Add TagName attribute for tag helpers.

- Made it so the TagHelperDescriptorFactory could pull tag name targets from TagNameAttributes on types.
- Tested the behavior between TagNameAttribute and TagHelperDescriptorFactory (the only consumer of the attribute).
- Made sure to test duplicate and inherited class scenarios.

#120

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

@@ -78,6 +78,22 @@ namespace Microsoft.AspNet.Razor.Runtime
             return string.Format(CultureInfo.CurrentCulture, GetString("ScopeManager_EndCannotBeCalledWithoutACallToBegin"), p0, p1, p2);
         }
 
+        /// <summary>
+        /// Parameter {0} must not contain null tag names.
+        /// </summary>
+        internal static string TagNameAttribute_AdditionalTagsCannotContainNull
+        {
+            get { return GetString("TagNameAttribute_AdditionalTagsCannotContainNull"); }
+        }
+
+        /// <summary>
+        /// Parameter {0} must not contain null tag names.
+        /// </summary>
+        internal static string FormatTagNameAttribute_AdditionalTagsCannotContainNull(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagNameAttribute_AdditionalTagsCannotContainNull"), p0);
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

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

@@ -131,4 +131,7 @@
   <data name="ScopeManager_EndCannotBeCalledWithoutACallToBegin" xml:space="preserve">
     <value>Must call '{2}.{1}' before calling '{2}.{0}'.</value>
   </data>
+  <data name="TagNameAttribute_AdditionalTagsCannotContainNull" xml:space="preserve">
+    <value>Parameter {0} must not contain null tag names.</value>
+  </data>
 </root>

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

@@ -24,30 +24,40 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// </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)
+        public static IEnumerable<TagHelperDescriptor> CreateDescriptors(Type type)
         {
-            var tagName = GetTagName(type);
+            var tagNames = GetTagNames(type);
             var typeName = type.FullName;
             var attributeDescriptors = GetAttributeDescriptors(type);
             var contentBehavior = GetContentBehavior(type);
 
-            return new TagHelperDescriptor(tagName,
-                                           typeName,
-                                           contentBehavior,
-                                           attributeDescriptors);
+            return tagNames.Select(tagName =>
+                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)
+        private static IEnumerable<string> GetTagNames(Type tagHelperType)
         {
-            var name = tagHelperType.Name;
+            var typeInfo = tagHelperType.GetTypeInfo();
+            var attributes = typeInfo.GetCustomAttributes<TagNameAttribute>(inherit: false);
 
-            if (name.EndsWith(TagHelperNameEnding, StringComparison.OrdinalIgnoreCase))
+            // If there isn't an attribute specifying the tag name derive it from the name
+            if (!attributes.Any())
             {
-                name = name.Substring(0, name.Length - TagHelperNameEnding.Length);
+                var name = typeInfo.Name;
+
+                if (name.EndsWith(TagHelperNameEnding, StringComparison.OrdinalIgnoreCase))
+                {
+                    name = name.Substring(0, name.Length - TagHelperNameEnding.Length);
+                }
+
+                return new[] { name };
             }
 
-            return name;
+            // Remove duplicate tag names.
+            return attributes.SelectMany(attribute => attribute.Tags).Distinct();
         }
 
         private static IEnumerable<TagHelperAttributeDescriptor> GetAttributeDescriptors(Type type)

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

@@ -48,7 +48,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             var tagHelperTypes = ResolveTagHelperTypes(lookupStrings);
 
             // Convert types to TagHelperDescriptors
-            var descriptors = tagHelperTypes.Select(TagHelperDescriptorFactory.CreateDescriptor);
+            var descriptors = tagHelperTypes.SelectMany(TagHelperDescriptorFactory.CreateDescriptors);
 
             return descriptors;
         }

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

@@ -0,0 +1,50 @@
+// 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;
+
+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 TagNameAttribute : Attribute
+    {
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagNameAttribute"/> class.
+        /// </summary>
+        /// <param name="tag">The HTML tag name for the <see cref="TagHelper"/> to target.</param>
+        public TagNameAttribute([NotNull] string tag)
+        {
+            Tags = new[] { tag };
+        }
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagNameAttribute"/> 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 TagNameAttribute([NotNull] string tag, [NotNull] params string[] additionalTags)
+        {
+            if (additionalTags.Contains(null))
+            {
+                throw new ArgumentNullException(
+                    nameof(additionalTags),
+                    Resources.FormatTagNameAttribute_AdditionalTagsCannotContainNull(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 set; }
+    }
+}

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

@@ -15,9 +15,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             var expectedDescriptor = new TagHelperDescriptor("Object", "System.Object", ContentBehavior.None);
 
             // Act
-            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(typeof(object));
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(object));
 
             // Assert
+            var descriptor = Assert.Single(descriptors);
             Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
         }
 
@@ -35,9 +36,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 });
 
             // Act
-            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(typeof(SingleAttributeTagHelper));
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(SingleAttributeTagHelper));
 
             // Assert
+            var descriptor = Assert.Single(descriptors);
             Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
         }
 
@@ -56,9 +58,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 });
 
             // Act
-            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(typeof(MissingAccessorTagHelper));
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(MissingAccessorTagHelper));
 
             // Assert
+            var descriptor = Assert.Single(descriptors);
             Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
         }
 
@@ -78,13 +81,13 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 });
 
             // Act
-            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(typeof(PrivateAccessorTagHelper));
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(PrivateAccessorTagHelper));
 
             // Assert
+            var descriptor = Assert.Single(descriptors);
             Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
         }
 
