Add TagHelper attribute targeting.

- Transitioned HtmlElementNameAttribute into a more generic TargetElementAttribute. Targeting an HTML element can be done by attribute, tag or both.
- Updated TagHelperDescriptor to track required attributes.
- Updated TagHelperProvider to ask for provided attributes when resolving TagHelperDescriptors, this is used to apply RequiredAttributes.
- Updated TagHelperParseTreeRewriter to properly track HTML elements that coincide with a TagHelper scope based on the presence of RequiredAttributes.

#311

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

@@ -75,19 +75,19 @@ namespace Microsoft.AspNet.Razor.Runtime
         }
 
         /// <summary>
-        /// Tag name cannot be null or whitespace.
+        /// {0} name cannot be null or whitespace.
         /// </summary>
-        internal static string HtmlElementNameAttribute_ElementNameCannotBeNullOrWhitespace
+        internal static string TargetElementAttribute_NameCannotBeNullOrWhitespace
         {
-            get { return GetString("HtmlElementNameAttribute_ElementNameCannotBeNullOrWhitespace"); }
+            get { return GetString("TargetElementAttribute_NameCannotBeNullOrWhitespace"); }
         }
 
         /// <summary>
-        /// Tag name cannot be null or whitespace.
+        /// {0} name cannot be null or whitespace.
         /// </summary>
-        internal static string FormatHtmlElementNameAttribute_ElementNameCannotBeNullOrWhitespace()
+        internal static string FormatTargetElementAttribute_NameCannotBeNullOrWhitespace(object p0)
         {
-            return GetString("HtmlElementNameAttribute_ElementNameCannotBeNullOrWhitespace");
+            return string.Format(CultureInfo.CurrentCulture, GetString("TargetElementAttribute_NameCannotBeNullOrWhitespace"), p0);
         }
 
         /// <summary>
@@ -123,19 +123,19 @@ namespace Microsoft.AspNet.Razor.Runtime
         }
 
         /// <summary>
-        /// Tag helpers cannot target element name '{0}' because it contains a '{1}' character.
+        /// Tag helpers cannot target {0} name '{1}' because it contains a '{2}' character.
         /// </summary>
-        internal static string HtmlElementNameAttribute_InvalidElementName
+        internal static string TargetElementAttribute_InvalidName
         {
-            get { return GetString("HtmlElementNameAttribute_InvalidElementName"); }
+            get { return GetString("TargetElementAttribute_InvalidName"); }
         }
 
         /// <summary>
-        /// Tag helpers cannot target element name '{0}' because it contains a '{1}' character.
+        /// Tag helpers cannot target {0} name '{1}' because it contains a '{2}' character.
         /// </summary>
-        internal static string FormatHtmlElementNameAttribute_InvalidElementName(object p0, object p1)
+        internal static string FormatTargetElementAttribute_InvalidName(object p0, object p1, object p2)
         {
-            return string.Format(CultureInfo.CurrentCulture, GetString("HtmlElementNameAttribute_InvalidElementName"), p0, p1);
+            return string.Format(CultureInfo.CurrentCulture, GetString("TargetElementAttribute_InvalidName"), p0, p1, p2);
         }
 
         /// <summary>
@@ -170,6 +170,38 @@ namespace Microsoft.AspNet.Razor.Runtime
             return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorResolver_InvalidTagHelperPrefixValue"), p0, p1, p2);
         }
 
+        /// <summary>
+        /// Attribute
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_Attribute
+        {
+            get { return GetString("TagHelperDescriptorFactory_Attribute"); }
+        }
+
+        /// <summary>
+        /// Attribute
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_Attribute()
+        {
+            return GetString("TagHelperDescriptorFactory_Attribute");
+        }
+
+        /// <summary>
+        /// Tag
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_Tag
+        {
+            get { return GetString("TagHelperDescriptorFactory_Tag"); }
+        }
+
+        /// <summary>
+        /// Tag
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_Tag()
+        {
+            return GetString("TagHelperDescriptorFactory_Tag");
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

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

@@ -129,8 +129,8 @@
   <data name="ScopeManager_EndCannotBeCalledWithoutACallToBegin" xml:space="preserve">
     <value>Must call '{2}.{1}' before calling '{2}.{0}'.</value>
   </data>
-  <data name="HtmlElementNameAttribute_ElementNameCannotBeNullOrWhitespace" xml:space="preserve">
+  <data name="TargetElementAttribute_NameCannotBeNullOrWhitespace" xml:space="preserve">
-    <value>Tag name cannot be null or whitespace.</value>
+    <value>{0} name cannot be null or whitespace.</value>
   </data>
   <data name="ArgumentCannotBeNullOrEmpty" xml:space="preserve">
     <value>The value cannot be null or empty.</value>
@@ -138,8 +138,8 @@
   <data name="TagHelperDescriptorResolver_EncounteredUnexpectedError" xml:space="preserve">
     <value>Encountered an unexpected error when attempting to resolve tag helper directive '{0}' with value '{1}'. Error: {2}</value>
   </data>
-  <data name="HtmlElementNameAttribute_InvalidElementName" xml:space="preserve">
+  <data name="TargetElementAttribute_InvalidName" xml:space="preserve">
-    <value>Tag helpers cannot target element name '{0}' because it contains a '{1}' character.</value>
+    <value>Tag helpers cannot target {0} name '{1}' because it contains a '{2}' 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>
@@ -147,4 +147,10 @@
   <data name="TagHelperDescriptorResolver_InvalidTagHelperPrefixValue" xml:space="preserve">
     <value>Invalid tag helper directive '{0}' value. '{1} is not allowed in prefix '{2}'.</value>
   </data>
+  <data name="TagHelperDescriptorFactory_Attribute" xml:space="preserve">
+    <value>Attribute</value>
+  </data>
+  <data name="TagHelperDescriptorFactory_Tag" xml:space="preserve">
+    <value>Tag</value>
+  </data>
 </root>

