Add TagHelperDescriptorResolver.

- This involved also adding a TagHelperTypeResolver and TagHelperDescriptorFactory.
- The TagHelperTypeResolver is responsible for determining the format of lookup text's used throughout the tag helper system.  By default it handles the following formats:
"assemblyName"
"specificType, assemblyName"
- It also restricts what types are considered TagHelpers.  In this implementation we only accept public, non-nested, non-abstract, non-generic TagHelpers.
- The TagHelperDescriptorFactory is responsible for converting a Type to a TagHelperDescriptor.
- Added tests to validate TagHelperDescriptorResolver, TagHelperTypeResolver and TagHelperDescriptorFactory.

#99
#158

src/Microsoft.AspNet.Razor.Runtime/AssemblyInfo.cs

@@ -0,0 +1,6 @@
+// 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.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("Microsoft.AspNet.Razor.Runtime.Test")]

src/Microsoft.AspNet.Razor.Runtime/NotNullArgument.cs

@@ -0,0 +1,12 @@
+// 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;
+
+namespace Microsoft.AspNet.Razor.Runtime
+{
+    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false)]
+    internal sealed class NotNullAttribute : Attribute
+    {
+    }
+}

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

@@ -0,0 +1,82 @@
+// <auto-generated />
+namespace Microsoft.AspNet.Razor.Runtime
+{
+    using System.Globalization;
+    using System.Reflection;
+    using System.Resources;
+
+    internal static class Resources
+    {
+        private static readonly ResourceManager _resourceManager
+            = new ResourceManager("Microsoft.AspNet.Razor.Runtime.Resources", typeof(Resources).GetTypeInfo().Assembly);
+
+        /// <summary>
+        /// Invalid tag helper directive look up text '{0}'. The correct look up text formats are:
+        /// "assemblyName"
+        /// "typeName, assemblyName"
+        /// </summary>
+        internal static string TagHelperDescriptorResolver_InvalidTagHelperLookupText
+        {
+            get { return GetString("TagHelperDescriptorResolver_InvalidTagHelperLookupText"); }
+        }
+
+        /// <summary>
+        /// Invalid tag helper directive look up text '{0}'. The correct look up text formats are:
+        /// "assemblyName"
+        /// "typeName, assemblyName"
+        /// </summary>
+        internal static string FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorResolver_InvalidTagHelperLookupText"), p0);
+        }
+
+        /// <summary>
+        /// Cannot resolve TagHelper containing assembly '{0}'. Error: '{1}'.
+        /// </summary>
+        internal static string TagHelperTypeResolver_CannotResolveTagHelperAssembly
+        {
+            get { return GetString("TagHelperTypeResolver_CannotResolveTagHelperAssembly"); }
+        }
+
+        /// <summary>
+        /// Cannot resolve TagHelper containing assembly '{0}'. Error: '{1}'.
+        /// </summary>
+        internal static string FormatTagHelperTypeResolver_CannotResolveTagHelperAssembly(object p0, object p1)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperTypeResolver_CannotResolveTagHelperAssembly"), p0, p1);
+        }
+
+        /// <summary>
+        /// Tag helper directive assembly name cannot be null or empty.
+        /// </summary>
+        internal static string TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull
+        {
+            get { return GetString("TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull"); }
+        }
+
+        /// <summary>
+        /// Tag helper directive assembly name cannot be null or empty.
+        /// </summary>
+        internal static string FormatTagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull()
+        {
+            return GetString("TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull");
+        }
+
+        private static string GetString(string name, params string[] formatterNames)
+        {
+            var value = _resourceManager.GetString(name);
+
+            System.Diagnostics.Debug.Assert(value != null);
+
+            if (formatterNames != null)
+            {
+                for (var i = 0; i < formatterNames.Length; i++)
+                {
+                    value = value.Replace("{" + formatterNames[i] + "}", "{" + i + "}");
+                }
+            }
+
+            return value;
+        }
+    }
+}

