Add @addtaghelper directive.

- Also added some infrastructure pieces for it such as the ITagHelperDescriptorResolver and the default implementation TagHelperDescriptorResolver which will be filled out in a later commit.
- Reworked some extensibility points to allow accessibility of the descriptor resolvers per offline discussions.

#111

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/CSharpCodeBuilder.cs

@@ -56,7 +56,7 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
 
                     new CSharpHelperVisitor(csharpCodeVisitor, writer, Context).Accept(Tree.Chunks);
                     new CSharpTypeMemberVisitor(csharpCodeVisitor, writer, Context).Accept(Tree.Chunks);
-                    new CSharpDesignTimeHelpersVisitor(writer, Context).AcceptTree(Tree);
+                    new CSharpDesignTimeHelpersVisitor(csharpCodeVisitor, writer, Context).AcceptTree(Tree);
                     new CSharpTagHelperFieldDeclarationVisitor(writer, Context).Accept(Tree.Chunks);
 
                     BuildConstructor(writer);

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/Visitors/CSharpCodeVisitor.cs

@@ -17,6 +17,7 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
         private const string TemplateWriterName = "__razor_template_writer";
 
         private CSharpPaddingBuilder _paddingBuilder;
+        private CSharpTagHelperCodeRenderer _tagHelperCodeRenderer;
 
         public CSharpCodeVisitor(CSharpCodeWriter writer, CodeBuilderContext context)
             : base(writer, context)
@@ -25,7 +26,22 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
             TagHelperRenderer = new CSharpTagHelperCodeRenderer(this, writer, context);
         }
 
-        public CSharpTagHelperCodeRenderer TagHelperRenderer { get; set; }
+        public CSharpTagHelperCodeRenderer TagHelperRenderer
+        {
+            get
+            {
+                return _tagHelperCodeRenderer;
+            }
+            set
+            {
+                if (value == null)
+                {
+                    throw new ArgumentNullException(nameof(TagHelperRenderer));
+                }
+
+                _tagHelperCodeRenderer = value;
+            }
+        }
 
         protected override void Visit(TagHelperChunk chunk)
         {

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/Visitors/CSharpDesignTimeHelpersVisitor.cs

@@ -1,6 +1,9 @@
 // Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System.Diagnostics;
+using System.Globalization;
+
 namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
 {
     public class CSharpDesignTimeHelpersVisitor : CodeVisitor<CSharpCodeWriter>
@@ -8,10 +11,21 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
         internal const string InheritsHelper = "__inheritsHelper";
         internal const string DesignTimeHelperMethodName = "__RazorDesignTimeHelpers__";
 
+        private const string TagHelperDirectiveSyntaxHelper = "__tagHelperDirectiveSyntaxHelper";
         private const int DisableVariableNamingWarnings = 219;
 
-        public CSharpDesignTimeHelpersVisitor(CSharpCodeWriter writer, CodeBuilderContext context)
+        private readonly CSharpCodeVisitor _csharpCodeVisitor;
-            : base(writer, context) { }
+
+        private bool _initializedTagHelperDirectiveSyntaxHelper;
+
+        public CSharpDesignTimeHelpersVisitor([NotNull] CSharpCodeVisitor csharpCodeVisitor,
+                                              [NotNull] CSharpCodeWriter writer,
+                                              [NotNull] CodeBuilderContext context)
+
+            : base(writer, context)
+        {
+            _csharpCodeVisitor = csharpCodeVisitor;
+        }
 
         public void AcceptTree(CodeTree tree)
         {
@@ -43,5 +57,29 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
                 }
             }
         }
+
+        protected override void Visit(AddTagHelperChunk chunk)
+        {
+            // We should always be in design time mode because of the calling AcceptTree method verification.
+            Debug.Assert(Context.Host.DesignTimeMode);
+
+            if (!_initializedTagHelperDirectiveSyntaxHelper)
+            {
+                _initializedTagHelperDirectiveSyntaxHelper = true;
+                Writer.WriteVariableDeclaration("string", TagHelperDirectiveSyntaxHelper, "null");
+            }
+
+            Writer.WriteStartAssignment(TagHelperDirectiveSyntaxHelper);
+
+            // The parsing mechanism for the AddTagHelperChunk (CSharpCodeParser.TagHelperDirective()) removes quotes
+            // that surround the chunk.LookupText.
+            _csharpCodeVisitor.CreateExpressionCodeMapping(
+                string.Format(
+                    CultureInfo.InvariantCulture,
+                    "\"{0}\"", chunk.LookupText),
+                chunk);
+
+            Writer.WriteLine(";");
+        }
     }
 }