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

@@ -1,70 +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;
-using System.Collections.Generic;
-using System.Linq;
-using Microsoft.Framework.Internal;
-
-namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
-{
-    /// <summary>
-    /// Used to override a <see cref="ITagHelper"/>'s default tag name target.
-    /// </summary>
-    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
-    public sealed class HtmlElementNameAttribute : Attribute
-    {
-        /// <summary>
-        /// Instantiates a new instance of the <see cref="HtmlElementNameAttribute"/> class.
-        /// </summary>
-        /// <param name="tag">The HTML tag name for the <see cref="TagHelper"/> to target.</param>
-        public HtmlElementNameAttribute([NotNull] string tag)
-        {
-            ValidateTagName(tag, nameof(tag));
-
-            Tags = new[] { tag };
-        }
-
-        /// <summary>
-        /// Instantiates a new instance of the <see cref="HtmlElementNameAttribute"/> class.
-        /// </summary>
-        /// <param name="tag">The HTML tag name for the <see cref="TagHelper"/> to target.</param>
-        /// <param name="additionalTags">Additional HTML tag names for the <see cref="TagHelper"/> to target.</param>
-        public HtmlElementNameAttribute([NotNull] string tag, [NotNull] params string[] additionalTags)
-        {
-            ValidateTagName(tag, nameof(tag));
-
-            foreach (var tagName in additionalTags)
-            {
-                ValidateTagName(tagName, nameof(additionalTags));
-            }
-
-            var allTags = new List<string>(additionalTags);
-            allTags.Add(tag);
-
-            Tags = allTags;
-        }
-
-        /// <summary>
-        /// An <see cref="IEnumerable{string}"/> of tag names for the <see cref="TagHelper"/> to target.
-        /// </summary>
-        public IEnumerable<string> Tags { get; }
-
-        private static void ValidateTagName(string tagName, string parameterName)
-        {
-            if (string.IsNullOrWhiteSpace(tagName))
-            {
-                throw new ArgumentException(
-                    Resources.HtmlElementNameAttribute_ElementNameCannotBeNullOrWhitespace,
-                    parameterName);
-            }
-
-            if (tagName.Contains('!'))
-            {
-                throw new ArgumentException(
-                    Resources.FormatHtmlElementNameAttribute_InvalidElementName(tagName, '!'),
-                    parameterName);
-            }
-        }
-    }
-}

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

@@ -6,7 +6,10 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 using System.Text.RegularExpressions;
+using Microsoft.AspNet.Razor.Parser;
 using Microsoft.AspNet.Razor.TagHelpers;
+using Microsoft.AspNet.Razor.Text;
+using Microsoft.Framework.Internal;
 
 namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 {
@@ -28,34 +31,52 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         // TODO: Investigate if we should cache TagHelperDescriptors for types:
         // https://github.com/aspnet/Razor/issues/165
 
+        public static ICollection<char> InvalidNonWhitespaceNameCharacters { get; } = new HashSet<char>(
+            new[] { '@', '!', '<', '/', '?', '[', '>', ']', '=', '"', '\'' });
+
         /// <summary>
         /// Creates a <see cref="TagHelperDescriptor"/> from the given <paramref name="type"/>.
         /// </summary>
         /// <param name="assemblyName">The assembly name that contains <paramref name="type"/>.</param>
         /// <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 IEnumerable<TagHelperDescriptor> CreateDescriptors(string assemblyName, Type type)
+        public static IEnumerable<TagHelperDescriptor> CreateDescriptors(
+            string assemblyName,
+            [NotNull] Type type,
+            [NotNull] ParserErrorSink errorSink)
         {
-            var tagNames = GetTagNames(type);
+            var typeInfo = type.GetTypeInfo();
-            var typeName = type.FullName;
             var attributeDescriptors = GetAttributeDescriptors(type);
+            var targetElementAttributes = GetValidTargetElementAttributes(typeInfo, errorSink);
+            var tagHelperDescriptors =
+                BuildTagHelperDescriptors(
+                    typeInfo,
+                    assemblyName,
+                    attributeDescriptors,
+                    targetElementAttributes);
 
-            return tagNames.Select(tagName =>
+            return tagHelperDescriptors.Distinct(TagHelperDescriptorComparer.Default);
-                new TagHelperDescriptor(
-                    prefix: string.Empty,
-                    tagName: tagName,
-                    typeName: typeName,
-                    assemblyName: assemblyName,
-                    attributes: attributeDescriptors));
         }
 