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

@@ -0,0 +1,131 @@
+<?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="TagHelperDescriptorResolver_InvalidTagHelperLookupText" xml:space="preserve">
+    <value>Invalid tag helper directive look up text '{0}'. The correct look up text formats are:
+"assemblyName"
+"typeName, assemblyName"</value>
+  </data>
+  <data name="TagHelperTypeResolver_CannotResolveTagHelperAssembly" xml:space="preserve">
+    <value>Cannot resolve TagHelper containing assembly '{0}'. Error: '{1}'.</value>
+  </data>
+  <data name="TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull" xml:space="preserve">
+    <value>Tag helper directive assembly name cannot be null or empty.</value>
+  </data>
+</root>

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

@@ -0,0 +1,12 @@
+// 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;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    public interface ITagHelper
+    {
+        // TODO: Will be implemented in https://github.com/aspnet/razor/issues/154
+    }
+}

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

@@ -0,0 +1,83 @@
+// 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 System.Linq;
+using System.Reflection;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    /// <summary>
+    /// Factory for <see cref="TagHelperDescriptor"/>s from <see cref="Type"/>s.
+    /// </summary>
+    public static class TagHelperDescriptorFactory
+    {
+        private const string TagHelperNameEnding = "TagHelper";
+
+        // TODO: Investigate if we should cache TagHelperDescriptors for types:
+        // https://github.com/aspnet/Razor/issues/165
+
+        /// <summary>
+        /// Creates a <see cref="TagHelperDescriptor"/> from the given <paramref name="type"/>.
+        /// </summary>
+        /// <param name="type">The type to create a <see cref="TagHelperDescriptor"/> from.</param>
+        /// <returns>A <see cref="TagHelperDescriptor"/> that describes the given <paramref name="type"/>.</returns>
+        public static TagHelperDescriptor CreateDescriptor(Type type)
+        {
+            var tagName = GetTagName(type);
+            var typeName = type.FullName;
+            var attributeDescriptors = GetAttributeDescriptors(type);
+            var contentBehavior = GetContentBehavior(type);
+
+            return new TagHelperDescriptor(tagName,
+                                           typeName,
+                                           contentBehavior,
+                                           attributeDescriptors);
+        }
+
+        // TODO: Make this method support TagNameAttribute tag names: https://github.com/aspnet/Razor/issues/120
+        private static string GetTagName(Type tagHelperType)
+        {
+            var name = tagHelperType.Name;
+
+            if (name.EndsWith(TagHelperNameEnding, StringComparison.OrdinalIgnoreCase))
+            {
+                name = name.Substring(0, name.Length - TagHelperNameEnding.Length);
+            }
+
+            return name;
+        }
+
+        private static IEnumerable<TagHelperAttributeDescriptor> GetAttributeDescriptors(Type type)
+        {
+            var typeInfo = type.GetTypeInfo();
+            var properties = typeInfo.DeclaredProperties.Where(IsValidProperty);
+            var attributeDescriptors = properties.Select(ToAttributeDescriptor);
+
+            return attributeDescriptors;
+        }
+
+        // TODO: Make the HTML attribute name support names from a AttributeNameAttribute: 
+        // https://github.com/aspnet/Razor/issues/121
+        private static TagHelperAttributeDescriptor ToAttributeDescriptor(PropertyInfo property)
+        {
+            return new TagHelperAttributeDescriptor(property.Name, property);
+        }
+
+        // TODO: Make the content behavior pull from a ContentBehaviorAttribute: https://github.com/aspnet/Razor/issues/122
+        private static ContentBehavior GetContentBehavior(Type type)
+        {
+            return ContentBehavior.None;
+        }
+
+        private static bool IsValidProperty(PropertyInfo property)
+        {
+            return property.GetMethod != null &&
+                   property.GetMethod.IsPublic &&
+                   property.SetMethod != null &&
+                   property.SetMethod.IsPublic;
+        }
+    }
+}