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/ChunkVisitor.cs

@@ -57,6 +57,10 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
             {
                 Visit((TagHelperChunk)chunk);
             }
+            else if (chunk is AddTagHelperChunk)
+            {
+                Visit((AddTagHelperChunk)chunk);
+            }
             else if(chunk is SetLayoutChunk)
             {
                 Visit((SetLayoutChunk)chunk);
@@ -115,6 +119,7 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
         protected abstract void Visit(ExpressionChunk chunk);
         protected abstract void Visit(StatementChunk chunk);
         protected abstract void Visit(TagHelperChunk chunk);
+        protected abstract void Visit(AddTagHelperChunk chunk);
         protected abstract void Visit(UsingChunk chunk);
         protected abstract void Visit(ChunkBlock chunk);
         protected abstract void Visit(DynamicCodeAttributeChunk chunk);

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CodeVisitor.cs

@@ -32,6 +32,9 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
         protected override void Visit(TagHelperChunk chunk)
         {
         }
+        protected override void Visit(AddTagHelperChunk chunk)
+        {
+        }
         protected override void Visit(LiteralCodeAttributeChunk chunk)
         {
         }

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeTree/Chunks/TagHelpers/AddTagHelperChunk.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNet.Razor.Generator.Compiler
+{
+    /// <summary>
+    /// A <see cref="Chunk"/> used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s.
+    /// </summary>
+    public class AddTagHelperChunk : Chunk
+    {
+        /// <summary>
+        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s.
+        /// </summary>
+        public string LookupText { get; set; }
+    }
+}

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeTree/CodeTreeBuilder.cs

@@ -39,6 +39,14 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
             }
         }
 
+        public void AddAddTagHelperChunk(string lookupText, SyntaxTreeNode association)
+        {
+            AddChunk(new AddTagHelperChunk
+            {
+                LookupText = lookupText
+            }, association);
+        }
+
         public void AddLiteralChunk(string literal, SyntaxTreeNode association)
         {
             // If the previous chunk was also a LiteralChunk, append the content of the current node to the previous one.

src/Microsoft.AspNet.Razor/Generator/TagHelpers/AddTagHelperCodeGenerator.cs

@@ -0,0 +1,42 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNet.Razor.Parser.SyntaxTree;
+
+namespace Microsoft.AspNet.Razor.Generator
+{
+    /// <summary>
+    /// A <see cref="SpanCodeGenerator"/> responsible for generating <see cref="Compiler.AddTagHelperChunk"/>s.
+    /// </summary>
+    public class AddTagHelperCodeGenerator : SpanCodeGenerator
+    {
+        /// <summary>
+        /// Instantiates a new <see cref="AddTagHelperCodeGenerator"/>.
+        /// </summary>
+        /// <param name="lookupText">
+        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s.
+        /// </param>
+        public AddTagHelperCodeGenerator(string lookupText)
+        {
+            LookupText = lookupText;
+        }
+
+        /// <summary>
+        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s.
+        /// </summary>
+        public string LookupText { get; private set; }
+
+        /// <summary>
+        /// Generates a <see cref="Compiler.AddTagHelperChunk"/>.
+        /// </summary>
+        /// <param name="target">
+        /// The <see cref="Span"/> responsible for this <see cref="AddTagHelperCodeGenerator"/>.
+        /// </param>
+        /// <param name="context">A <see cref="CodeGeneratorContext"/> instance that contains information about 
+        /// the current code generation process.</param>
+        public override void GenerateCode(Span target, CodeGeneratorContext context)
+        {
+            context.CodeTreeBuilder.AddAddTagHelperChunk(LookupText, target);
+        }
+    }
+}

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.Directives.cs

@@ -19,6 +19,7 @@ namespace Microsoft.AspNet.Razor.Parser
     {
         private void SetupDirectives()
         {
+            MapDirectives(AddTagHelperDirective, SyntaxConstants.CSharp.AddTagHelperKeyword);
             MapDirectives(InheritsDirective, SyntaxConstants.CSharp.InheritsKeyword);
             MapDirectives(FunctionsDirective, SyntaxConstants.CSharp.FunctionsKeyword);
             MapDirectives(SectionDirective, SyntaxConstants.CSharp.SectionKeyword);
@@ -27,6 +28,14 @@ namespace Microsoft.AspNet.Razor.Parser
             MapDirectives(SessionStateDirective, SyntaxConstants.CSharp.SessionStateKeyword);
         }
 
