Add TagHelperPrefix directive.

- Updated TagHelperDescriptor to have a Prefix property. This enables new tooling scenarios such as refactoring prefixes or even giving them their own classification.
- Added invalid prefix cases.
- Added TagHelperPrefix chunks, codegenerator, parsing logic to flow the directive through the Razor pipeline.

#309

src/Microsoft.AspNet.Razor.Runtime/Properties/Resources.Designer.cs

@@ -138,6 +138,38 @@ namespace Microsoft.AspNet.Razor.Runtime
             return string.Format(CultureInfo.CurrentCulture, GetString("HtmlElementNameAttribute_InvalidElementName"), p0, p1);
         }
 
+        /// <summary>
+        /// Invalid tag helper directive '{0}'. Cannot have multiple '{0}' directives on a page.
+        /// </summary>
+        internal static string TagHelperDescriptorResolver_InvalidTagHelperDirective
+        {
+            get { return GetString("TagHelperDescriptorResolver_InvalidTagHelperDirective"); }
+        }
+
+        /// <summary>
+        /// Invalid tag helper directive '{0}'. Cannot have multiple '{0}' directives on a page.
+        /// </summary>
+        internal static string FormatTagHelperDescriptorResolver_InvalidTagHelperDirective(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorResolver_InvalidTagHelperDirective"), p0);
+        }
+
+        /// <summary>
+        /// Invalid tag helper directive '{0}' value. '{1} is not allowed in prefix '{2}'.
+        /// </summary>
+        internal static string TagHelperDescriptorResolver_InvalidTagHelperPrefixValue
+        {
+            get { return GetString("TagHelperDescriptorResolver_InvalidTagHelperPrefixValue"); }
+        }
+
+        /// <summary>
+        /// Invalid tag helper directive '{0}' value. '{1} is not allowed in prefix '{2}'.
+        /// </summary>
+        internal static string FormatTagHelperDescriptorResolver_InvalidTagHelperPrefixValue(object p0, object p1, object p2)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorResolver_InvalidTagHelperPrefixValue"), p0, p1, p2);
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNet.Razor.Runtime/Resources.resx

@@ -141,4 +141,10 @@
   <data name="HtmlElementNameAttribute_InvalidElementName" xml:space="preserve">
     <value>Tag helpers cannot target element name '{0}' because it contains a '{1}' character.</value>
   </data>
+  <data name="TagHelperDescriptorResolver_InvalidTagHelperDirective" xml:space="preserve">
+    <value>Invalid tag helper directive '{0}'. Cannot have multiple '{0}' directives on a page.</value>
+  </data>
+  <data name="TagHelperDescriptorResolver_InvalidTagHelperPrefixValue" xml:space="preserve">
+    <value>Invalid tag helper directive '{0}' value. '{1} is not allowed in prefix '{2}'.</value>
+  </data>
 </root>

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

@@ -23,7 +23,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             {
                 { TagHelperDirectiveType.AddTagHelper, SyntaxConstants.CSharp.AddTagHelperKeyword },
                 { TagHelperDirectiveType.RemoveTagHelper, SyntaxConstants.CSharp.RemoveTagHelperKeyword },
+                { TagHelperDirectiveType.TagHelperPrefix, SyntaxConstants.CSharp.TagHelperPrefixKeyword },
             };
+        private static readonly HashSet<char> InvalidNonWhitespacePrefixCharacters =
+            new HashSet<char>(new[] { '@', '!', '<', '!', '/', '?', '[', '>', ']', '=', '"', '\'' });
 
         private readonly TagHelperTypeResolver _typeResolver;
 
@@ -51,7 +54,12 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         {
             var resolvedDescriptors = new HashSet<TagHelperDescriptor>(TagHelperDescriptorComparer.Default);
 
