {
+                                writer.WritePropertyName("line");
+                                writer.WriteValue(token.Line);
+                            }
+                            if (token.Column != null)
+                            {
+                                writer.WritePropertyName("col");
+                                writer.WriteValue(token.Column);
+                            }
+                            writer.WritePropertyName("bool");
+                            writer.WriteValue(booleanToken.Value);
+                            writer.WriteEndObject();
+                        }
+                        return;
+
+                    case TokenType.Number:
+                        var numberToken = token as NumberToken;
+                        if (token.FileId == null && token.Line == null && token.Column == null)
+                        {
+                            writer.WriteValue(numberToken.Value);
+                        }
+                        else
+                        {
+                            writer.WriteStartObject();
+                            writer.WritePropertyName("type");
+                            writer.WriteValue(token.Type);
+                            if (token.FileId != null)
+                            {
+                                writer.WritePropertyName("file");
+                                writer.WriteValue(token.FileId);
+                            }
+                            if (token.Line != null)
+                            {
+                                writer.WritePropertyName("line");
+                                writer.WriteValue(token.Line);
+                            }
+                            if (token.Column != null)
+                            {
+                                writer.WritePropertyName("col");
+                                writer.WriteValue(token.Column);
+                            }
+                            writer.WritePropertyName("num");
+                            writer.WriteValue(numberToken.Value);
+                            writer.WriteEndObject();
+                        }
+                        return;
+
+                    case TokenType.String:
+                        var stringToken = token as StringToken;
+                        if (token.FileId == null && token.Line == null && token.Column == null)
+                        {
+                            writer.WriteValue(stringToken.Value);
+                        }
+                        else
+                        {
+                            writer.WriteStartObject();
+                            writer.WritePropertyName("type");
+                            writer.WriteValue(token.Type);
+                            if (token.FileId != null)
+                            {
+                                writer.WritePropertyName("file");
+                                writer.WriteValue(token.FileId);
+                            }
+                            if (token.Line != null)
+                            {
+                                writer.WritePropertyName("line");
+                                writer.WriteValue(token.Line);
+                            }
+                            if (token.Column != null)
+                            {
+                                writer.WritePropertyName("col");
+                                writer.WriteValue(token.Column);
+                            }
+                            writer.WritePropertyName("lit");
+                            writer.WriteValue(stringToken.Value);
+                            writer.WriteEndObject();
+                        }
+                        return;
+
+                    case TokenType.BasicExpression:
+                        var basicExpressionToken = token as BasicExpressionToken;
+                        writer.WriteStartObject();
+                        writer.WritePropertyName("type");
+                        writer.WriteValue(token.Type);
+                        if (token.FileId != null)
+                        {
+                            writer.WritePropertyName("file");
+                            writer.WriteValue(token.FileId);
+                        }
+                        if (token.Line != null)
+                        {
+                            writer.WritePropertyName("line");
+                            writer.WriteValue(token.Line);
+                        }
+                        if (token.Column != null)
+                        {
+                            writer.WritePropertyName("col");
+                            writer.WriteValue(token.Column);
+                        }
+                        if (!String.IsNullOrEmpty(basicExpressionToken.Expression))
+                        {
+                            writer.WritePropertyName("expr");
+                            writer.WriteValue(basicExpressionToken.Expression);
+                        }
+                        writer.WriteEndObject();
+                        return;
+
+                    case TokenType.InsertExpression:
+                        var insertExpressionToken = token as InsertExpressionToken;
+                        writer.WriteStartObject();
+                        writer.WritePropertyName("type");
+                        writer.WriteValue(token.Type);
+                        if (token.FileId != null)
+                        {
+                            writer.WritePropertyName("file");
+                            writer.WriteValue(token.FileId);
+                        }
+                        if (token.Line != null)
+                        {
+                            writer.WritePropertyName("line");
+                            writer.WriteValue(token.Line);
+                        }
+                        if (token.Column != null)
+                        {
+                            writer.WritePropertyName("col");
+                            writer.WriteValue(token.Column);
+                        }
+                        writer.WritePropertyName("directive");
+                        writer.WriteValue(insertExpressionToken.Directive);
+                        writer.WriteEndObject();
+                        return;
+
+                    case TokenType.Sequence:
+                        var sequenceToken = token as SequenceToken;
+                        writer.WriteStartObject();
+                        writer.WritePropertyName("type");
+                        writer.WriteValue(token.Type);
+                        if (token.FileId != null)
+                        {
+                            writer.WritePropertyName("file");
+                            writer.WriteValue(token.FileId);
+                        }
+                        if (token.Line != null)
+                        {
+                            writer.WritePropertyName("line");
+                            writer.WriteValue(token.Line);
+                        }
+                        if (token.Column != null)
+                        {
+                            writer.WritePropertyName("col");
+                            writer.WriteValue(token.Column);
+                        }
+                        if (sequenceToken.Count > 0)
+                        {
+                            writer.WritePropertyName("seq");
+                            writer.WriteStartArray();
+                            foreach (var item in sequenceToken)
+                            {
+                                serializer.Serialize(writer, item);
+                            }
+                            writer.WriteEndArray();
+                        }
+                        writer.WriteEndObject();
+                        return;
+
+                    case TokenType.Mapping:
+                        var mappingToken = token as MappingToken;
+                        writer.WriteStartObject();
+                        writer.WritePropertyName("type");
+                        writer.WriteValue(token.Type);
+                        if (token.FileId != null)
+                        {
+                            writer.WritePropertyName("file");
+                            writer.WriteValue(token.FileId);
+                        }
+                        if (token.Line != null)
+                        {
+                            writer.WritePropertyName("line");
+                            writer.WriteValue(token.Line);
+                        }
+                        if (token.Column != null)
+                        {
+                            writer.WritePropertyName("col");
+                            writer.WriteValue(token.Column);
+                        }
+                        if (mappingToken.Count > 0)
+                        {
+                            writer.WritePropertyName("map");
+                            writer.WriteStartArray();
+                            foreach (var item in mappingToken)
+                            {
+                                serializer.Serialize(writer, item);
+                            }
+                            writer.WriteEndArray();
+                        }
+                        writer.WriteEndObject();
+                        return;
+                }
+            }
+
+            throw new NotSupportedException($"Unexpected type '{value?.GetType().FullName}' when serializing template token");
+        }
+    }
+}

src/Sdk/WorkflowParser/ObjectTemplating/Tokens/TokenType.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace GitHub.Actions.WorkflowParser.ObjectTemplating.Tokens
+{
+    internal static class TokenType
+    {
+        internal const Int32 String = 0;
+
+        internal const Int32 Sequence = 1;
+
+        internal const Int32 Mapping = 2;
+
+        internal const Int32 BasicExpression = 3;
+
+        internal const Int32 InsertExpression = 4;
+
+        internal const Int32 Boolean = 5;
+
+        internal const Int32 Number = 6;
+
+        internal const Int32 Null = 7;
+    }
+}