-        private static IEnumerable<string> GetTagNames(Type tagHelperType)
+        private static IEnumerable<TargetElementAttribute> GetValidTargetElementAttributes(
+            TypeInfo typeInfo,
+            ParserErrorSink errorSink)
         {
-            var typeInfo = tagHelperType.GetTypeInfo();
+            var targetElementAttributes = typeInfo.GetCustomAttributes<TargetElementAttribute>(inherit: false);
-            var attributes = typeInfo.GetCustomAttributes<HtmlElementNameAttribute>(inherit: false);
+
+            return targetElementAttributes.Where(attribute => ValidTargetElementAttributeNames(attribute, errorSink));
+        }
+
+        private static IEnumerable<TagHelperDescriptor> BuildTagHelperDescriptors(
+            TypeInfo typeInfo,
+            string assemblyName,
+            IEnumerable<TagHelperAttributeDescriptor> attributeDescriptors,
+            IEnumerable<TargetElementAttribute> targetElementAttributes)
+        {
+            var typeName = typeInfo.FullName;
 
             // If there isn't an attribute specifying the tag name derive it from the name
-            if (!attributes.Any())
+            if (!targetElementAttributes.Any())
             {
                 var name = typeInfo.Name;
 
@@ -64,11 +85,122 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     name = name.Substring(0, name.Length - TagHelperNameEnding.Length);
                 }
 
-                return new[] { ToHtmlCase(name) };
+                return new[]
+                {
+                    BuildTagHelperDescriptor(
+                        ToHtmlCase(name),
+                        typeName,
+                        assemblyName,
+                        attributeDescriptors,
+                        requiredAttributes: Enumerable.Empty<string>())
+                };
             }
 
-            // Remove duplicate tag names.
+            return targetElementAttributes.Select(
-            return attributes.SelectMany(attribute => attribute.Tags).Distinct();
+                attribute => BuildTagHelperDescriptor(typeName, assemblyName, attributeDescriptors, attribute));
+        }
+
+        private static TagHelperDescriptor BuildTagHelperDescriptor(
+            string typeName,
+            string assemblyName,
+            IEnumerable<TagHelperAttributeDescriptor> attributeDescriptors,
+            TargetElementAttribute targetElementAttribute)
+        {
+            var requiredAttributes = GetCommaSeparatedValues(targetElementAttribute.Attributes);
+
+            return BuildTagHelperDescriptor(
+                targetElementAttribute.Tag,
+                typeName,
+                assemblyName,
+                attributeDescriptors,
+                requiredAttributes);
+        }
+
+        private static TagHelperDescriptor BuildTagHelperDescriptor(
+            string tagName,
+            string typeName,
+            string assemblyName,
+            IEnumerable<TagHelperAttributeDescriptor> attributeDescriptors,
+            IEnumerable<string> requiredAttributes)
+        {
+            return new TagHelperDescriptor(
+                prefix: string.Empty,
+                tagName: tagName,
+                typeName: typeName,
+                assemblyName: assemblyName,
+                attributes: attributeDescriptors,
+                requiredAttributes: requiredAttributes);
+        }
+
+        /// <summary>
+        /// Internal for testing.
+        /// </summary>
+        internal static IEnumerable<string> GetCommaSeparatedValues(string text)
+        {
+            // We don't want to remove empty entries, need to notify users of invalid values.
+            return text?.Split(',').Select(tagName => tagName.Trim()) ?? Enumerable.Empty<string>();
+        }
+
+        /// <summary>
+        /// Internal for testing.
+        /// </summary>
+        internal static bool ValidTargetElementAttributeNames(
+            TargetElementAttribute attribute,
+            ParserErrorSink errorSink)
+        {
+            var validTagName = ValidateName(attribute.Tag, targetingAttributes: false, errorSink: errorSink);
+            var validAttributeNames = true;
+            var attributeNames = GetCommaSeparatedValues(attribute.Attributes);
+
+            foreach (var attributeName in attributeNames)
+            {
+                if (!ValidateName(attributeName, targetingAttributes: true, errorSink: errorSink))
+                {
+                    validAttributeNames = false;
+                }
+            }
+
+            return validTagName && validAttributeNames;
+        }
+
+        private static bool ValidateName(
+            string name,
+            bool targetingAttributes,
+            ParserErrorSink errorSink)
+        {
+            var targetName = targetingAttributes ?
+                Resources.TagHelperDescriptorFactory_Attribute :
+                Resources.TagHelperDescriptorFactory_Tag;
+            var validName = true;
+
+            if (string.IsNullOrWhiteSpace(name))
+            {
+                errorSink.OnError(
+                    SourceLocation.Zero,
+                    Resources.FormatTargetElementAttribute_NameCannotBeNullOrWhitespace(targetName));
+
+                validName = false;
+            }
+            else
+            {
+                foreach (var character in name)
+                {
+                    if (char.IsWhiteSpace(character) ||
+                        InvalidNonWhitespaceNameCharacters.Contains(character))
+                    {
+                        errorSink.OnError(
+                            SourceLocation.Zero,
+                            Resources.FormatTargetElementAttribute_InvalidName(
+                                targetName.ToLower(),
+                                name,
+                                character));
+
+                        validName = false;
+                    }
+                }
+            }
+
+            return validName;
         }
 
         private static IEnumerable<TagHelperAttributeDescriptor> GetAttributeDescriptors(Type type)

