Add ModelExpression code generation.

- Sealed the ModelExpression.
- We use the stringified version of the ModelExpression type name to detect ModelExpression properties on TagHelpers. This is so the MvcRazorHost can work in tooling and in runtime.
- Created a GeneratedTagHelperAttributeContext to represent the specific stringified versions of the ModelExpression assets.
- Created an MvcTagHelperAttributeValueCodeRenderer to modify rendering of ModelExpression properties.

#1241

src/Microsoft.AspNet.Mvc.Core/Rendering/ModelExpression.cs

@@ -10,7 +10,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
     /// <summary>
     /// Describes an <see cref="System.Linq.Expressions.Expression"/> passed to a tag helper.
     /// </summary>
-    public class ModelExpression
+    public sealed class ModelExpression
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="ModelExpression"/> class.

src/Microsoft.AspNet.Mvc.Razor.Host/GeneratedTagHelperAttributeContext.cs

@@ -0,0 +1,23 @@
+// 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.Mvc.Razor
+{
+    /// <summary>
+    /// Contains information for the <see cref="ITagHelper"/> attribute code generation process.
+    /// </summary>
+    public class GeneratedTagHelperAttributeContext
+    {
+        /// <summary>
+        /// Name of the model expression type.
+        /// </summary>
+        public string ModelExpressionTypeName { get; set; }
+
+        /// <summary>
+        /// Name the method to create <c>ModelExpression</c>s.
+        /// </summary>
+        public string CreateModelExpressionMethodName { get; set; }
+    }
+}

src/Microsoft.AspNet.Mvc.Razor.Host/MvcCSharpCodeBuilder.cs

@@ -11,20 +11,34 @@ namespace Microsoft.AspNet.Mvc.Razor
 {
     public class MvcCSharpCodeBuilder : CSharpCodeBuilder
     {
+        private readonly GeneratedTagHelperAttributeContext _tagHelperAttributeContext;
         private readonly string _defaultModel;
         private readonly string _activateAttribute;
 
         public MvcCSharpCodeBuilder([NotNull] CodeBuilderContext context,
                                     [NotNull] string defaultModel,
-                                    [NotNull] string activateAttribute)
+                                    [NotNull] string activateAttribute,
+                                    [NotNull] GeneratedTagHelperAttributeContext tagHelperAttributeContext)
             : base(context)
         {
+            _tagHelperAttributeContext = tagHelperAttributeContext;
             _defaultModel = defaultModel;
             _activateAttribute = activateAttribute;
         }
 
         private string Model { get; set; }
 
+        protected override CSharpCodeVisitor CreateCSharpCodeVisitor([NotNull] CSharpCodeWriter writer,
+                                                                     [NotNull] CodeBuilderContext context)
+        {
+            var csharpCodeVisitor = base.CreateCSharpCodeVisitor(writer, context);
+
+            csharpCodeVisitor.TagHelperRenderer.AttributeValueCodeRenderer =
+                new MvcTagHelperAttributeValueCodeRenderer(_tagHelperAttributeContext);
+
+            return csharpCodeVisitor;
+        }
+
         protected override CSharpCodeWritingScope BuildClassDeclaration(CSharpCodeWriter writer)
         {
             // Grab the last model chunk so it gets intellisense.

src/Microsoft.AspNet.Mvc.Razor.Host/MvcRazorHost.cs

@@ -153,6 +153,22 @@ namespace Microsoft.AspNet.Mvc.Razor
             get { return "Microsoft.AspNet.Mvc.ActivateAttribute"; }
         }
 
+        /// <summary>
+        /// Gets the type name used to represent <see cref="ITagHelper"/> model expression properties.
+        /// </summary>
+        public virtual string ModelExpressionType
+        {
+            get { return "Microsoft.AspNet.Mvc.Rendering.ModelExpression"; }
+        }
+
+        /// <summary>
+        /// Gets the method name used to create model expressions.
+        /// </summary>
+        public virtual string CreateModelExpressionMethod
+        {
+            get { return "CreateModelExpression"; }
+        }
+
         /// <inheritdoc />
         public GeneratorResults GenerateCode(string rootRelativePath, Stream inputStream)
         {
@@ -173,7 +189,15 @@ namespace Microsoft.AspNet.Mvc.Razor
                                                         [NotNull] CodeBuilderContext context)
         {
             UpdateCodeBuilder(context);
-            return new MvcCSharpCodeBuilder(context, DefaultModel, ActivateAttribute);
+
+            return new MvcCSharpCodeBuilder(context,
+                                            DefaultModel, 
+                                            ActivateAttribute,
+                                            new GeneratedTagHelperAttributeContext
+                                            {
+                                                ModelExpressionTypeName = ModelExpressionType,
+                                                CreateModelExpressionMethodName = CreateModelExpressionMethod
+                                            });
         }
 
         private void UpdateCodeBuilder(CodeGeneratorContext context)

src/Microsoft.AspNet.Mvc.Razor.Host/MvcTagHelperAttributeValueCodeRenderer.cs

@@ -0,0 +1,57 @@
+// 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.Generator;
+using Microsoft.AspNet.Razor.Generator.Compiler.CSharp;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Mvc.Razor
+{
+    /// <inheritdoc />
+    public class MvcTagHelperAttributeValueCodeRenderer : TagHelperAttributeValueCodeRenderer
+    {
+        private const string ModelLambdaVariableName = "__model";
+
+        private readonly GeneratedTagHelperAttributeContext _context;
+
+        /// <summary>
+        /// Instantiates a new instance of <see cref="MvcTagHelperAttributeValueCodeRenderer"/>.
+        /// </summary>
+        /// <param name="context">Contains code generation information for rendering attribute values.</param>
+        public MvcTagHelperAttributeValueCodeRenderer([NotNull] GeneratedTagHelperAttributeContext context)
+        {
+            _context = context;
+        }
+
+        /// <inheritdoc />
+        /// <remarks>If the attribute being rendered is of the type 
+        /// <see cref="GeneratedTagHelperAttributeContext.ModelExpressionTypeName"/> then a model expression will be
+        /// created by calling into <see cref="GeneratedTagHelperAttributeContext.CreateModelExpressionMethodName"/>.
+        /// </remarks>
+        public override void RenderAttributeValue([NotNull] TagHelperAttributeDescriptor attributeDescriptor,
+                                                  [NotNull] CSharpCodeWriter writer,
+                                                  [NotNull] CodeBuilderContext codeBuilderContext,
+                                                  [NotNull] Action<CSharpCodeWriter> renderAttributeValue)
+        {
+            var propertyType = attributeDescriptor.PropertyInfo.PropertyType;
+
+            if (propertyType.FullName.Equals(_context.ModelExpressionTypeName, StringComparison.Ordinal))
+            {
+                writer.WriteStartMethodInvocation(_context.CreateModelExpressionMethodName)
+                      .Write(ModelLambdaVariableName)
+                      .Write(" => ")
+                      .Write(ModelLambdaVariableName)
+                      .Write(".");
+
+                renderAttributeValue(writer);
+
+                writer.WriteEndMethodInvocation(endLine: false);
+            }
+            else
+            {
+                base.RenderAttributeValue(attributeDescriptor, writer, codeBuilderContext, renderAttributeValue);
+            }
+        }
+    }
+}