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

@@ -0,0 +1,78 @@
+// 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 System.Linq;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    /// <summary>
+    /// Used to resolve <see cref="TagHelperDescriptor"/>s.
+    /// </summary>
+    public class TagHelperDescriptorResolver : ITagHelperDescriptorResolver
+    {
+        private readonly TagHelperTypeResolver _typeResolver;
+
+        // internal for testing
+        internal TagHelperDescriptorResolver(TagHelperTypeResolver typeResolver)
+        {
+            _typeResolver = typeResolver;
+        }
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagHelperDescriptorResolver"/> class.
+        /// </summary>
+        public TagHelperDescriptorResolver()
+            : this(new TagHelperTypeResolver())
+        {
+        }
+
+        /// <inheritdoc />
+        public IEnumerable<TagHelperDescriptor> Resolve(string lookupText)
+        {
+            var lookupStrings = lookupText?.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
+
+            // Ensure that we have valid lookupStrings to work with. Valid formats are:
+            // "assemblyName"
+            // "typeName, assemblyName"
+            if (string.IsNullOrEmpty(lookupText) ||
+                (lookupStrings.Length != 1 && lookupStrings.Length != 2))
+            {
+                throw new ArgumentException(
+                    Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText),
+                    nameof(lookupText));
+            }
+
+            var tagHelperTypes = ResolveTagHelperTypes(lookupStrings);
+
+            // Convert types to TagHelperDescriptors
+            var descriptors = tagHelperTypes.Select(TagHelperDescriptorFactory.CreateDescriptor);
+
+            return descriptors;
+        }
+
+        private IEnumerable<Type> ResolveTagHelperTypes(string[] lookupStrings)
+        {
+            // Grab the assembly name from the lookup text strings. Due to our supported lookupText formats it will 
+            // always be the last element provided.
+            var assemblyName = lookupStrings.Last().Trim();
+
+            // Resolve valid tag helper types from the assembly.
+            var types = _typeResolver.Resolve(assemblyName);
+
+            // Check if the lookupText specifies a type to search for.
+            if (lookupStrings.Length == 2)
+            {
+                // The user provided a type name retrieve the value and trim it.
+                var typeName = lookupStrings[0].Trim();
+
+                types = types.Where(type => 
+                    string.Equals(type.Namespace + "." + type.Name, typeName, StringComparison.Ordinal));
+            }
+
+            return types;
+        }
+    }
+}

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

@@ -0,0 +1,74 @@
+// 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 System.Linq;
+using System.Reflection;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    /// <summary>
+    /// Class that locates valid <see cref="ITagHelper"/>s within an assembly.
+    /// </summary>
+    public class TagHelperTypeResolver
+    {
+        private static readonly TypeInfo ITagHelperTypeInfo = typeof(ITagHelper).GetTypeInfo();
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagHelperTypeResolver"/> class.
+        /// </summary>
+        public TagHelperTypeResolver()
+        {
+        }
+
+        /// <summary>
+        /// Loads an <see cref="Assembly"/> using the given <paramref name="name"/> and resolves
+        /// all valid <see cref="ITagHelper"/> <see cref="Type"/>s.
+        /// </summary>
+        /// <param name="name">The name of an <see cref="Assembly"/> to search.</param>
+        /// <returns>An <see cref="IEnumerable{Type}"/> of valid <see cref="ITagHelper"/> <see cref="Type"/>s.</returns>        
+        public IEnumerable<Type> Resolve(string name)
+        {
+            if (string.IsNullOrEmpty(name))
+            {
+                throw new ArgumentException(
+                    Resources.TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull,
+                    nameof(name));
+            }
+
+            var assemblyName = new AssemblyName(name);
+            var libraryTypes = GetLibraryDefinedTypes(assemblyName);
+            var validTagHelpers = libraryTypes.Where(IsTagHelper);
+
+            // Convert from TypeInfo[] to Type[]
+            return validTagHelpers.Select(type => type.AsType());
+        }
+
+        // Internal for testing, don't want to be loading assemblies during a test.
+        internal virtual IEnumerable<TypeInfo> GetLibraryDefinedTypes(AssemblyName assemblyName)
+        {
+            try
+            {
+                var assembly = Assembly.Load(assemblyName);
+
+                return assembly.DefinedTypes;
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException(
+                    Resources.FormatTagHelperTypeResolver_CannotResolveTagHelperAssembly(assemblyName.Name,
+                                                                                         ex.Message));
+            }
+        }
+
+        private static bool IsTagHelper(TypeInfo typeInfo)
+        {
+            return typeInfo.IsPublic &&
+                   !typeInfo.IsAbstract &&
+                   !typeInfo.IsGenericType &&
+                   !typeInfo.IsNested &&
+                   ITagHelperTypeInfo.IsAssignableFrom(typeInfo);
+        }
+    }
+}