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

@@ -25,8 +25,6 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 { TagHelperDirectiveType.RemoveTagHelper, SyntaxConstants.CSharp.RemoveTagHelperKeyword },
                 { TagHelperDirectiveType.TagHelperPrefix, SyntaxConstants.CSharp.TagHelperPrefixKeyword },
             };
-        private static readonly HashSet<char> InvalidNonWhitespacePrefixCharacters =
-            new HashSet<char>(new[] { '@', '!', '<', '!', '/', '?', '[', '>', ']', '=', '"', '\'' });
 
         private readonly TagHelperTypeResolver _typeResolver;
 
@@ -131,7 +129,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 
             // Convert types to TagHelperDescriptors
             var descriptors = tagHelperTypes.SelectMany(
-                type => TagHelperDescriptorFactory.CreateDescriptors(assemblyName, type));
+                type => TagHelperDescriptorFactory.CreateDescriptors(assemblyName, type, errorSink));
 
             return descriptors;
         }
@@ -150,7 +148,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         descriptor.TagName,
                         descriptor.TypeName,
                         descriptor.AssemblyName,
-                        descriptor.Attributes));
+                        descriptor.Attributes,
+                        descriptor.RequiredAttributes));
             }
 
             return descriptors;
@@ -198,7 +197,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             {
                 // Prefixes are correlated with tag names, tag names cannot have whitespace.
                 if (char.IsWhiteSpace(character) ||
-                    InvalidNonWhitespacePrefixCharacters.Contains(character))
+                    TagHelperDescriptorFactory.InvalidNonWhitespaceNameCharacters.Contains(character))
                 {
                     errorSink.OnError(
                         directiveLocation,

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

@@ -0,0 +1,54 @@
+// 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 Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    /// <summary>
+    /// Provides an <see cref="ITagHelper"/>'s target.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
+    public sealed class TargetElementAttribute : Attribute
+    {
+        public const string CatchAllDescriptorTarget = TagHelperDescriptorProvider.CatchAllDescriptorTarget;
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TargetElementAttribute"/> class with <see cref="Tag"/>
+        /// set to <c>*</c>.
+        /// </summary>
+        /// <remarks>A <c>*</c> <see cref="Tag"/> value indicates an <see cref="ITagHelper"/> 
+        /// that targets all HTML elements with the required <see cref="Attributes"/>.</remarks>
+        public TargetElementAttribute()
+            : this(CatchAllDescriptorTarget)
+        {
+        }
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TargetElementAttribute"/> class.
+        /// </summary>
+        /// <param name="tag">
+        /// The HTML tag the <see cref="ITagHelper"/> targets.
+        /// </param>
+        /// <remarks>A <c>*</c> <paramref name="tag"/> value indicates an <see cref="ITagHelper"/> 
+        /// that targets all HTML elements with the required <see cref="Attributes"/>.</remarks>
+        public TargetElementAttribute(string tag)
+        {
+            Tag = tag;
+        }
+
+        /// <summary>
+        /// The HTML tag the <see cref="ITagHelper"/> targets.
+        /// </summary>
+        /// <remarks>A <c>*</c> <see cref="Tag"/> value indicates an <see cref="ITagHelper"/> 
+        /// that targets all HTML elements with the required <see cref="Attributes"/>.</remarks>
+        public string Tag { get; }
+
+        /// <summary>
+        /// A comma-separated <see cref="string"/> of attributes the HTML element must contain for the 
+        /// <see cref="ITagHelper"/> to run.
+        /// </summary>
+        public string Attributes { get; set; }
+    }
+}

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

@@ -224,9 +224,16 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
             if (name == null)
             {
-                errorSink.OnError(span.Start,
+                // We couldn't find a name, if the original span content was whitespace it ultimately means the tag
-                                  RazorResources.TagHelperBlockRewriter_TagHelperAttributesMustBeWelformed,
+                // that owns this "attribute" is malformed and is expecting a user to type a new attribute.
-                                  span.Content.Length);
+                // ex: <myTH class="btn"| |
+                if (!string.IsNullOrWhiteSpace(span.Content))
+                {
+                    errorSink.OnError(
+                        span.Start,
+                        RazorResources.TagHelperBlockRewriter_TagHelperAttributesMustBeWelformed,
+                        span.Content.Length);
+                }
 
                 attribute = default(KeyValuePair<string, SyntaxTreeNode>);
 

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

@@ -14,14 +14,14 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
     public class TagHelperParseTreeRewriter : ISyntaxTreeRewriter
     {
         private TagHelperDescriptorProvider _provider;
-        private Stack<TagHelperBlockBuilder> _tagStack;
+        private Stack<TagHelperBlockTracker> _trackerStack;
         private Stack<BlockBuilder> _blockStack;
         private BlockBuilder _currentBlock;
 
         public TagHelperParseTreeRewriter(TagHelperDescriptorProvider provider)
         {
             _provider = provider;
-            _tagStack = new Stack<TagHelperBlockBuilder>();
+            _trackerStack = new Stack<TagHelperBlockTracker>();
             _blockStack = new Stack<BlockBuilder>();
         }
 
@@ -41,7 +41,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 CodeGenerator = input.CodeGenerator
             });
 