-            foreach (var directiveDescriptor in context.DirectiveDescriptors)
+            // tagHelperPrefix directives do not affect which TagHelperDescriptors are added or removed from the final
+            // list, need to remove them.
+            var actionableDirectiveDescriptors = context.DirectiveDescriptors.Where(
+                directive => directive.DirectiveType != TagHelperDirectiveType.TagHelperPrefix);
+
+            foreach (var directiveDescriptor in actionableDirectiveDescriptors)
             {
                 try
                 {
@@ -69,9 +77,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     }
                     else if (directiveDescriptor.DirectiveType == TagHelperDirectiveType.AddTagHelper)
                     {
-                        var descriptors = ResolveDescriptorsInAssembly(lookupInfo.AssemblyName,
-                                                                       directiveDescriptor.Location,
-                                                                       context.ErrorSink);
+                        var descriptors = ResolveDescriptorsInAssembly(
+                            lookupInfo.AssemblyName,
+                            directiveDescriptor.Location,
+                            context.ErrorSink);
 
                         // Only use descriptors that match our lookup info
                         descriptors = descriptors.Where(descriptor => MatchesLookupInfo(descriptor, lookupInfo));
@@ -89,12 +98,14 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         directiveDescriptor.Location,
                         Resources.FormatTagHelperDescriptorResolver_EncounteredUnexpectedError(
                             "@" + directiveName,
-                            directiveDescriptor.LookupText,
+                            directiveDescriptor.DirectiveText,
                             ex.Message));
                 }
             }
 
-            return resolvedDescriptors;
+            var prefixedDescriptors = PrefixDescriptors(context, resolvedDescriptors);
+
+            return prefixedDescriptors;
         }
 
         /// <summary>
@@ -110,9 +121,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// <returns><see cref="TagHelperDescriptor"/>s for <see cref="ITagHelper"/>s from the given
         /// <paramref name="assemblyName"/>.</returns>
         // This is meant to be overridden by tooling to enable assembly level caching.
-        protected virtual IEnumerable<TagHelperDescriptor> ResolveDescriptorsInAssembly(string assemblyName,
-                                                                                        SourceLocation documentLocation,
-                                                                                        ParserErrorSink errorSink)
+        protected virtual IEnumerable<TagHelperDescriptor> ResolveDescriptorsInAssembly(
+            string assemblyName,
+            SourceLocation documentLocation,
+            ParserErrorSink errorSink)
         {
             // Resolve valid tag helper types from the assembly.
             var tagHelperTypes = _typeResolver.Resolve(assemblyName, documentLocation, errorSink);
@@ -124,6 +136,84 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             return descriptors;
         }
 
+        private static IEnumerable<TagHelperDescriptor> PrefixDescriptors(
+            TagHelperDescriptorResolutionContext context,
+            IEnumerable<TagHelperDescriptor> descriptors)
+        {
+            var tagHelperPrefix = ResolveTagHelperPrefix(context);
+
+            if (!string.IsNullOrEmpty(tagHelperPrefix))
+            {
+                return descriptors.Select(descriptor =>
+                    new TagHelperDescriptor(
+                        tagHelperPrefix,
+                        descriptor.TagName,
+                        descriptor.TypeName,
+                        descriptor.AssemblyName,
+                        descriptor.Attributes));
+            }
+
+            return descriptors;
+        }
+
+        private static string ResolveTagHelperPrefix(TagHelperDescriptorResolutionContext context)
+        {
+            var prefixDirectiveDescriptors = context.DirectiveDescriptors.Where(
+                descriptor => descriptor.DirectiveType == TagHelperDirectiveType.TagHelperPrefix);
+
+            TagHelperDirectiveDescriptor prefixDirective = null;
+
+            foreach (var directive in prefixDirectiveDescriptors)
+            {
+                if (prefixDirective == null)
+                {
+                    prefixDirective = directive;
+                }
+                else
+                {
+                    // For each invalid @tagHelperPrefix we need to create an error.
+                    context.ErrorSink.OnError(
+                        directive.Location,
+                        Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperDirective(
+                            SyntaxConstants.CSharp.TagHelperPrefixKeyword));
+                }
+            }
+
+            var prefix = prefixDirective?.DirectiveText;
+
+            if (prefix != null && !EnsureValidPrefix(prefix, prefixDirective.Location, context.ErrorSink))
+            {
+                prefix = null;
+            }
+
+            return prefix;
+        }
+
+        private static bool EnsureValidPrefix(
+            string prefix,
+            SourceLocation directiveLocation,
+            ParserErrorSink errorSink)
+        {
+            foreach (var character in prefix)
+            {
+                // Prefixes are correlated with tag names, tag names cannot have whitespace.
+                if (char.IsWhiteSpace(character) ||
+                    InvalidNonWhitespacePrefixCharacters.Contains(character))
+                {
+                    errorSink.OnError(
+                        directiveLocation,
+                        Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperPrefixValue(
+                            SyntaxConstants.CSharp.TagHelperPrefixKeyword,
+                            character,
+                            prefix));
+
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
         private static bool MatchesLookupInfo(TagHelperDescriptor descriptor, LookupInfo lookupInfo)
         {
             if (!string.Equals(descriptor.AssemblyName, lookupInfo.AssemblyName, StringComparison.Ordinal))
@@ -146,7 +236,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         private static LookupInfo GetLookupInfo(TagHelperDirectiveDescriptor directiveDescriptor,
                                                 ParserErrorSink errorSink)
         {
-            var lookupText = directiveDescriptor.LookupText;
+            var lookupText = directiveDescriptor.DirectiveText;
             var lookupStrings = lookupText?.Split(new[] { ',' });
 
