Make `CompositeValueProvider` consistent with `IBindingSourceValueProvider` contract

- #2907
- return `null` if `Filter()` finds no matching value provider in collection
- fix lack of defensiveness in model binders' use of `IBindingSourceValueProvider` implementations
- remove test workaround for unusual `CompositeValueProvider` behaviour

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.Core;
@@ -39,6 +40,12 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 !string.IsNullOrEmpty(bindingContext.ModelName);
 
             var newBindingContext = CreateNewBindingContext(bindingContext, bindingContext.ModelName);
+            if (newBindingContext == null)
+            {
+                // Unable to find a value provider for this binding source. Binding will fail.
+                return null;
+            }
+
             newBindingContext.IsFirstChanceBinding = isFirstChanceBinding;
             var modelBindingResult = await TryBind(newBindingContext);
 
@@ -46,12 +53,15 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             {
                 // Fall back to empty prefix.
                 newBindingContext = CreateNewBindingContext(bindingContext, modelName: string.Empty);
+                Debug.Assert(newBindingContext != null, "Should have failed on first attempt.");
+
                 modelBindingResult = await TryBind(newBindingContext);
             }
 
             if (modelBindingResult == null)
             {
-                return null; // something went wrong
+                // Unable to bind or something went wrong.
+                return null;
             }
 
             bindingContext.OperationBindingContext.BodyBindingState =
@@ -176,6 +186,11 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 if (valueProvider != null)
                 {
                     newBindingContext.ValueProvider = valueProvider.Filter(bindingSource);
+                    if (newBindingContext.ValueProvider == null)
+                    {
+                        // Unable to find a value provider for this binding source.
+                        return null;
+                    }
                 }
             }
 

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

@@ -137,6 +137,12 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 }
             }
 
+            if (filteredValueProviders.Count == 0)
+            {
+                // Do not create an empty CompositeValueProvider.
+                return null;
+            }
+
             if (filteredValueProviders.Count == Count)
             {
                 // No need for a new CompositeValueProvider.

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

@@ -115,7 +115,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
         private async Task<bool> CanValueBindAnyModelProperties(MutableObjectBinderContext context)
         {
-            // If there are no properties on the model, there is nothing to bind. We are here means this is not a top 
+            // If there are no properties on the model, there is nothing to bind. We are here means this is not a top
             // level object. So we return false.
             if (context.PropertyMetadata == null || context.PropertyMetadata.Count == 0)
             {
@@ -196,6 +196,11 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 if (rootValueProvider != null)
                 {
                     valueProvider = rootValueProvider.Filter(bindingSource);
+                    if (valueProvider == null)
+                    {
+                        // Unable to find a value provider for this binding source. Binding will fail.
+                        return false;
+                    }
                 }
             }
 

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

@@ -28,14 +28,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             return new CompositeValueProvider(new[] { emptyValueProvider, valueProvider });
         }
 
-        protected override void CheckFilterExcludeResult(IValueProvider result)
-        {
-            // CompositeValueProvider returns an empty instance rather than null. CompositeModelBinder and
-            // MutableObjectModelBinder depend on this empty instance.
-            var compositeProvider = Assert.IsType<CompositeValueProvider>(result);
-            Assert.Empty(compositeProvider);
-        }
-
 #if DNX451
         [Fact]
         public async Task GetKeysFromPrefixAsync_ReturnsResultFromFirstValueProviderThatReturnsValues()

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

@@ -273,11 +273,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var result = provider.Filter(bindingSource);
 
             // Assert
-            CheckFilterExcludeResult(result);
-        }
-
-        protected virtual void CheckFilterExcludeResult(IValueProvider result)
-        {
             Assert.Null(result);
         }