Reducing allocations in value providers

- Don't go async in formdata providers unless we need to
- Remove unnecessary defensive copy in CompositeValueProvider

src/Microsoft.AspNet.Mvc.Core/Internal/TaskCacheOfT.cs

@@ -0,0 +1,18 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNet.Mvc.Internal
+{
+    public static class TaskCache<T>
+    {
+        private static readonly Task<T> _defaultCompletedTask = Task.FromResult(default(T));
+
+        /// <summary>
+        /// Gets a completed <see cref="Task"/> with the value of <c>default(T)</c>.
+        /// </summary>
+        public static Task<T> DefaultCompletedTask => _defaultCompletedTask;
+    }
+
+}

src/Microsoft.AspNet.Mvc.Core/ModelBinding/CompositeValueProvider.cs

@@ -30,8 +30,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         /// </summary>
         /// <param name="valueProviders">The sequence of <see cref="IValueProvider"/> to add to this instance of
         /// <see cref="CompositeValueProvider"/>.</param>
-        public CompositeValueProvider(IEnumerable<IValueProvider> valueProviders)
+        protected CompositeValueProvider(IList<IValueProvider> valueProviders)
-            : base(valueProviders.ToList())
+            : base(valueProviders)
         {
         }
 

src/Microsoft.AspNet.Mvc.Core/ModelBinding/FormValueProviderFactory.cs

@@ -5,12 +5,13 @@ using System;
 using System.Globalization;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
+using Microsoft.AspNet.Mvc.Internal;
 
 namespace Microsoft.AspNet.Mvc.ModelBinding
 {
     public class FormValueProviderFactory : IValueProviderFactory
     {
-        public async Task<IValueProvider> GetValueProviderAsync(ValueProviderFactoryContext context)
+        public Task<IValueProvider> GetValueProviderAsync(ValueProviderFactoryContext context)
         {
             if (context == null)
             {
@@ -18,23 +19,20 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             }
 
             var request = context.HttpContext.Request;
-
             if (request.HasFormContentType)
             {
-                var culture = GetCultureInfo(request);
+                return CreateValueProviderAsync(request);
-
-                return new ReadableStringCollectionValueProvider(
-                    BindingSource.Form,
-                    await request.ReadFormAsync(),
-                    culture);
             }
 
-            return null;
+            return TaskCache<IValueProvider>.DefaultCompletedTask;
         }
 
-        private static CultureInfo GetCultureInfo(HttpRequest request)
+        private static async Task<IValueProvider> CreateValueProviderAsync(HttpRequest request)
         {
-            return CultureInfo.CurrentCulture;
+            return new ReadableStringCollectionValueProvider(
+                BindingSource.Form,
+                await request.ReadFormAsync(),
+                CultureInfo.CurrentCulture);
         }
     }
 }

src/Microsoft.AspNet.Mvc.Core/ModelBinding/JQueryFormValueProviderFactory.cs

@@ -8,30 +8,35 @@ using System.Text;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.Core;
+using Microsoft.AspNet.Mvc.Internal;
 using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNet.Mvc.ModelBinding
 {
     public class JQueryFormValueProviderFactory : IValueProviderFactory
     {
-        public async Task<IValueProvider> GetValueProviderAsync(ValueProviderFactoryContext context)
+        public Task<IValueProvider> GetValueProviderAsync(ValueProviderFactoryContext context)
         {
             if (context == null)
             {
                 throw new ArgumentNullException(nameof(context));
             }
-
+            
             var request = context.HttpContext.Request;
-
             if (request.HasFormContentType)
             {
-                return new JQueryFormValueProvider(
+                return CreateValueProviderAsync(request);
+            }
+
+            return TaskCache<IValueProvider>.DefaultCompletedTask;
+        }
+
+        private static async Task<IValueProvider> CreateValueProviderAsync(HttpRequest request)
+        {
+            return new JQueryFormValueProvider(
                     BindingSource.Form,
                     await GetValueCollectionAsync(request),
                     CultureInfo.CurrentCulture);
-            }
-
-            return null;
         }
 
         private static async Task<IDictionary<string, StringValues>> GetValueCollectionAsync(HttpRequest request)

test/Microsoft.AspNet.Mvc.Core.Test/ModelBinding/CompositeValueProviderTest.cs

@@ -26,7 +26,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 new JQueryFormValueProvider(bindingSource, new Dictionary<string, StringValues>(), culture);
             var valueProvider = new JQueryFormValueProvider(bindingSource, values, culture);
 
-            return new CompositeValueProvider(new[] { emptyValueProvider, valueProvider });
+            return new CompositeValueProvider() { emptyValueProvider, valueProvider };
         }
 
 #if DNX451
@@ -43,7 +43,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             provider2.Setup(p => p.GetKeysFromPrefix("prefix"))
                      .Returns(dictionary)
                      .Verifiable();
-            var provider = new CompositeValueProvider(new[] { provider1, provider2.Object });
+            var provider = new CompositeValueProvider() { provider1, provider2.Object };
 
             // Act
             var values = provider.GetKeysFromPrefix("prefix");
@@ -61,7 +61,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // Arrange
             var provider1 = Mock.Of<IValueProvider>();
             var provider2 = Mock.Of<IValueProvider>();
-            var provider = new CompositeValueProvider(new[] { provider1, provider2 });
+            var provider = new CompositeValueProvider() { provider1, provider2 };
 
             // Act
             var values = provider.GetKeysFromPrefix("prefix");
@@ -89,7 +89,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var valueProvider1 = GetMockValueProvider("Test");
             var valueProvider2 = GetMockValueProvider("Unrelated");
 
-            var provider = new CompositeValueProvider(new List<IValueProvider>() { valueProvider1.Object, valueProvider2.Object });
+            var provider = new CompositeValueProvider() { valueProvider1.Object, valueProvider2.Object };
 
             // Act
             var result = provider.Filter(metadata.BindingSource);