-            var activeTagHelpers = _tagStack.Count;
+            var activeTagHelpers = _trackerStack.Count;
 
             foreach (var child in input.Children)
             {
@@ -76,14 +76,14 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
             // We captured the number of active tag helpers at the start of our logic, it should be the same. If not
             // it means that there are malformed tag helpers at the top of our stack.
-            if (activeTagHelpers != _tagStack.Count)
+            if (activeTagHelpers != _trackerStack.Count)
             {
                 // Malformed tag helpers built here will be tag helpers that do not have end tags in the current block 
                 // scope. Block scopes are special cases in Razor such as @<p> would cause an error because there's no
                 // matching end </p> tag in the template block scope and therefore doesn't make sense as a tag helper.
-                BuildMalformedTagHelpers(_tagStack.Count - activeTagHelpers, context);
+                BuildMalformedTagHelpers(_trackerStack.Count - activeTagHelpers, context);
 
-                Debug.Assert(activeTagHelpers == _tagStack.Count);
+                Debug.Assert(activeTagHelpers == _trackerStack.Count);
             }
 
             BuildCurrentlyTrackedBlock();
@@ -91,8 +91,6 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
         private bool TryRewriteTagHelper(Block tagBlock, RewritingContext context)
         {
-            // TODO: Fully handle malformed tags: https://github.com/aspnet/Razor/issues/104
-
             // Get tag name of the current block (doesn't matter if it's an end or start tag)
             var tagName = GetTagName(tagBlock);
 
@@ -104,22 +102,39 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
             var descriptors = Enumerable.Empty<TagHelperDescriptor>();
 
-            if (IsPotentialTagHelper(tagName, tagBlock))
+            if (!IsPotentialTagHelper(tagName, tagBlock))
-            {
-                descriptors = _provider.GetTagHelpers(tagName);
-            }
-
-            // If there aren't any TagHelperDescriptors registered then we aren't a TagHelper
-            if (!descriptors.Any())
             {
                 return false;
             }
 
+            var tracker = _trackerStack.Count > 0 ? _trackerStack.Peek() : null;
+            var tagNameScope = tracker?.Builder.TagName ?? string.Empty;
+
             if (!IsEndTag(tagBlock))
             {
-                // We're in a begin tag helper block
+                // We're now in a start tag block, we first need to see if the tag block is a tag helper.
+                var providedAttributes = GetAttributeNames(tagBlock);
 
-                var validTagStructure = ValidTagStructure(tagName, tagBlock, context);
+                descriptors = _provider.GetDescriptors(tagName, providedAttributes);
+
+                // If there aren't any TagHelperDescriptors registered then we aren't a TagHelper
+                if (!descriptors.Any())
+                {
+                    // If the current tag matches the current TagHelper scope it means the parent TagHelper matched
+                    // all the required attributes but the current one did not; therefore, we need to increment the 
+                    // OpenMatchingTags counter for current the TagHelperBlock so we don't end it too early.
+                    // ex: <myth req="..."><myth></myth></myth> We don't want the first myth to close on the inside
+                    // tag.
+                    if (string.Equals(tagNameScope, tagName, StringComparison.OrdinalIgnoreCase))
+                    {
+                        tracker.OpenMatchingTags++;
+                    }
+
+                    return false;
+                }
+
+                // We're in a start TagHelper block.
+                var validTagStructure = ValidateTagStructure(tagName, tagBlock, context);
 
                 var builder = TagHelperBlockRewriter.Rewrite(
                     tagName,
@@ -144,32 +159,43 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             }
             else
             {
-                // We're in an end tag helper block.
+                // Validate that our end tag matches the currently scoped tag, if not we may need to error.
-
-                var tagNameScope = _tagStack.Count > 0 ? _tagStack.Peek().TagName : string.Empty;
-
-                // Validate that our end tag helper matches the currently scoped tag helper, if not we
-                // need to error.
                 if (tagNameScope.Equals(tagName, StringComparison.OrdinalIgnoreCase))
                 {
-                    ValidTagStructure(tagName, tagBlock, context);
+                    // If there are additional end tags required before we can build our block it means we're in a
+                    // situation like this: <myth req="..."><myth></myth></myth> where we're at the inside </myth>.
+                    if (tracker.OpenMatchingTags > 0)
+                    {
+                        tracker.OpenMatchingTags--;
+
+                        return false;
+                    }
+
+                    ValidateTagStructure(tagName, tagBlock, context);
 
                     BuildCurrentlyTrackedTagHelperBlock(tagBlock);
                 }
                 else
                 {
+                    // If there are not TagHelperDescriptors associated with the end tag block that also have no 
+                    // required attributes then it means we can't be a TagHelper, bail out.
+                    if (!_provider.GetDescriptors(tagName, attributeNames: Enumerable.Empty<string>()).Any())
+                    {
+                        return false;
+                    }
+
                     // Current tag helper scope does not match the end tag. Attempt to recover the tag 
                     // helper by looking up the previous tag helper scopes for a matching tag. If we 
-                    // can't recover it means there was no corresponding tag helper begin tag.
+                    // can't recover it means there was no corresponding tag helper start tag.
                     if (TryRecoverTagHelper(tagName, tagBlock, context))
                     {
-                        ValidTagStructure(tagName, tagBlock, context);
+                        ValidateTagStructure(tagName, tagBlock, context);
 