             // Ensure that we have valid lookupStrings to work with. Valid formats are:

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

@@ -58,17 +58,22 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
             }
         }
 
+        protected override void Visit(TagHelperPrefixDirectiveChunk chunk)
+        {
+            VisitTagHelperDirectiveChunk(chunk.Prefix, chunk);
+        }
+
         protected override void Visit(AddTagHelperChunk chunk)
         {
-            VisitAddOrRemoveTagHelperChunk(chunk.LookupText, chunk);
+            VisitTagHelperDirectiveChunk(chunk.LookupText, chunk);
         }
 
         protected override void Visit(RemoveTagHelperChunk chunk)
         {
-            VisitAddOrRemoveTagHelperChunk(chunk.LookupText, chunk);
+            VisitTagHelperDirectiveChunk(chunk.LookupText, chunk);
         }
 
-        private void VisitAddOrRemoveTagHelperChunk(string lookupText, Chunk chunk)
+        private void VisitTagHelperDirectiveChunk(string text, Chunk chunk)
         {
             // We should always be in design time mode because of the calling AcceptTree method verification.
             Debug.Assert(Context.Host.DesignTimeMode);
@@ -81,12 +86,10 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
 
             Writer.WriteStartAssignment(TagHelperDirectiveSyntaxHelper);
 
-            // The parsing mechanism for the add/remove TagHelper chunk (CSharpCodeParser.TagHelperDirective())
-            // removes quotes that surround the lookupText.
+            // The parsing mechanism for a TagHelper directive chunk (CSharpCodeParser.TagHelperDirective())
+            // removes quotes that surround the text.
             _csharpCodeVisitor.CreateExpressionCodeMapping(
-                string.Format(
-                    CultureInfo.InvariantCulture,
-                    "\"{0}\"", lookupText),
+                string.Format(CultureInfo.InvariantCulture, "\"{0}\"", text),
                 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 TagHelperPrefixDirectiveChunk)
+            {
+                Visit((TagHelperPrefixDirectiveChunk)chunk);
+            }
             else if (chunk is AddTagHelperChunk)
             {
                 Visit((AddTagHelperChunk)chunk);
@@ -119,6 +123,7 @@ 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(TagHelperPrefixDirectiveChunk chunk);
         protected abstract void Visit(AddTagHelperChunk chunk);
         protected abstract void Visit(RemoveTagHelperChunk chunk);
         protected abstract void Visit(UsingChunk chunk);

