Add TagHelper registration system.

- This registration system accepts tag descriptors that it then uses to feed requests for tag helpers.
- Added some infrastructure pieces that are used to build up valid tag helper descriptors.

#70

src/Microsoft.AspNet.Razor/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
+{
+    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false)]
+    internal sealed class NotNullAttribute : Attribute
+    {
+    }
+}

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

@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    /// <summary>
+    /// Defines how a tag helper will utilize its inner HTML.
+    /// </summary>
+    public enum ContentBehavior
+    {
+        /// <summary>
+        /// Indicates that the tag helper will not modify its inner HTML in any way. This is the default
+        /// <see cref="ContentBehavior"/>.
+        /// </summary>
+        /// <remarks>Children of the current tag helper will execute after the current tag helper.</remarks>
+        None,
+
+        /// <summary>
+        /// Indicates that the tag helper wants anything within its tag builder's inner HTML to be
+        /// appended to content its children generate.
+        /// </summary>
+        /// <remarks>Children of the current tag helper will execute before the current tag helper.</remarks>
+        Append,
+
+        /// <summary>
+        /// Indicates that the tag helper will modify its HTML content. Therefore this <see cref="ContentBehavior"/>
+        /// enables the tag helper to examine the content its children generate.
+        /// </summary>
+        /// <remarks>Children of the current tag helper will execute before the current tag helper.</remarks>
+        Modify,
+
+        /// <summary>
+        /// Indicates that the tag helper wants anything within its tag builder's inner HTML to be
+        /// prepended to the content its children generate.
+        /// </summary>
+        /// <remarks>Children of the current tag helper will execute after the current tag helper.</remarks>
+        Prepend,
+
+        /// <summary>
+        /// Indicates that the tag helper wants anything within its tag builder's inner HTML to
+        /// replace any HTML inside of it.
+        /// </summary>
+        /// <remarks>Children of the current tag helper will not execute.</remarks>
+        Replace,
+    }
+}

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

@@ -0,0 +1,47 @@
+// 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.Reflection;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    /// <summary>
+    /// A metadata class describing a tag helper attribute.
+    /// </summary>
+    public class TagHelperAttributeDescriptor
+    {
+        /// <summary>
+        /// Instantiates a new <see cref="TagHelperAttributeDescriptor"/> class.
+        /// </summary>
+        /// <param name="attributeName">The HTML attribute name.</param>
+        /// <param name="propertyInfo">The <see cref="System.Reflection.PropertyInfo"/> for the tag
+        /// helper attribute</param>
+        public TagHelperAttributeDescriptor(string attributeName,
+                                            PropertyInfo propertyInfo)
+        {
+            AttributeName = attributeName;
+            PropertyInfo = propertyInfo;
+        }
+
+        /// <summary>
+        /// The HTML attribute name.
+        /// </summary>
+        public string AttributeName { get; private set; }
+
+        /// <summary>
+        /// The name of the CLR property name that corresponds to the HTML attribute name.
+        /// </summary>
+        public string AttributePropertyName
+        {
+            get
+            {
+                return PropertyInfo.Name;
+            }
+        }
+
+        /// <summary>
+        /// The <see cref="System.Reflection.PropertyInfo"/> for the tag helper attribute
+        /// </summary>
+        public PropertyInfo PropertyInfo { get; private set; }
+    }
+}

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

@@ -0,0 +1,53 @@
+// 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 Microsoft.Internal.Web.Utils;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    /// <summary>
+    /// A metadata class describing a tag helper.
+    /// </summary>
+    public class TagHelperDescriptor
+    {
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagHelperDescriptor"/> class.
+        /// </summary>
+        /// <param name="tagName">The tag name that the tag helper targets. '*' indicates a catch-all
+        /// <see cref="TagHelperDescriptor"/> which applies to every HTML tag.</param>
+        /// <param name="tagHelperName">The full name of the tag helper class.</param>
+        /// <param name="contentBehavior">The <see cref="TagHelpers.ContentBehavior"/>
+        /// of the tag helper.</param>
+        public TagHelperDescriptor(string tagName,
+                                   string tagHelperName,
+                                   ContentBehavior contentBehavior)
+        {
+            TagName = tagName;
+            TagHelperName = tagHelperName;
+            ContentBehavior = contentBehavior;
+            Attributes = new List<TagHelperAttributeDescriptor>();
+        }
+
+        /// <summary>
+        /// The tag name that the tag helper should target.
+        /// </summary>
+        public string TagName { get; private set; }
+
+        /// <summary>
+        /// The full name of the tag helper class.
+        /// </summary>
+        public string TagHelperName { get; private set; }
+
+        /// <summary>
+        /// The <see cref="TagHelpers.ContentBehavior"/> of the tag helper.
+        /// </summary>
+        public ContentBehavior ContentBehavior { get; private set; }
+
+        /// <summary>
+        /// The list of attributes the tag helper expects.
+        /// </summary>
+        public virtual List<TagHelperAttributeDescriptor> Attributes { get; private set; }
+    }
+}