                         // Successfully recovered, move onto the next element.
                     }
                     else
                     {
-                        // Could not recover, the end tag helper has no corresponding begin tag, create
+                        // Could not recover, the end tag helper has no corresponding start tag, create
                         // an error based on the current childBlock.
                         context.ErrorSink.OnError(
                             tagBlock.Start,
@@ -183,13 +209,61 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             return true;
         }
 
-        private static bool ValidTagStructure(string tagName, Block tag, RewritingContext context)
+        private IEnumerable<string> GetAttributeNames(Block tagBlock)
+        {
+            // Need to calculate how many children we should take that represent the attributes.
+            var childrenOffset = IsPartialTag(tagBlock) ? 1 : 2;
+            var attributeChildren = tagBlock.Children.Skip(1).Take(tagBlock.Children.Count() - childrenOffset);
+            var attributeNames = new List<string>();
+
+            foreach (var child in attributeChildren)
+            {
+                Span childSpan;
+
+                if (child.IsBlock)
+                {
+                    childSpan = ((Block)child).FindFirstDescendentSpan();
+
+                    if (childSpan == null)
+                    {
+                        continue;
+                    }
+                }
+                else
+                {
+                    childSpan = child as Span;
+                }
+
+                var attributeName = childSpan
+                    .Content
+                    .Split(separator: new[] { '=' }, count: 2)[0]
+                    .TrimStart();
+
+                attributeNames.Add(attributeName);
+            }
+
+            return attributeNames;
+        }
+
+        private static bool ValidateTagStructure(string tagName, Block tag, RewritingContext context)
         {
             // We assume an invalid structure until we verify that the tag meets all of our "valid structure" criteria.
-            var invalidStructure = true;
+            if (IsPartialTag(tag))
+            {
+                context.ErrorSink.OnError(
+                    tag.Start,
+                    RazorResources.FormatTagHelpersParseTreeRewriter_MissingCloseAngle(tagName));
 
+                return false;
+            }
+
+            return true;
+        }
+
+        private static bool IsPartialTag(Block tagBlock)
+        {
             // No need to validate the tag end because in order to be a tag block it must start with '<'.
-            var tagEnd = tag.Children.Last() as Span;
+            var tagEnd = tagBlock.Children.Last() as Span;
 
             // If our tag end is not a markup span it means it's some sort of code SyntaxTreeNode (not a valid format)
             if (tagEnd != null && tagEnd.Kind == SpanKind.Markup)
@@ -198,18 +272,11 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
                 if (endSymbol != null && endSymbol.Type == HtmlSymbolType.CloseAngle)
                 {
-                    invalidStructure = false;
+                    return false;
                 }
             }
 
-            if (invalidStructure)
+            return true;
-            {
-                context.ErrorSink.OnError(
-                    tag.Start,
-                    RazorResources.FormatTagHelpersParseTreeRewriter_MissingCloseAngle(tagName));
-            }
-
-            return !invalidStructure;
         }
 
         private void BuildCurrentlyTrackedBlock()
@@ -239,7 +306,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
         {
             // Track the original end tag so the editor knows where each piece of the TagHelperBlock lies 
             // for formatting.
-            _tagStack.Pop().SourceEndTag = endTag;
+            _trackerStack.Pop().Builder.SourceEndTag = endTag;
 
             BuildCurrentlyTrackedBlock();
         }
@@ -256,11 +323,6 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                    childSpan.Kind != SpanKind.Transition;
         }
 
-        private bool IsRegisteredTagHelper(string tagName)
-        {
-            return _provider.GetTagHelpers(tagName).Any();
-        }
-
         private void TrackBlock(BlockBuilder builder)
         {
             _currentBlock = builder;
@@ -270,7 +332,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
         private void TrackTagHelperBlock(TagHelperBlockBuilder builder)
         {
-            _tagStack.Push(builder);
+            _trackerStack.Push(new TagHelperBlockTracker(builder));
 
             TrackBlock(builder);
         }
@@ -279,9 +341,9 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
         {
             var malformedTagHelperCount = 0;
 
-            foreach (var tag in _tagStack)
+            foreach (var tracker in _trackerStack)
             {
-                if (tag.TagName.Equals(tagName, StringComparison.OrdinalIgnoreCase))
+                if (tracker.Builder.TagName.Equals(tagName, StringComparison.OrdinalIgnoreCase))
                 {
                     break;
                 }
@@ -289,9 +351,9 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 malformedTagHelperCount++;
             }
 
-            // If the malformedTagHelperCount == _tagStack.Count it means we couldn't find a begin tag for the tag 
+            // If the malformedTagHelperCount == _tagStack.Count it means we couldn't find a start tag for the tag 
             // helper, can't recover.
-            if (malformedTagHelperCount != _tagStack.Count)
+            if (malformedTagHelperCount != _trackerStack.Count)
             {
                 BuildMalformedTagHelpers(malformedTagHelperCount, context);
 
@@ -302,7 +364,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 return true;
             }
 
-            // Could not recover tag helper. Aka we found a tag helper end tag without a corresponding begin tag.
+            // Could not recover tag helper. Aka we found a tag helper end tag without a corresponding start tag.
             return false;
         }
 