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

@@ -33,6 +33,9 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
         protected override void Visit(TagHelperChunk chunk)
         {
         }
+        protected override void Visit(TagHelperPrefixDirectiveChunk chunk)
+        {
+        }
         protected override void Visit(AddTagHelperChunk chunk)
         {
         }

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

@@ -0,0 +1,17 @@
+// 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"/> for the <c>@tagHelperPrefix</c> directive.
+    /// </summary>
+    public class TagHelperPrefixDirectiveChunk : Chunk
+    {
+        /// <summary>
+        /// Text used as a required prefix when matching HTML start and end tags in the Razor source to available 
+        /// tag helpers.
+        /// </summary>
+        public string Prefix { get; set; }
+    }
+}

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

@@ -37,6 +37,17 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
             }
         }
 
+        public void AddTagHelperPrefixDirectiveChunk(string prefix, SyntaxTreeNode association)
+        {
+            AddChunk(
+                new TagHelperPrefixDirectiveChunk
+                {
+                    Prefix = prefix
+                },
+                association,
+                topLevel: true);
+        }
+
         public void AddAddTagHelperChunk(string lookupText, SyntaxTreeNode association)
         {
             AddChunk(new AddTagHelperChunk

src/Microsoft.AspNet.Razor/Generator/TagHelperCodeGenerator.cs

@@ -75,9 +75,11 @@ namespace Microsoft.AspNet.Razor.Generator
                 codeGenerator.Context.CodeTreeBuilder = new CodeTreeBuilder();
             }
 
+            var unprefixedTagName = tagHelperBlock.TagName.Substring(_tagHelperDescriptors.First().Prefix.Length);
+
             context.CodeTreeBuilder.StartChunkBlock(
                 new TagHelperChunk(
-                    tagHelperBlock.TagName,
+                    unprefixedTagName,
                     tagHelperBlock.SelfClosing,
                     attributes,
                     _tagHelperDescriptors),

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

@@ -0,0 +1,43 @@
+// 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.TagHelperPrefixDirectiveChunk"/>s.
+    /// </summary>
+    public class TagHelperPrefixDirectiveCodeGenerator : SpanCodeGenerator
+    {
+        /// <summary>
+        /// Instantiates a new <see cref="TagHelperPrefixDirectiveCodeGenerator"/>.
+        /// </summary>
+        /// <param name="prefix">
+        /// Text used as a required prefix when matching HTML.
+        /// </param>
+        public TagHelperPrefixDirectiveCodeGenerator(string prefix)
+        {
+            Prefix = prefix;
+        }
+
+        /// <summary>
+        /// Text used as a required prefix when matching HTML.
+        /// </summary>
+        public string Prefix { get; }
+
+        /// <summary>
+        /// Generates <see cref="Compiler.TagHelperPrefixDirectiveChunk"/>s.
+        /// </summary>
+        /// <param name="target">
+        /// The <see cref="Span"/> responsible for this <see cref="TagHelperPrefixDirectiveCodeGenerator"/>.
+        /// </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.AddTagHelperPrefixDirectiveChunk(Prefix, target);
+        }
+    }
+}

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

@@ -18,6 +18,7 @@ namespace Microsoft.AspNet.Razor.Parser
     {
         private void SetupDirectives()
         {
+            MapDirectives(TagHelperPrefixDirective, SyntaxConstants.CSharp.TagHelperPrefixKeyword);
             MapDirectives(AddTagHelperDirective, SyntaxConstants.CSharp.AddTagHelperKeyword);
             MapDirectives(RemoveTagHelperDirective, SyntaxConstants.CSharp.RemoveTagHelperKeyword);
             MapDirectives(InheritsDirective, SyntaxConstants.CSharp.InheritsKeyword);
@@ -27,14 +28,27 @@ namespace Microsoft.AspNet.Razor.Parser
             MapDirectives(LayoutDirective, SyntaxConstants.CSharp.LayoutKeyword);
         }
 