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

@@ -40,7 +40,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                     throw new InvalidOperationException(
                         RazorResources.FormatTagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver(
                             SyntaxConstants.CSharp.AddTagHelperKeyword,
-                            nameof(TagHelperDescriptorResolver),
+                            nameof(ITagHelperDescriptorResolver),
                             nameof(RazorParser)));
                 }
 

src/Microsoft.AspNet.Razor/Properties/AssemblyInfo.cs

@@ -3,4 +3,5 @@
 
 using System.Runtime.CompilerServices;
 
-[assembly: InternalsVisibleTo("Microsoft.AspNet.Razor.Test")]
+[assembly: InternalsVisibleTo("Microsoft.AspNet.Razor.Test")]
+[assembly: InternalsVisibleTo("Microsoft.AspNet.Razor.Runtime.Test")]

src/Microsoft.AspNet.Razor/RazorEngineHost.cs

@@ -113,6 +113,11 @@ namespace Microsoft.AspNet.Razor
         /// </summary>
         public virtual RazorCodeLanguage CodeLanguage { get; protected set; }
 
+        /// <summary>
+        /// The <see cref="ITagHelperDescriptorResolver"/> used to resolve <see cref="TagHelperDescriptor"/>s.
+        /// </summary>
+        public virtual ITagHelperDescriptorResolver TagHelperDescriptorResolver { get; protected set; }
+
         /// <summary>
         /// Boolean indicating if instrumentation code should be injected into the output page
         /// </summary>
@@ -161,12 +166,6 @@ 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/RazorTemplateEngine.cs

@@ -271,11 +271,10 @@ namespace Microsoft.AspNet.Razor
         {
             var codeParser = Host.CodeLanguage.CreateCodeParser();
             var markupParser = Host.CreateMarkupParser();
-            var tagHelperDescriptorResolver = Host.CreateTagHelperDescriptorResolver();
 
             return new RazorParser(Host.DecorateCodeParser(codeParser),
                                    Host.DecorateMarkupParser(markupParser),
-                                   tagHelperDescriptorResolver)
+                                   Host.TagHelperDescriptorResolver)
             {
                 DesignTimeMode = Host.DesignTimeMode
             };

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

@@ -5,10 +5,19 @@ using System.Collections.Generic;
 
 namespace Microsoft.AspNet.Razor.TagHelpers
 {
-    // TODO: Document this class as part of https://github.com/aspnet/Razor/issues/99
+    /// <summary>
-
+    /// Contract used to resolve <see cref="TagHelperDescriptor"/>s.
+    /// </summary>
     public interface ITagHelperDescriptorResolver
     {
+        /// <summary>
+        /// Resolves <see cref="TagHelperDescriptor"/>s matching the given <paramref name="lookupText"/>.
+        /// </summary>
+        /// <param name="lookupText">
+        /// A <see cref="string"/> used to find tag helper <see cref="Type"/>s.
+        /// </param>
+        /// <returns>An <see cref="IEnumerable{TagHelperDescriptor}"/> of <see cref="TagHelperDescriptor"/>s matching 
+        /// the given <paramref name="lookupText"/>.</returns>
         IEnumerable<TagHelperDescriptor> Resolve(string lookupText);
     }
 }

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

@@ -8,7 +8,7 @@ using Microsoft.Internal.Web.Utils;
 namespace Microsoft.AspNet.Razor.TagHelpers
 {
     /// <summary>
-    /// Defines a an <see cref="IEqualityComparer{TagHelperDescriptor}"/> that is used to check equality between
+    /// An <see cref="IEqualityComparer{TagHelperDescriptor}"/> used to check equality between
     /// two <see cref="TagHelperDescriptor"/>s.
     /// </summary>
     public class TagHelperDescriptorComparer : IEqualityComparer<TagHelperDescriptor>
@@ -18,7 +18,8 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// </summary>
         public static readonly TagHelperDescriptorComparer Default = new TagHelperDescriptorComparer();
 
-        private TagHelperDescriptorComparer()
+        // Internal for testing
+        internal TagHelperDescriptorComparer()
         {
         }
 

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

@@ -1,18 +0,0 @@
-// 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>();
-        }
-    }
-}