+        protected virtual void AddTagHelperDirective()
+        {
+            TagHelperDirective(SyntaxConstants.CSharp.AddTagHelperKeyword, (lookupText) =>
+            {
+                return new AddTagHelperCodeGenerator(lookupText);
+            });
+        }
+
         protected virtual void LayoutDirective()
         {
             AssertDirective(SyntaxConstants.CSharp.LayoutKeyword);
@@ -498,5 +507,65 @@ namespace Microsoft.AspNet.Razor.Parser
             CompleteBlock();
             Output(SpanKind.Code);
         }
+
+        private void TagHelperDirective(string keyword, Func<string, SpanCodeGenerator> codeGeneratorBuilder)
+        {
+            AssertDirective(keyword);
+
+            // Accept the directive name
+            AcceptAndMoveNext();
+
+            // Set the block type
+            Context.CurrentBlock.Type = BlockType.Directive;
+
+            var foundWhitespace = At(CSharpSymbolType.WhiteSpace);
+            AcceptWhile(CSharpSymbolType.WhiteSpace);
+
+            // If we found whitespace then any content placed within the whitespace MAY cause a destructive change
+            // to the document.  We can't accept it.
+            Output(SpanKind.MetaCode, foundWhitespace ? AcceptedCharacters.None : AcceptedCharacters.Any);
+
+            if (EndOfFile || At(CSharpSymbolType.NewLine))
+            {
+                Context.OnError(CurrentLocation, RazorResources.FormatParseError_DirectiveMustHaveValue(keyword));
+            }
+            else
+            {
+                // Need to grab the current location before we accept until the end of the line.
+                var startLocation = CurrentLocation;
+
+                // Parse to the end of the line. Essentially accepts anything until end of line, comments, invalid code
+                // etc.
+                AcceptUntil(CSharpSymbolType.NewLine);
+
+                // Pull out the value minus the spaces at the end
+                var rawValue = Span.GetContent().Value.TrimEnd();
+                var startsWithQuote = rawValue.StartsWith("\"", StringComparison.OrdinalIgnoreCase);
+
+                // If the value starts with a quote then we should generate appropriate C# code to colorize the value.
+                if (startsWithQuote)
+                {
+                    // Set up code generation
+                    // The generated chunk of this code generator is picked up by CSharpDesignTimeHelpersVisitor which
+                    // renders the C# to colorize the user provided value. We trim the quotes around the user's value
+                    // so when we render the code we can project the users value into double quotes to not invoke C# 
+                    // IntelliSense. 
+                    Span.CodeGenerator = codeGeneratorBuilder(rawValue.Trim('"'));
+                }
+
+                // We expect the directive to be surrounded in quotes.
+                // The format for taghelper directives are: @directivename "SomeValue"
+                if (!startsWithQuote ||
+                    !rawValue.EndsWith("\"", StringComparison.OrdinalIgnoreCase))
+                {
+                    Context.OnError(startLocation, 
+                                    RazorResources.FormatParseError_DirectiveMustBeSurroundedByQuotes(keyword));
+                }
+            }
+
+            // Output the span and finish the block
+            CompleteBlock();
+            Output(SpanKind.Code);
+        }
     }
 }

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.cs

@@ -19,6 +19,7 @@ namespace Microsoft.AspNet.Razor.Parser
 
         internal static ISet<string> DefaultKeywords = new HashSet<string>()
         {
+            SyntaxConstants.CSharp.AddTagHelperKeyword,
             "if",
             "do",
             "try",

src/Microsoft.AspNet.Razor/Parser/RazorParser.cs

@@ -17,25 +17,16 @@ namespace Microsoft.AspNet.Razor.Parser
 {
     public class RazorParser
     {
-        public RazorParser(ParserBase codeParser, ParserBase markupParser)
+        private ITagHelperDescriptorResolver _tagHelperDescriptorResolver;
-        {
-            if (codeParser == null)
-            {
-                throw new ArgumentNullException("codeParser");
-            }
-            if (markupParser == null)
-            {
-                throw new ArgumentNullException("markupParser");
-            }
 
+        public RazorParser([NotNull] ParserBase codeParser,
+                           [NotNull] ParserBase markupParser,
+                           ITagHelperDescriptorResolver tagHelperDescriptorResolver)
+        {
+            _tagHelperDescriptorResolver = tagHelperDescriptorResolver;
             MarkupParser = markupParser;
             CodeParser = codeParser;
 