+        protected virtual void TagHelperPrefixDirective()
+        {
+            TagHelperDirective(
+                SyntaxConstants.CSharp.TagHelperPrefixKeyword,
+                prefix => new TagHelperPrefixDirectiveCodeGenerator(prefix));
+        }
+
         protected virtual void AddTagHelperDirective()
         {
-            TagHelperDirective(SyntaxConstants.CSharp.AddTagHelperKeyword, removeTagHelperDescriptors: false);
+            TagHelperDirective(
+                SyntaxConstants.CSharp.AddTagHelperKeyword,
+                lookupText =>
+                    new AddOrRemoveTagHelperCodeGenerator(removeTagHelperDescriptors: false, lookupText: lookupText));
         }
 
         protected virtual void RemoveTagHelperDirective()
         {
-            TagHelperDirective(SyntaxConstants.CSharp.RemoveTagHelperKeyword, removeTagHelperDescriptors: true);
+            TagHelperDirective(
+                SyntaxConstants.CSharp.RemoveTagHelperKeyword, 
+                lookupText =>
+                    new AddOrRemoveTagHelperCodeGenerator(removeTagHelperDescriptors: true, lookupText: lookupText));
         }
 
         protected virtual void LayoutDirective()
@@ -450,7 +464,7 @@ namespace Microsoft.AspNet.Razor.Parser
             Output(SpanKind.Code);
         }
 
-        private void TagHelperDirective(string keyword, bool removeTagHelperDescriptors)
+        private void TagHelperDirective(string keyword, Func<string, ISpanCodeGenerator> buildCodeGenerator)
         {
             AssertDirective(keyword);
 
@@ -492,8 +506,7 @@ namespace Microsoft.AspNet.Razor.Parser
                     // 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 =
-                        new AddOrRemoveTagHelperCodeGenerator(removeTagHelperDescriptors, rawValue.Trim('"'));
+                    Span.CodeGenerator = buildCodeGenerator(rawValue.Trim('"'));
                 }
 
                 // We expect the directive to be surrounded in quotes.

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.TagHelperPrefixKeyword,
             SyntaxConstants.CSharp.AddTagHelperKeyword,
             SyntaxConstants.CSharp.RemoveTagHelperKeyword,
             "if",

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

@@ -213,7 +213,7 @@ namespace Microsoft.AspNet.Razor.Parser
                                                                                    [NotNull] ParserErrorSink errorSink)
         {
             var addOrRemoveTagHelperSpanVisitor = 
-                new AddOrRemoveTagHelperSpanVisitor(TagHelperDescriptorResolver, errorSink);
+                new TagHelperDirectiveSpanVisitor(TagHelperDescriptorResolver, errorSink);
             return addOrRemoveTagHelperSpanVisitor.GetDescriptors(documentRoot);
         }
 

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 TagHelperPrefixKeyword = "tagHelperPrefix";
             public static readonly string AddTagHelperKeyword = "addTagHelper";
             public static readonly string RemoveTagHelperKeyword = "removeTagHelper";
             public static readonly string InheritsKeyword = "inherits";

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

@@ -12,7 +12,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers
     /// A <see cref="ParserVisitor"/> that generates <see cref="TagHelperDescriptor"/>s from
     /// tag helper code generators in a Razor document.
     /// </summary>
-    public class AddOrRemoveTagHelperSpanVisitor : ParserVisitor
+    public class TagHelperDirectiveSpanVisitor : ParserVisitor
     {
         private readonly ITagHelperDescriptorResolver _descriptorResolver;
         private readonly ParserErrorSink _errorSink;
@@ -20,12 +20,12 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers
         private List<TagHelperDirectiveDescriptor> _directiveDescriptors;
 
         // Internal for testing use
-        internal AddOrRemoveTagHelperSpanVisitor(ITagHelperDescriptorResolver descriptorResolver)
+        internal TagHelperDirectiveSpanVisitor(ITagHelperDescriptorResolver descriptorResolver)
             : this(descriptorResolver, new ParserErrorSink())
         {
         }
 