-
         [Fact]
         public void CreateDescriptor_ResolvesCustomContentBehavior()
         {
@@ -95,9 +98,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 ContentBehavior.Append);
 
             // Act
-            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(typeof(CustomContentBehaviorTagHelper));
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(CustomContentBehaviorTagHelper));
 
             // Assert
+            var descriptor = Assert.Single(descriptors);
             Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
         }
 
@@ -111,13 +115,114 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 ContentBehavior.None);
 
             // Act
-            var descriptor = TagHelperDescriptorFactory.CreateDescriptor(
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(
                 typeof(InheritedCustomContentBehaviorTagHelper));
 
             // Assert
+            var descriptor = Assert.Single(descriptors);
             Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
         }
 
+        [Fact]
+        public void CreateDescriptor_ResolvesMultipleTagHelperDescriptorsFromSingleType()
+        {
+            // Arrange
+            var validProp = typeof(MultiTagTagHelper).GetProperty(nameof(MultiTagTagHelper.ValidAttribute));
+            var expectedDescriptors = new[] {
+                new TagHelperDescriptor(
+                    "div",
+                    typeof(MultiTagTagHelper).FullName,
+                    ContentBehavior.None,
+                    new[] {
+                        new TagHelperAttributeDescriptor(nameof(MultiTagTagHelper.ValidAttribute), validProp)
+                    }),
+                new TagHelperDescriptor(
+                    "p",
+                    typeof(MultiTagTagHelper).FullName,
+                    ContentBehavior.None,
+                    new[] {
+                        new TagHelperAttributeDescriptor(nameof(MultiTagTagHelper.ValidAttribute), validProp)
+                    })
+            };
+
+            // Act
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(MultiTagTagHelper));
+
+            // Assert
+            Assert.Equal(descriptors, expectedDescriptors, CompleteTagHelperDescriptorComparer.Default);
+        }
+
+        [Fact]
+        public void CreateDescriptor_DoesntResolveInheritedTagNames()
+        {
+            // Arrange
+            var validProp = typeof(InheritedMultiTagTagHelper).GetProperty(nameof(InheritedMultiTagTagHelper.ValidAttribute));
+            var expectedDescriptor = new TagHelperDescriptor(
+                    "InheritedMultiTag",
+                    typeof(InheritedMultiTagTagHelper).FullName,
+                    ContentBehavior.None,
+                    new[] {
+                        new TagHelperAttributeDescriptor(nameof(InheritedMultiTagTagHelper.ValidAttribute), validProp)
+                    });
+
+            // Act
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(InheritedMultiTagTagHelper));
+
+            // Assert
+            var descriptor = Assert.Single(descriptors);
+            Assert.Equal(descriptor, expectedDescriptor, CompleteTagHelperDescriptorComparer.Default);
+        }
+
+        [Fact]
+        public void CreateDescriptor_IgnoresDuplicateTagNamesFromAttribute()
+        {
+            // Arrange
+            var expectedDescriptors = new[] {
+                new TagHelperDescriptor("p", typeof(DuplicateTagNameTagHelper).FullName, ContentBehavior.None),
+                new TagHelperDescriptor("div", typeof(DuplicateTagNameTagHelper).FullName, ContentBehavior.None)
+            };
+
+            // Act
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(DuplicateTagNameTagHelper));
+
+            // Assert
+            Assert.Equal(descriptors, expectedDescriptors, CompleteTagHelperDescriptorComparer.Default);
+        }
+
+        [Fact]
+        public void CreateDescriptor_OverridesTagNameFromAttribute()
+        {
+            // Arrange
+            var expectedDescriptors = new[] {
+                new TagHelperDescriptor("data-condition", 
+                                        typeof(OverrideNameTagHelper).FullName, 
+                                        ContentBehavior.None),
+            };
+
+            // Act
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(OverrideNameTagHelper));
+
+            // Assert
+            Assert.Equal(descriptors, expectedDescriptors, CompleteTagHelperDescriptorComparer.Default);
+        }
+
+        [Fact]
+        public void CreateDescriptor_GetsTagNamesFromMultipleAttributes()
+        {
+            // Arrange
+            var expectedDescriptors = new[] {
+                new TagHelperDescriptor("span", typeof(MultipleAttributeTagHelper).FullName, ContentBehavior.None),
+                new TagHelperDescriptor("p", typeof(MultipleAttributeTagHelper).FullName, ContentBehavior.None),
+                new TagHelperDescriptor("div", typeof(MultipleAttributeTagHelper).FullName, ContentBehavior.None)
+            };
+
+            // Act
+            var descriptors = TagHelperDescriptorFactory.CreateDescriptors(typeof(MultipleAttributeTagHelper));
+
+            // Assert
+            Assert.Equal(descriptors, expectedDescriptors, CompleteTagHelperDescriptorComparer.Default);
+        }
+
         [ContentBehavior(ContentBehavior.Append)]
         private class CustomContentBehaviorTagHelper
         {
@@ -126,5 +231,31 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         private class InheritedCustomContentBehaviorTagHelper : CustomContentBehaviorTagHelper
         {
         }
+
+        [TagName("p", "div")]
+        private class MultiTagTagHelper
+        {
+            public string ValidAttribute { get; set; }
+        }
+
+        private class InheritedMultiTagTagHelper : MultiTagTagHelper
+        {
+        }
+
+        [TagName("p", "p", "div", "div")]
+        private class DuplicateTagNameTagHelper
+        {
+        }
+
+        [TagName("data-condition")]
+        private class OverrideNameTagHelper
+        {
+        }
+
+        [TagName("span")]
+        [TagName("div", "p")]
+        private class MultipleAttributeTagHelper
+        {
+        }
     }
 }