@@ -310,7 +372,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
         {
             for (var i = 0; i < count; i++)
             {
-                var malformedTagHelper = _tagStack.Peek();
+                var malformedTagHelper = _trackerStack.Peek().Builder;
 
                 context.ErrorSink.OnError(
                     malformedTagHelper.Start,
@@ -357,5 +419,17 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             Debug.Assert(tagBlock.Type == BlockType.Tag);
             Debug.Assert(tagBlock.Children.First() is Span);
         }
+
+        private class TagHelperBlockTracker
+        {
+            public TagHelperBlockTracker(TagHelperBlockBuilder builder)
+            {
+                Builder = builder;
+            }
+
+            public TagHelperBlockBuilder Builder { get; }
+
+            public uint OpenMatchingTags { get; set; }
+        }
     }
 }

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

@@ -14,9 +14,10 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <summary>
         /// Internal for testing.
         /// </summary>
-        internal TagHelperDescriptor([NotNull] string tagName,
+        internal TagHelperDescriptor(
-                                     [NotNull] string typeName,
+            [NotNull] string tagName,
-                                     [NotNull] string assemblyName)
+            [NotNull] string typeName,
+            [NotNull] string assemblyName)
             : this(
                 tagName,
                 typeName,
@@ -33,12 +34,31 @@ namespace Microsoft.AspNet.Razor.TagHelpers
             [NotNull] string typeName,
             [NotNull] string assemblyName,
             [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes)
+            : this(
+                tagName,
+                typeName,
+                assemblyName,
+                attributes,
+                requiredAttributes: Enumerable.Empty<string>())
+        {
+        }
+
+        /// <summary>
+        /// Internal for testing.
+        /// </summary>
+        internal TagHelperDescriptor(
+            [NotNull] string tagName,
+            [NotNull] string typeName,
+            [NotNull] string assemblyName,
+            [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes,
+            [NotNull] IEnumerable<string> requiredAttributes)
             : this(
                 prefix: string.Empty,
                 tagName: tagName,
                 typeName: typeName,
                 assemblyName: assemblyName,
-                attributes: attributes)
+                attributes: attributes,
+                requiredAttributes: requiredAttributes)
         {
         }
 
@@ -57,12 +77,16 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <param name="attributes">
         /// The <see cref="TagHelperAttributeDescriptor"/>s to request from the HTML tag.
         /// </param>
+        /// <param name="requiredAttributes">
+        /// The attribute names required for the tag helper to target the HTML tag.
+        /// </param>
         public TagHelperDescriptor(
             string prefix,
             [NotNull] string tagName,
             [NotNull] string typeName,
             [NotNull] string assemblyName,
-            [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes)
+            [NotNull] IEnumerable<TagHelperAttributeDescriptor> attributes,
+            [NotNull] IEnumerable<string> requiredAttributes)
         {
             Prefix = prefix ?? string.Empty;
             TagName = tagName;
@@ -70,6 +94,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
             TypeName = typeName;
             AssemblyName = assemblyName;
             Attributes = new List<TagHelperAttributeDescriptor>(attributes);
+            RequiredAttributes = new List<string>(requiredAttributes);
         }
 
         /// <summary>
@@ -102,6 +127,11 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <summary>
         /// The list of attributes the tag helper expects.
         /// </summary>
-        public virtual List<TagHelperAttributeDescriptor> Attributes { get; private set; }
+        public IList<TagHelperAttributeDescriptor> Attributes { get; private set; }
+
+        /// <summary>
+        /// The list of required attribute names the tag helper expects to target an element.
+        /// </summary>
+        public IList<string> RequiredAttributes { get; private set; }
     }
 }

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.Internal.Web.Utils;
 
 namespace Microsoft.AspNet.Razor.TagHelpers
@@ -29,15 +30,22 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <c>false</c> otherwise.</returns>
         /// <remarks>
         /// Determines equality based on <see cref="TagHelperDescriptor.TypeName"/>,
-        /// <see cref="TagHelperDescriptor.AssemblyName"/>, <see cref="TagHelperDescriptor.TagName"/> and
+        /// <see cref="TagHelperDescriptor.AssemblyName"/>, <see cref="TagHelperDescriptor.TagName"/>,
-        /// <see cref="TagHelperDescriptor.Prefix"/>.
+        /// and <see cref="TagHelperDescriptor.RequiredAttributes"/>.
         /// </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) &&
-                   string.Equals(descriptorX.AssemblyName, descriptorY.AssemblyName, StringComparison.Ordinal);
+                   Enumerable.SequenceEqual(
+                       descriptorX.RequiredAttributes.OrderBy(
+                           attribute => attribute, 
+                           StringComparer.OrdinalIgnoreCase),
+                       descriptorY.RequiredAttributes.OrderBy(
+                           attribute => attribute, 
+                           StringComparer.OrdinalIgnoreCase),
+                       StringComparer.OrdinalIgnoreCase);
         }
 