-        public AddOrRemoveTagHelperSpanVisitor([NotNull] ITagHelperDescriptorResolver descriptorResolver,
+        public TagHelperDirectiveSpanVisitor([NotNull] ITagHelperDescriptorResolver descriptorResolver,
                                                [NotNull] ParserErrorSink errorSink)
         {
             _descriptorResolver = descriptorResolver;
@@ -61,13 +61,26 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers
             {
                 var codeGenerator = (AddOrRemoveTagHelperCodeGenerator)span.CodeGenerator;
 
-                var directive = codeGenerator.RemoveTagHelperDescriptors ?
-                                TagHelperDirectiveType.RemoveTagHelper :
-                                TagHelperDirectiveType.AddTagHelper;
+                var directive = 
+                    codeGenerator.RemoveTagHelperDescriptors ?
+                    TagHelperDirectiveType.RemoveTagHelper :
+                    TagHelperDirectiveType.AddTagHelper;
 
-                var directiveDescriptor = new TagHelperDirectiveDescriptor(codeGenerator.LookupText, 
-                                                                           span.Start, 
-                                                                           directive);
+                var directiveDescriptor = new TagHelperDirectiveDescriptor(
+                    codeGenerator.LookupText, 
+                    span.Start, 
+                    directive);
+
+                _directiveDescriptors.Add(directiveDescriptor);
+            }
+            else if (span.CodeGenerator is TagHelperPrefixDirectiveCodeGenerator)
+            {
+                var codeGenerator = (TagHelperPrefixDirectiveCodeGenerator)span.CodeGenerator;
+
+                var directiveDescriptor = new TagHelperDirectiveDescriptor(
+                    codeGenerator.Prefix,
+                    span.Start,
+                    TagHelperDirectiveType.TagHelperPrefix);
 
                 _directiveDescriptors.Add(directiveDescriptor);
             }

src/Microsoft.AspNet.Razor/ParserResults.cs

@@ -54,6 +54,7 @@ namespace Microsoft.AspNet.Razor
             TagHelperDescriptors = tagHelperDescriptors;
             ErrorSink = errorSink;
             ParserErrors = errorSink.Errors;
+            Prefix = tagHelperDescriptors.FirstOrDefault()?.Prefix;
         }
 
         /// <summary>
@@ -81,5 +82,10 @@ namespace Microsoft.AspNet.Razor
         /// The <see cref="TagHelperDescriptor"/>s found for the current Razor document.
         /// </summary>
         public IEnumerable<TagHelperDescriptor> TagHelperDescriptors { get; }
+
+        /// <summary>
+        /// Text used as a required prefix when matching HTML.
+        /// </summary>
+        public string Prefix { get; }
     }
 }

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

@@ -17,10 +17,11 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         internal TagHelperDescriptor([NotNull] string tagName,
                                      [NotNull] string typeName,
                                      [NotNull] string assemblyName)
-            : this(tagName,
-                   typeName,
-                   assemblyName,
-                   Enumerable.Empty<TagHelperAttributeDescriptor>())
+            : this(
+                tagName,
+                typeName,
+                assemblyName,
+                attributes: Enumerable.Empty<TagHelperAttributeDescriptor>())
         {
         }
 
