Tag Helpers: add `ModelExpression` class to support `Expression<Func<TModel, TValue>>` attributes

- includes new `RazorPage<TModel>.CreateModelExpression<TValue>()` method
- #1240

nit:
- regenerating the resources reordered Microsoft.AspNet.Mvc.Core's Resources.designer.cs

src/Microsoft.AspNet.Mvc.Core/Properties/Resources.Designer.cs

@@ -1482,22 +1482,6 @@ namespace Microsoft.AspNet.Mvc.Core
             return GetString("AttributeRoute_NullTemplateRepresentation");
         }
 
-        /// <summary>
-        /// "The path to the file must be absolute: {0}"
-        /// </summary>
-        internal static string FileResult_InvalidPathType_RelativeOrVirtualPath
-        {
-            get { return GetString("FileResult_InvalidPathType_RelativeOrVirtualPath"); }
-        }
-
-        /// <summary>
-        /// "The path to the file must be absolute: {0}"
-        /// </summary>
-        internal static string FormatFileResult_InvalidPathType_RelativeOrVirtualPath(object p0)
-        {
-            return string.Format(CultureInfo.CurrentCulture, GetString("FileResult_InvalidPathType_RelativeOrVirtualPath"), p0);
-        }
-
         /// <summary>
         /// Multiple actions matched. The following actions matched route data and had all constraints satisfied:{0}{0}{1}
         /// </summary>
@@ -1514,6 +1498,22 @@ namespace Microsoft.AspNet.Mvc.Core
             return string.Format(CultureInfo.CurrentCulture, GetString("DefaultActionSelector_AmbiguousActions"), p0, p1);
         }
 
+        /// <summary>
+        /// "The path to the file must be absolute: {0}"
+        /// </summary>
+        internal static string FileResult_InvalidPathType_RelativeOrVirtualPath
+        {
+            get { return GetString("FileResult_InvalidPathType_RelativeOrVirtualPath"); }
+        }
+
+        /// <summary>
+        /// "The path to the file must be absolute: {0}"
+        /// </summary>
+        internal static string FormatFileResult_InvalidPathType_RelativeOrVirtualPath(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("FileResult_InvalidPathType_RelativeOrVirtualPath"), p0);
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

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

@@ -0,0 +1,47 @@
+
+using System;
+using Microsoft.AspNet.Mvc.Core;
+using Microsoft.AspNet.Mvc.ModelBinding;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    /// <summary>
+    /// Describes an <see cref="System.Linq.Expressions.Expression"/> passed to a tag helper.
+    /// </summary>
+    public class ModelExpression
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ModelExpression"/> class.
+        /// </summary>
+        /// <param name="name">
+        /// String representation of the <see cref="System.Linq.Expressions.Expression"/> of interest.
+        /// </param>
+        /// <param name="metadata">
+        /// Metadata about the <see cref="System.Linq.Expressions.Expression"/> of interest.
+        /// </param>
+        public ModelExpression(string name, [NotNull] ModelMetadata metadata)
+        {
+            if (string.IsNullOrEmpty(name))
+            {
+                throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(name));
+            }
+
+            Name = name;
+            Metadata = metadata;
+        }
+
+        /// <summary>
+        /// String representation of the <see cref="System.Linq.Expressions.Expression"/> of interest.
+        /// </summary>
+        public string Name { get; private set; }
+
+        /// <summary>
+        /// Metadata about the <see cref="System.Linq.Expressions.Expression"/> of interest.
+        /// </summary>
+        /// <remarks>
+        /// Getting <see cref="ModelMetadata.Model"/> will evaluate a compiled version of the original
+        /// <see cref="System.Linq.Expressions.Expression"/>.
+        /// </remarks>
+        public ModelMetadata Metadata { get; private set; }
+    }
+}

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

@@ -186,6 +186,22 @@ namespace Microsoft.AspNet.Mvc.Razor
             return GetString("RazorPage_YouCannotFlushWhileInAWritingScope");
         }
 