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

@@ -0,0 +1,107 @@
+// 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.Internal.Web.Utils;
+
+namespace Microsoft.AspNet.Razor.TagHelpers
+{
+    /// <summary>
+    /// Enables retrieval of <see cref="TagHelperDescriptor"/>'s.
+    /// </summary>
+    public class TagHelperDescriptorProvider
+    {
+        private const string CatchAllDescriptorTarget = "*";
+
+        private static readonly TagHelperDescriptorComparer DefaultTagHelperDescriptorComparer = 
+            new TagHelperDescriptorComparer();
+
+        private IDictionary<string, HashSet<TagHelperDescriptor>> _registrations;
+
+        /// <summary>
+        /// Instantiates a new instance of the <see cref="TagHelperDescriptorProvider"/>.
+        /// </summary>
+        /// <param name="descriptors">The descriptors that the <see cref="TagHelperDescriptorProvider"/> will pull from.</param>
+        public TagHelperDescriptorProvider(IEnumerable<TagHelperDescriptor> descriptors)
+        {
+            _registrations = new Dictionary<string, HashSet<TagHelperDescriptor>>(StringComparer.OrdinalIgnoreCase);
+
+            // Populate our registrations
+            foreach (var descriptor in descriptors)
+            {
+                Register(descriptor);
+            }
+        }
+
+        /// <summary>
+        /// Gets all tag helpers that match the given <paramref name="tagName"/>.
+        /// </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>
+        /// <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)
+        {
+            HashSet<TagHelperDescriptor> descriptors;
+
+            // Ensure there's an ISet to use.
+            if (!_registrations.TryGetValue(CatchAllDescriptorTarget, out descriptors))
+            {
+                descriptors = new HashSet<TagHelperDescriptor>();
+            }
+
+            // If the requested tag name is the catch-all target, we should short circuit.
+            if (tagName.Equals(CatchAllDescriptorTarget, StringComparison.OrdinalIgnoreCase))
+            {
+                return descriptors;
+            }
+
+            // If we have a tag name associated with the requested name, return the descriptors +
+            // all of the catch-all descriptors.
+            if (_registrations.TryGetValue(tagName, out var matchingDescriptors))
+            {
+                return matchingDescriptors.Concat(descriptors);
+            }
+            
+            // We couldn't any descriptors associated with the requested tag name, return all
+            // of the "catch-all" tag descriptors (there may not be any).
+            return descriptors;
+        }
+
+        private void Register(TagHelperDescriptor descriptor)
+        {
+            HashSet<TagHelperDescriptor> descriptorSet;
+
+            // Ensure there's a List to add the descriptor to.
+            if (!_registrations.TryGetValue(descriptor.TagName, out descriptorSet))
+            {
+                descriptorSet = new HashSet<TagHelperDescriptor>(DefaultTagHelperDescriptorComparer);
+                _registrations[descriptor.TagName] = descriptorSet;
+            }
+
+            descriptorSet.Add(descriptor);
+        }
+
+        private class TagHelperDescriptorComparer : IEqualityComparer<TagHelperDescriptor>
+        {
+            public bool Equals(TagHelperDescriptor descriptorX, TagHelperDescriptor descriptorY)
+            {
+                return descriptorX.TagHelperName == descriptorY.TagHelperName &&
+                       descriptorX.TagName == descriptorY.TagName &&
+                       descriptorX.ContentBehavior == descriptorY.ContentBehavior;
+            }
+
+            public int GetHashCode(TagHelperDescriptor descriptor)
+            {
+                return HashCodeCombiner.Start()
+                                       .Add(descriptor.TagName)
+                                       .Add(descriptor.TagHelperName)
+                                       .Add(descriptor.ContentBehavior)
+                                       .CombinedHash;
+            }
+        }
+    }
+}