Addressed code review comments.

Author: N. Taylor Mullen

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

@@ -117,15 +117,15 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
-  <data name="TagHelperDescriptorResolver_InvalidAddTagHelperLookupText" xml:space="preserve">
-    <value>Invalid `@addtaghelper` lookup text '{0}'. The correct lookup text formats are:
+  <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_AddTagHelperAssemblyNameCannotBeEmptyOrNull" xml:space="preserve">
-    <value>`@addtaghelper` assembly name cannot be null or empty.</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/Resources.resx

@@ -23,7 +23,7 @@ public static class TagHelperDescriptorFactory
         /// 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 represents the given <paramref name="type"/>.</returns>
+        /// <returns>A <see cref="TagHelperDescriptor"/> that describes the given <paramref name="type"/>.</returns>
         public static TagHelperDescriptor CreateDescriptor(Type type)
         {
             var tagName = GetTagName(type);

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

@@ -32,47 +32,42 @@ public TagHelperDescriptorResolver()
         /// <inheritdoc />
         public IEnumerable<TagHelperDescriptor> Resolve(string lookupText)
         {
-            if (string.IsNullOrEmpty(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 InvalidOperationException(
-                    Resources.FormatTagHelperDescriptorResolver_InvalidAddTagHelperLookupText(lookupText));
+                throw new ArgumentException(
+                    Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText),
+                    nameof(lookupText));
             }
 
-            var tagHelperTypes = ResolveTagHelperTypes(lookupText);
+            var tagHelperTypes = ResolveTagHelperTypes(lookupStrings);
 
             // Convert types to TagHelperDescriptors
             var descriptors = tagHelperTypes.Select(TagHelperDescriptorFactory.CreateDescriptor);
 
             return descriptors;
         }
 
-        private IEnumerable<Type> ResolveTagHelperTypes(string lookupText)
+        private IEnumerable<Type> ResolveTagHelperTypes(string[] lookupStrings)
         {
-            var lookupStrings = lookupText.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
-
-            // Ensure that we have valid lookupStrings to work with. Valid formats are:
-            // "assemblyName"
-            // "typeName, assemblyName"
-            if (lookupStrings.Length != 1 && lookupStrings.Length != 2)
-            {
-                throw new InvalidOperationException(
-                    Resources.FormatTagHelperDescriptorResolver_InvalidAddTagHelperLookupText(lookupText));
-            }
-
             // 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 form the assembly.
+            // Resolve valid tag helper types from the assembly.
             var types = _typeResolver.Resolve(assemblyName);
 
-            // If the user provided a type name (lookupStrings.Length == 2) then we need to retrieve the value and 
-            // trim it.
-            var typeName = lookupStrings.Length == 2 ? lookupStrings[0].Trim() : null;
-
             // Check if the lookupText specifies a type to search for.
-            if (typeName != null)
+            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));
             }

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

@@ -9,7 +9,7 @@
 namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 {
     /// <summary>
-    /// Defines a class that locates valid <see cref="ITagHelper"/>s within an assembly.
+    /// Class that locates valid <see cref="ITagHelper"/>s within an assembly.
     /// </summary>
     public class TagHelperTypeResolver
     {
@@ -32,8 +32,9 @@ public IEnumerable<Type> Resolve(string name)
         {
             if (string.IsNullOrEmpty(name))
             {
-                throw new InvalidOperationException(
-                    Resources.TagHelperTypeResolver_AddTagHelperAssemblyNameCannotBeEmptyOrNull);
+                throw new ArgumentException(
+                    Resources.TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull,
+                    nameof(name));
             }
 
             var assemblyName = new AssemblyName(name);

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

@@ -39,7 +39,7 @@ public override void VisitSpan(Span span)
                     throw new InvalidOperationException(
                         RazorResources.FormatTagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver(
                             SyntaxConstants.CSharp.AddTagHelperKeyword,
-                            nameof(TagHelperDescriptorResolver),
+                            nameof(ITagHelperDescriptorResolver),
                             nameof(RazorParser)));
                 }
 

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

@@ -6,7 +6,7 @@
 namespace Microsoft.AspNet.Razor.TagHelpers
 {
     /// <summary>
-    /// Defines a contract used to resolve <see cref="TagHelperDescriptor"/>s.
+    /// Contract used to resolve <see cref="TagHelperDescriptor"/>s.
     /// </summary>
     public interface ITagHelperDescriptorResolver
     {

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

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

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

@@ -1,9 +1,7 @@
 // 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.Runtime.TagHelpers;
-
-namespace Microsoft.AspNet.Razor.Runtime.Test.TagHelpers
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 {
     public class Valid_PlainTagHelper : ITagHelper
     {

test/Microsoft.AspNet.Razor.Runtime.Test/TagHelpers/CommonTagHelpers.cs

@@ -0,0 +1,59 @@
+// 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;
+using Microsoft.AspNet.Razor.TagHelpers;
+using Microsoft.Internal.Web.Utils;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    public class CompleteTagHelperDescriptorComparer : TagHelperDescriptorComparer
+    {
+        public new static readonly CompleteTagHelperDescriptorComparer Default =
+            new CompleteTagHelperDescriptorComparer();
+
+        private CompleteTagHelperDescriptorComparer()
+        {
+        }
+
+        public new bool Equals(TagHelperDescriptor descriptorX, TagHelperDescriptor descriptorY)
+        {
+            return base.Equals(descriptorX, descriptorY) &&
+                   descriptorX.Attributes.SequenceEqual(descriptorY.Attributes,
+                                                        CompleteTagHelperAttributeDescriptorComparer.Default);
+        }
+
+        public new int GetHashCode(TagHelperDescriptor descriptor)
+        {
+            return HashCodeCombiner.Start()
+                                   .Add(base.GetHashCode())
+                                   .Add(descriptor.Attributes)
+                                   .CombinedHash;
+        }
+
+        private class CompleteTagHelperAttributeDescriptorComparer : IEqualityComparer<TagHelperAttributeDescriptor>
+        {
+            public static readonly CompleteTagHelperAttributeDescriptorComparer Default =
+                new CompleteTagHelperAttributeDescriptorComparer();
+
+            private CompleteTagHelperAttributeDescriptorComparer()
+            {
+            }
+
+            public bool Equals(TagHelperAttributeDescriptor descriptorX, TagHelperAttributeDescriptor descriptorY)
+            {
+                return descriptorX.AttributeName == descriptorY.AttributeName &&
+                       descriptorX.AttributePropertyName == descriptorY.AttributePropertyName;
+            }
+
+            public int GetHashCode(TagHelperAttributeDescriptor descriptor)
+            {
+                return HashCodeCombiner.Start()
+                                       .Add(descriptor.AttributeName)
+                                       .Add(descriptor.AttributePropertyName)
+                                       .CombinedHash;
+            }
+        }
+    }
+}