         /// <summary>
@@ -47,11 +55,22 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <returns>An <see cref="int"/> that uniquely identifies the given <paramref name="descriptor"/>.</returns>
         public int GetHashCode(TagHelperDescriptor descriptor)
         {
-            return HashCodeCombiner.Start()
+            var hashCodeCombiner = HashCodeCombiner
-                                   .Add(descriptor.TagName, StringComparer.OrdinalIgnoreCase)
+                .Start()
-                                   .Add(descriptor.TypeName, StringComparer.Ordinal)
+                .Add(descriptor.TypeName, StringComparer.Ordinal)
-                                   .Add(descriptor.AssemblyName, StringComparer.Ordinal)
+                .Add(descriptor.TagName, StringComparer.OrdinalIgnoreCase)
-                                   .CombinedHash;
+                .Add(descriptor.AssemblyName, StringComparer.Ordinal);
+
+            var attributes = descriptor.RequiredAttributes.OrderBy(
+                attribute => attribute, 
+                StringComparer.OrdinalIgnoreCase);
+
+            foreach (var attribute in attributes)
+            {
+                hashCodeCombiner.Add(attributes);
+            }
+
+            return hashCodeCombiner.CombinedHash;
         }
     }
 }

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

@@ -12,7 +12,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
     /// </summary>
     public class TagHelperDescriptorProvider
     {
-        private const string CatchAllDescriptorTarget = "*";
+        public const string CatchAllDescriptorTarget = "*";
 
         private IDictionary<string, HashSet<TagHelperDescriptor>> _registrations;
         private string _tagHelperPrefix;
@@ -37,13 +37,12 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// </summary>
         /// <param name="tagName">The name of the HTML tag to match. Providing a '*' tag name
         /// retrieves catch-all <see cref="TagHelperDescriptor"/>s (descriptors that target every tag).</param>
+        /// <param name="attributeNames">Attributes the HTML element must contain to match.</param>
         /// <returns><see cref="TagHelperDescriptor"/>s that apply to the given <paramref name="tagName"/>.
         /// Will return an empty <see cref="Enumerable" /> if no <see cref="TagHelperDescriptor"/>s are
         /// found.</returns>
-        public IEnumerable<TagHelperDescriptor> GetTagHelpers(string tagName)
+        public IEnumerable<TagHelperDescriptor> GetDescriptors(string tagName, IEnumerable<string> attributeNames)
         {
-            HashSet<TagHelperDescriptor> descriptors;
-
             if (!string.IsNullOrEmpty(_tagHelperPrefix) &&
                 (tagName.Length <= _tagHelperPrefix.Length ||
                 !tagName.StartsWith(_tagHelperPrefix, StringComparison.OrdinalIgnoreCase)))
@@ -52,29 +51,54 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 return Enumerable.Empty<TagHelperDescriptor>();
             }
 
+            HashSet<TagHelperDescriptor> catchAllDescriptors;
+            IEnumerable<TagHelperDescriptor> descriptors;
+
             // Ensure there's a HashSet to use.
-            if (!_registrations.TryGetValue(CatchAllDescriptorTarget, out descriptors))
+            if (!_registrations.TryGetValue(CatchAllDescriptorTarget, out catchAllDescriptors))
             {
                 descriptors = new HashSet<TagHelperDescriptor>(TagHelperDescriptorComparer.Default);
             }
-
+            else
-            // If the requested tag name is the catch-all target, we should short circuit.
-            if (tagName.Equals(CatchAllDescriptorTarget, StringComparison.OrdinalIgnoreCase))
             {
-                return descriptors;
+                descriptors = catchAllDescriptors;
             }
 
-            // If we have a tag name associated with the requested name, return the descriptors +
+            // If the requested tag name is the catch-all target, we shouldn't do the work of concatenating extra
-            // all of the catch-all descriptors.
+            // descriptors.
-            HashSet<TagHelperDescriptor> matchingDescriptors;
+            if (!tagName.Equals(CatchAllDescriptorTarget, StringComparison.OrdinalIgnoreCase))
-            if (_registrations.TryGetValue(tagName, out matchingDescriptors))
             {
-                return matchingDescriptors.Concat(descriptors);
+                // If we have a tag name associated with the requested name, we need to combine matchingDescriptors 
+                // with all the catch-all descriptors.
+                HashSet<TagHelperDescriptor> matchingDescriptors;
+                if (_registrations.TryGetValue(tagName, out matchingDescriptors))
+                {
+                    descriptors = matchingDescriptors.Concat(descriptors);
+                }
             }
 
-            // We couldn't any descriptors associated with the requested tag name, return all
+            var applicableDescriptors = ApplyRequiredAttributes(descriptors, attributeNames);
-            // of the "catch-all" tag descriptors (there may not be any).
+
-            return descriptors;
+            return applicableDescriptors;
+        }
+
+        private IEnumerable<TagHelperDescriptor> ApplyRequiredAttributes(
+            IEnumerable<TagHelperDescriptor> descriptors,
+            IEnumerable<string> attributeNames)
+        {
+            return descriptors.Where(
+                descriptor =>
+                {
+                    foreach (var requiredAttribute in descriptor.RequiredAttributes)
+                    {
+                        if (!attributeNames.Contains(requiredAttribute, StringComparer.OrdinalIgnoreCase))
+                        {
+                            return false;
+                        }
+                    }
+
+                    return true;
+                });
         }
 
         private void Register(TagHelperDescriptor descriptor)