@@ -35,22 +36,67 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <param name="attributes">
         /// The <see cref="TagHelperAttributeDescriptor"/>s to request from the HTML tag.
         /// </param>
-        public TagHelperDescriptor([NotNull] string tagName,
-                                   [NotNull] string typeName,
-                                   [NotNull] string assemblyName,
-                                   [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes)
+        public TagHelperDescriptor(
+            [NotNull] string tagName,
+            [NotNull] string typeName,
+            [NotNull] string assemblyName,
+            [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes)
+            : this(
+                prefix: string.Empty,
+                tagName: tagName, 
+                typeName: typeName, 
+                assemblyName: assemblyName, 
+                attributes: attributes)
         {
+        }
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagHelperDescriptor"/> class with the given
+        /// <paramref name="attributes"/>.
+        /// </summary>
+        /// <param name="prefix">
+        /// Text used as a required prefix when matching HTML start and end tags in the Razor source to available 
+        /// tag helpers.
+        /// </param>
+        /// <param name="tagName">The tag name that the tag helper targets. '*' indicates a catch-all
+        /// <see cref="TagHelperDescriptor"/> which applies to every HTML tag.</param>
+        /// <param name="typeName">The full name of the tag helper class.</param>
+        /// <param name="assemblyName">The name of the assembly containing the tag helper class.</param>
+        /// <param name="attributes">
+        /// The <see cref="TagHelperAttributeDescriptor"/>s to request from the HTML tag.
+        /// </param>
+        public TagHelperDescriptor(
+            string prefix,
+            [NotNull] string tagName,
+            [NotNull] string typeName,
+            [NotNull] string assemblyName,
+            [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes)
+        {
+            Prefix = prefix ?? string.Empty;
             TagName = tagName;
+            FullTagName = Prefix + TagName;
             TypeName = typeName;
             AssemblyName = assemblyName;
             Attributes = new List<TagHelperAttributeDescriptor>(attributes);
         }
 
+        /// <summary>
+        /// Text used as a required prefix when matching HTML start and end tags in the Razor source to available 
+        /// tag helpers.
+        /// </summary>
+        public string Prefix { get; private set; }
+
         /// <summary>
         /// The tag name that the tag helper should target.
         /// </summary>
         public string TagName { get; private set; }
 
+        /// <summary>
+        /// The full tag name that is required for the tag helper to target an HTML element.
+        /// </summary>
+        /// <remarks>This is equivalent to <see cref="Prefix"/> and <see cref="TagName"/> concatenated.</remarks>
+        public string FullTagName { get; private set; }
+
         /// <summary>
         /// The full name of the tag helper class.
         /// </summary>

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

@@ -29,12 +29,14 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <c>false</c> otherwise.</returns>
         /// <remarks>
         /// Determines equality based on <see cref="TagHelperDescriptor.TypeName"/>,
-        /// <see cref="TagHelperDescriptor.AssemblyName"/> and <see cref="TagHelperDescriptor.TagName"/>.
+        /// <see cref="TagHelperDescriptor.AssemblyName"/>, <see cref="TagHelperDescriptor.TagName"/> and
+        /// <see cref="TagHelperDescriptor.Prefix"/>.
         /// </remarks>
         public bool Equals(TagHelperDescriptor descriptorX, TagHelperDescriptor descriptorY)
         {
             return string.Equals(descriptorX.TypeName, descriptorY.TypeName, StringComparison.Ordinal) &&
                    string.Equals(descriptorX.TagName, descriptorY.TagName, StringComparison.OrdinalIgnoreCase) &&
+                   string.Equals(descriptorX.Prefix, descriptorY.Prefix, StringComparison.OrdinalIgnoreCase) &&
                    string.Equals(descriptorX.AssemblyName, descriptorY.AssemblyName, StringComparison.Ordinal);
         }
 

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

@@ -15,6 +15,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         private const string CatchAllDescriptorTarget = "*";
 
         private IDictionary<string, HashSet<TagHelperDescriptor>> _registrations;
+        private string _tagHelperPrefix;
 
         /// <summary>
         /// Instantiates a new instance of the <see cref="TagHelperDescriptorProvider"/>.
@@ -43,10 +44,18 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         {
             HashSet<TagHelperDescriptor> descriptors;
 
-            // Ensure there's an ISet to use.
+            if (!string.IsNullOrEmpty(_tagHelperPrefix) &&
+                (tagName.Length <= _tagHelperPrefix.Length ||
+                !tagName.StartsWith(_tagHelperPrefix, StringComparison.OrdinalIgnoreCase)))
+            {
+                // The tagName doesn't have the tag helper prefix, we can short circuit.
+                return Enumerable.Empty<TagHelperDescriptor>();
+            }
+
+            // Ensure there's a HashSet to use.
             if (!_registrations.TryGetValue(CatchAllDescriptorTarget, out descriptors))
             {
-                descriptors = new HashSet<TagHelperDescriptor>();
+                descriptors = new HashSet<TagHelperDescriptor>(TagHelperDescriptorComparer.Default);
             }
 