-            // TODO: As part of https://github.com/aspnet/Razor/issues/111 and 
-            // https://github.com/aspnet/Razor/issues/112 pull the provider from some sort of tag helper locator 
-            // object.
-            var provider = new TagHelperDescriptorProvider(Enumerable.Empty<TagHelperDescriptor>());
-
             Optimizers = new List<ISyntaxTreeRewriter>()
             {
                 // TODO: Modify the below WhiteSpaceRewriter & ConditionalAttributeCollapser to handle 
@@ -45,8 +36,6 @@ namespace Microsoft.AspNet.Razor.Parser
                 new WhiteSpaceRewriter(MarkupParser.BuildSpan),
                 // Collapse conditional attributes where the entire value is literal
                 new ConditionalAttributeCollapser(MarkupParser.BuildSpan),
-                // Enables tag helpers
-                new TagHelperParseTreeRewriter(provider),
             };
         }
 
@@ -153,6 +142,16 @@ namespace Microsoft.AspNet.Razor.Parser
                 current = rewriter.Rewrite(current);
             }
 
+            if (_tagHelperDescriptorResolver != null)
+            {
+                var tagHelperRegistrationVisitor = new TagHelperRegistrationVisitor(_tagHelperDescriptorResolver);
+                var tagHelperProvider = tagHelperRegistrationVisitor.CreateProvider(current);
+
+                var tagHelperParseTreeRewriter = new TagHelperParseTreeRewriter(tagHelperProvider);
+                // Rewrite the document to utilize tag helpers
+                current = tagHelperParseTreeRewriter.Rewrite(current);
+            }
+
             // Link the leaf nodes into a chain
             Span prev = null;
             foreach (Span node in current.Flatten())

src/Microsoft.AspNet.Razor/Parser/SyntaxConstants.cs

@@ -18,6 +18,7 @@ namespace Microsoft.AspNet.Razor.Parser
         public static class CSharp
         {
             public static readonly int UsingKeywordLength = 5;
+            public static readonly string AddTagHelperKeyword = "addtaghelper";
             public static readonly string InheritsKeyword = "inherits";
             public static readonly string FunctionsKeyword = "functions";
             public static readonly string SectionKeyword = "section";

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperRegistrationVisitor.cs

@@ -0,0 +1,60 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using Microsoft.AspNet.Razor.Generator;
+using Microsoft.AspNet.Razor.Parser.SyntaxTree;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
+{
+    public class TagHelperRegistrationVisitor : ParserVisitor
+    {
+        private readonly ITagHelperDescriptorResolver _descriptorResolver;
+
+        private HashSet<TagHelperDescriptor> _descriptors;
+
+        public TagHelperRegistrationVisitor(ITagHelperDescriptorResolver descriptorResolver)
+        {
+            _descriptorResolver = descriptorResolver;
+        }
+
+        public TagHelperDescriptorProvider CreateProvider(Block root)
+        {
+            _descriptors = new HashSet<TagHelperDescriptor>(TagHelperDescriptorComparer.Default);
+
+            // This will recurse through the syntax tree.
+            VisitBlock(root);
+
+            return new TagHelperDescriptorProvider(_descriptors);
+        }
+
+        public override void VisitSpan(Span span)
+        {
+            // We're only interested in spans with an AddTagHelperCodeGenerator.
+            if (span.CodeGenerator is AddTagHelperCodeGenerator)
+            {
+                if (_descriptorResolver == null)
+                {
+                    throw new InvalidOperationException(
+                        RazorResources.FormatTagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver(
+                            SyntaxConstants.CSharp.AddTagHelperKeyword,
+                            nameof(TagHelperDescriptorResolver),
+                            nameof(RazorParser)));
+                }
+
+                var addGenerator = (AddTagHelperCodeGenerator)span.CodeGenerator;
+
+                // Look up all the descriptors associated with the "LookupText".
+                var descriptors = _descriptorResolver.Resolve(addGenerator.LookupText);
+
+                // Add all the found descriptors to our HashSet.
+                foreach (var descriptor in descriptors)
+                {
+                    _descriptors.Add(descriptor);
+                }
+            }
+        }
+    }
+}

src/Microsoft.AspNet.Razor/Properties/RazorResources.Designer.cs

@@ -1494,6 +1494,54 @@ namespace Microsoft.AspNet.Razor
             return string.Format(CultureInfo.CurrentCulture, GetString("TagHelpers_AttributesThatAreNotStringsMustNotContainAtSymbols"), p0);
         }
 
