Make `ModelBinderAttribute.BindingSource` setter `protected`

- #3428
- the `public` setter was not useful when this class is used as an attribute
- make property `virtual` to support other override patterns
- update existing test to use both override patterns

src/Microsoft.AspNet.Mvc.Core/ModelBinderAttribute.cs

@@ -32,7 +32,7 @@ namespace Microsoft.AspNet.Mvc
         public Type BinderType { get; set; }
 
         /// <inheritdoc />
-        public BindingSource BindingSource
+        public virtual BindingSource BindingSource
         {
             get
             {
@@ -43,7 +43,7 @@ namespace Microsoft.AspNet.Mvc
 
                 return _bindingSource;
             }
-            set
+            protected set
             {
                 _bindingSource = value;
             }

test/Microsoft.AspNet.Mvc.Core.Test/ModelBinding/Metadata/DefaultBindingMetadataProviderTest.cs

@@ -108,8 +108,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Metadata
             // Arrange
             var attributes = new object[]
             {
-                new ModelBinderAttribute() { BindingSource = BindingSource.Body },
-                new ModelBinderAttribute() { BindingSource = BindingSource.Query },
+                new BindingSourceModelBinderAttribute(BindingSource.Body),
+                new BindingSourceModelBinderAttribute(BindingSource.Query),
             };
 
             var context = new BindingMetadataProviderContext(
@@ -132,8 +132,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Metadata
             var attributes = new object[]
             {
                 new ModelBinderAttribute(),
-                new ModelBinderAttribute() { BindingSource = BindingSource.Body },
-                new ModelBinderAttribute() { BindingSource = BindingSource.Query },
+                new BindingSourceModelBinderAttribute(BindingSource.Body),
+                new BindingSourceModelBinderAttribute(BindingSource.Query),
             };
 
             var context = new BindingMetadataProviderContext(
@@ -545,5 +545,13 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Metadata
         private class BindRequiredOverridesInheritedBindNever : BindNeverOnClass
         {
         }
+
+        private class BindingSourceModelBinderAttribute : ModelBinderAttribute
+        {
+            public BindingSourceModelBinderAttribute(BindingSource bindingSource)
+            {
+                BindingSource = bindingSource;
+            }
+        }
     }
 }

test/Microsoft.AspNet.Mvc.Core.Test/ModelBinding/Metadata/ModelBinderAttributeTest.cs

@@ -1,7 +1,6 @@
 // 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;
 using Xunit;
 
 namespace Microsoft.AspNet.Mvc.ModelBinding
@@ -39,8 +38,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         public void BinderType_SettingBindingSource_OverridesDefaultCustomBindingSource()
         {
             // Arrange
-            var attribute = new ModelBinderAttribute();
-            attribute.BindingSource = BindingSource.Query;
+            var attribute = new FromQueryModelBinderAttribute();
             attribute.BinderType = typeof(ByteArrayModelBinder);
 
             // Act
@@ -49,5 +47,10 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // Assert
             Assert.Equal(BindingSource.Query, source);
         }
+
+        private class FromQueryModelBinderAttribute : ModelBinderAttribute
+        {
+            public override BindingSource BindingSource => BindingSource.Query;
+        }
     }
 }