+        /// <summary>
+        /// The {0} was unable to provide metadata for expression '{1}'.
+        /// </summary>
+        internal static string RazorPage_NullModelMetadata
+        {
+            get { return GetString("RazorPage_NullModelMetadata"); }
+        }
+
+        /// <summary>
+        /// The {0} was unable to provide metadata for expression '{1}'.
+        /// </summary>
+        internal static string FormatRazorPage_NullModelMetadata(object p0, object p1)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("RazorPage_NullModelMetadata"), p0, p1);
+        }
+
         /// <summary>
         /// {0} can only be called from a layout page.
         /// </summary>

src/Microsoft.AspNet.Mvc.Razor/RazorPageOfT.cs

@@ -1,6 +1,13 @@
 // 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.Linq.Expressions;
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.AspNet.Mvc.Rendering.Expressions;
+using Microsoft.Framework.DependencyInjection;
+
 namespace Microsoft.AspNet.Mvc.Razor
 {
     /// <summary>
@@ -9,6 +16,8 @@ namespace Microsoft.AspNet.Mvc.Razor
     /// <typeparam name="TModel">The type of the view data model.</typeparam>
     public abstract class RazorPage<TModel> : RazorPage
     {
+        IModelMetadataProvider _provider;
+
         public TModel Model
         {
             get
@@ -19,5 +28,33 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         [Activate]
         public ViewDataDictionary<TModel> ViewData { get; set; }
+
+        /// <summary>
+        /// Returns a <see cref="ModelExpression"/> instance describing the given <paramref name="expression"/>.
+        /// </summary>
+        /// <typeparam name="TValue">The type of the <paramref name="expression"/> result.</typeparam>
+        /// <param name="expression">An expression to be evaluated against the current model.</param>
+        /// <returns>A new <see cref="ModelExpression"/> instance describing the given <paramref name="expression"/>.
+        /// </returns>
+        /// <remarks>
+        /// Compiler normally infers <typeparamref name="TValue"/> from the given <paramref name="expression"/>.
+        /// </remarks>
+        public ModelExpression CreateModelExpression<TValue>([NotNull] Expression<Func<TModel, TValue>> expression)
+        {
+            if (_provider == null)
+            {
+                _provider = Context.RequestServices.GetService<IModelMetadataProvider>();
+            }
+
+            var name = ExpressionHelper.GetExpressionText(expression);
+            var metadata = ExpressionMetadataProvider.FromLambdaExpression(expression, ViewData, _provider);
+            if (metadata == null)
+            {
+                throw new InvalidOperationException(
+                    Resources.FormatRazorPage_NullModelMetadata(nameof(IModelMetadataProvider), name));
+            }
+
+            return new ModelExpression(name, metadata);
+        }
     }
 }

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

@@ -150,6 +150,9 @@
   <data name="RazorPage_YouCannotFlushWhileInAWritingScope" xml:space="preserve">
     <value>You cannot flush while inside a writing scope.</value>
   </data>
+  <data name="RazorPage_NullModelMetadata" xml:space="preserve">
+    <value>The {0} was unable to provide metadata for expression '{1}'.</value>
+  </data>
   <data name="RenderBodyCannotBeCalled" xml:space="preserve">
     <value>{0} can only be called from a layout page.</value>
   </data>

test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageCreateModelExpressionTest.cs