+        /// <summary>
+        /// Directive '{0}' must have a value.
+        /// </summary>
+        internal static string ParseError_DirectiveMustHaveValue
+        {
+            get { return GetString("ParseError_DirectiveMustHaveValue"); }
+        }
+
+        /// <summary>
+        /// Directive '{0}' must have a value.
+        /// </summary>
+        internal static string FormatParseError_DirectiveMustHaveValue(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("ParseError_DirectiveMustHaveValue"), p0);
+        }
+
+        /// <summary>
+        /// Directive '{0}'s value must be surrounded in double quotes.
+        /// </summary>
+        internal static string ParseError_DirectiveMustBeSurroundedByQuotes
+        {
+            get { return GetString("ParseError_DirectiveMustBeSurroundedByQuotes"); }
+        }
+
+        /// <summary>
+        /// Directive '{0}'s value must be surrounded in double quotes.
+        /// </summary>
+        internal static string FormatParseError_DirectiveMustBeSurroundedByQuotes(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("ParseError_DirectiveMustBeSurroundedByQuotes"), p0);
+        }
+
+        /// <summary>
+        /// Cannot use directive '{0}' when a {1} has not been provided to the {2}.
+        /// </summary>
+        internal static string TagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver
+        {
+            get { return GetString("TagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver"); }
+        }
+
+        /// <summary>
+        /// Cannot use directive '{0}' when a {1} has not been provided to the {2}.
+        /// </summary>
+        internal static string FormatTagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver(object p0, object p1, object p2)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver"), p0, p1, p2);
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNet.Razor/RazorEngineHost.cs

@@ -8,6 +8,7 @@ using Microsoft.AspNet.Razor.Generator;
 using Microsoft.AspNet.Razor.Generator.Compiler;
 using Microsoft.AspNet.Razor.Generator.Compiler.CSharp;
 using Microsoft.AspNet.Razor.Parser;
+using Microsoft.AspNet.Razor.TagHelpers;
 
 namespace Microsoft.AspNet.Razor
 {
@@ -160,6 +161,12 @@ namespace Microsoft.AspNet.Razor
             return null;
         }
 
+        // TODO: Document this as part of https://github.com/aspnet/Razor/issues/99
+        public virtual ITagHelperDescriptorResolver CreateTagHelperDescriptorResolver()
+        {
+            return new TagHelperDescriptorResolver();
+        }
+
         /// <summary>
         /// Gets an instance of the code parser and is provided an opportunity to decorate or replace it
         /// </summary>

src/Microsoft.AspNet.Razor/RazorResources.resx

@@ -415,4 +415,13 @@ Instead, wrap the contents of the block in "{{}}":
   <data name="TagHelpers_AttributesThatAreNotStringsMustNotContainAtSymbols" xml:space="preserve">
     <value>TagHelper attributes that do not expect strings must not have @ symbols within them. Found attribute '{0}' with an invalid value.</value>
   </data>
+  <data name="ParseError_DirectiveMustHaveValue" xml:space="preserve">
+    <value>Directive '{0}' must have a value.</value>
+  </data>
+  <data name="ParseError_DirectiveMustBeSurroundedByQuotes" xml:space="preserve">
+    <value>Directive '{0}'s value must be surrounded in double quotes.</value>
+  </data>
+  <data name="TagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver" xml:space="preserve">
+    <value>Cannot use directive '{0}' when a {1} has not been provided to the {2}.</value>
+  </data>
 </root>

src/Microsoft.AspNet.Razor/RazorTemplateEngine.cs

@@ -269,11 +269,13 @@ namespace Microsoft.AspNet.Razor
 
         protected internal virtual RazorParser CreateParser()
         {
-            ParserBase codeParser = Host.CodeLanguage.CreateCodeParser();
+            var codeParser = Host.CodeLanguage.CreateCodeParser();
-            ParserBase markupParser = Host.CreateMarkupParser();
+            var markupParser = Host.CreateMarkupParser();
+            var tagHelperDescriptorResolver = Host.CreateTagHelperDescriptorResolver();
 
             return new RazorParser(Host.DecorateCodeParser(codeParser),
-                                   Host.DecorateMarkupParser(markupParser))
+                                   Host.DecorateMarkupParser(markupParser),
+                                   tagHelperDescriptorResolver)
             {
                 DesignTimeMode = Host.DesignTimeMode
             };

src/Microsoft.AspNet.Razor/TagHelpers/ITagHelperDescriptorResolver.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    // TODO: Document this class as part of https://github.com/aspnet/Razor/issues/99
+
+    public interface ITagHelperDescriptorResolver
+    {
+        IEnumerable<TagHelperDescriptor> Resolve(string lookupText);
+    }
+}

src/Microsoft.AspNet.Razor/TagHelpers/TagHelperDescriptorResolver.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    // TODO: Implement this class as part of https://github.com/aspnet/Razor/issues/99
+
+    public class TagHelperDescriptorResolver : ITagHelperDescriptorResolver
+    {
+        public IEnumerable<TagHelperDescriptor> Resolve(string lookupText)
+        {
+            return Enumerable.Empty<TagHelperDescriptor>();
+        }
+    }
+}