             // If the requested tag name is the catch-all target, we should short circuit.
@@ -72,11 +81,21 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         {
             HashSet<TagHelperDescriptor> descriptorSet;
 
-            // Ensure there's a List to add the descriptor to.
-            if (!_registrations.TryGetValue(descriptor.TagName, out descriptorSet))
+            if (_tagHelperPrefix == null)
+            {
+                _tagHelperPrefix = descriptor.Prefix;
+            }
+
+            var registrationKey =
+                string.Equals(descriptor.TagName, CatchAllDescriptorTarget, StringComparison.Ordinal) ?
+                CatchAllDescriptorTarget :
+                descriptor.FullTagName;
+
+            // Ensure there's a HashSet to add the descriptor to.
+            if (!_registrations.TryGetValue(registrationKey, out descriptorSet))
             {
                 descriptorSet = new HashSet<TagHelperDescriptor>(TagHelperDescriptorComparer.Default);
-                _registrations[descriptor.TagName] = descriptorSet;
+                _registrations[registrationKey] = descriptorSet;
             }
 
             descriptorSet.Add(descriptor);

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

@@ -11,23 +11,24 @@ namespace Microsoft.AspNet.Razor.TagHelpers
     public class TagHelperDirectiveDescriptor
     {
         // Internal for testing purposes.
-        internal TagHelperDirectiveDescriptor(string lookupText,
+        internal TagHelperDirectiveDescriptor(string directiveText,
                                               TagHelperDirectiveType directiveType)
-            : this(lookupText, SourceLocation.Zero, directiveType)
+            : this(directiveText, SourceLocation.Zero, directiveType)
         {
         }
 
         /// <summary>
         /// Instantiates a new instance of <see cref="TagHelperDirectiveDescriptor"/>.
         /// </summary>
-        /// <param name="lookupText">A <see cref="string"/> used to find tag helper <see cref="System.Type"/>s.</param>
+        /// <param name="directiveText">A <see cref="string"/> used to understand tag helper 
+        /// <see cref="System.Type"/>s.</param>
         /// <param name="location">The <see cref="SourceLocation"/> of the directive.</param>
         /// <param name="directiveType">The <see cref="TagHelperDirectiveType"/> of this directive.</param>
-        public TagHelperDirectiveDescriptor([NotNull] string lookupText, 
+        public TagHelperDirectiveDescriptor([NotNull] string directiveText, 
                                             SourceLocation location, 
                                             TagHelperDirectiveType directiveType)
         {
-            LookupText = lookupText;
+            DirectiveText = directiveText;
             Location = location;
             DirectiveType = directiveType;
         }
@@ -35,7 +36,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <summary>
         /// A <see cref="string"/> used to find tag helper <see cref="System.Type"/>s.
         /// </summary>
-        public string LookupText { get; private set; }
+        public string DirectiveText { get; private set; }
 
         /// <summary>
         /// The <see cref="TagHelperDirectiveType"/> of this directive.

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

@@ -9,13 +9,18 @@ namespace Microsoft.AspNet.Razor.TagHelpers
     public enum TagHelperDirectiveType
     {
         /// <summary>
-        /// An @addTagHelper directive.
+        /// An <c>@addTagHelper</c> directive.
         /// </summary>
         AddTagHelper,
 
         /// <summary>
-        /// A @removeTagHelper directive.
+        /// A <c>@removeTagHelper</c> directive.
         /// </summary>
-        RemoveTagHelper
+        RemoveTagHelper,
+
+        /// <summary>
+        /// A <c>@tagHelperPrefix</c> directive.
+        /// </summary>
+        TagHelperPrefix
     }
 }