@@ -0,0 +1,162 @@
+
+using System;
+using System.IO;
+using System.Linq.Expressions;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Http;
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.AspNet.Routing;
+using Moq;
+using Xunit;
+
+namespace Microsoft.AspNet.Mvc.Razor
+{
+    public class RazorPageCreateModelExpressionTest
+    {
+        public static TheoryData<Expression<Func<RazorPageCreateModelExpressionModel, int>>, string> IntExpressions
+        {
+            get
+            {
+                var somethingElse = 23;
+                return new TheoryData<Expression<Func<RazorPageCreateModelExpressionModel, int>>, string>
+                {
+                    { model => somethingElse, "somethingElse" },
+                    { model => model.Id, "Id" },
+                    { model => model.SubModel.Id, "SubModel.Id" },
+                    { model => model.SubModel.SubSubModel.Id, "SubModel.SubSubModel.Id" },
+                };
+            }
+        }
+
+        public static TheoryData<Expression<Func<RazorPageCreateModelExpressionModel, string>>, string> StringExpressions
+        {
+            get
+            {
+                var somethingElse = "This is something else";
+                return new TheoryData<Expression<Func<RazorPageCreateModelExpressionModel, string>>, string>
+                {
+                    { model => somethingElse, "somethingElse" },
+                    { model => model.Name, "Name" },
+                    { model => model.SubModel.Name, "SubModel.Name" },
+                    { model => model.SubModel.SubSubModel.Name, "SubModel.SubSubModel.Name" },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(IntExpressions))]
+        public void CreateModelExpression_ReturnsExpectedMetadata_IntExpressions(
+            Expression<Func<RazorPageCreateModelExpressionModel, int>> expression,
+            string expectedName)
+        {
+            // Arrange
+            var viewContext = CreateViewContext(model: null);
+            var page = CreatePage(viewContext);
+
+            // Act
+            var result = page.CreateModelExpression(expression);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.NotNull(result.Metadata);
+            Assert.Equal(typeof(int), result.Metadata.ModelType);
+            Assert.Equal(expectedName, result.Name);
+        }
+
+        [Theory]
+        [MemberData(nameof(StringExpressions))]
+        public void CreateModelExpression_ReturnsExpectedMetadata_StringExpressions(
+            Expression<Func<RazorPageCreateModelExpressionModel, string>> expression,
+            string expectedName)
+        {
+            // Arrange
+            var viewContext = CreateViewContext(model: null);
+            var page = CreatePage(viewContext);
+
+            // Act
+            var result = page.CreateModelExpression(expression);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.NotNull(result.Metadata);
+            Assert.Equal(typeof(string), result.Metadata.ModelType);
+            Assert.Equal(expectedName, result.Name);
+        }
+
+        private static TestRazorPage CreatePage(ViewContext viewContext)
+        {
+            return new TestRazorPage
+            {
+                ViewContext = viewContext,
+                ViewData = (ViewDataDictionary<RazorPageCreateModelExpressionModel>)viewContext.ViewData,
+            };
+        }
+
+        private static ViewContext CreateViewContext(RazorPageCreateModelExpressionModel model)
+        {
+            return CreateViewContext(model, new DataAnnotationsModelMetadataProvider());
+        }
+
+        private static ViewContext CreateViewContext(
+            RazorPageCreateModelExpressionModel model,
+            IModelMetadataProvider provider)
+        {
+            var viewData = new ViewDataDictionary<RazorPageCreateModelExpressionModel>(provider)
+            {
+                Model = model,
+            };
+
+            var serviceProvider = new Mock<IServiceProvider>();
+            serviceProvider
+                .Setup(real => real.GetService(typeof(IModelMetadataProvider)))
+                .Returns(provider);
+
+            var httpContext = new Mock<HttpContext>();
+            httpContext
+                .SetupGet(real => real.RequestServices)
+                .Returns(serviceProvider.Object);
+
+            var actionContext = new ActionContext(httpContext.Object, new RouteData(), new ActionDescriptor());
+
+            return new ViewContext(
+                actionContext,
+                view: Mock.Of<IView>(),
+                viewData: viewData,
+                writer: new StringWriter());
+        }
+
+        private class TestRazorPage : RazorPage<RazorPageCreateModelExpressionModel>
+        {
+            public override Task ExecuteAsync()
+            {
+                throw new NotImplementedException();
+            }
+        }
+
+        public class RazorPageCreateModelExpressionModel
+        {
+            public int Id { get; set; }
+
+            public string Name { get; set; }
+
+            public RazorPageCreateModelExpressionSubModel SubModel { get; set; }
+        }
+
+        public class RazorPageCreateModelExpressionSubModel
+        {
+            public int Id { get; set; }
+
+            public string Name { get; set; }
+
+            public RazorPageCreateModelExpressionSubSubModel SubSubModel { get; set; }
+        }
+
+        public class RazorPageCreateModelExpressionSubSubModel
+        {
+            public int Id { get; set; }
+
+            public string Name { get; set; }
+        }
+    }
+}