Change ModelBinding to use a single pass

src/Microsoft.AspNet.Mvc.Abstractions/ModelBinding/ModelBindingContext.cs

@@ -11,9 +11,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
     /// </summary>
     public class ModelBindingContext
     {
-        private static readonly Func<ModelBindingContext, string, bool>
-            _defaultPropertyFilter = (context, propertyName) => true;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="ModelBindingContext"/> class.
         /// </summary>
@@ -167,19 +164,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         /// <remarks>Passed into the model binding system.</remarks>
         public bool IsTopLevelObject { get; set; }
 
-        /// <summary>
-        /// Gets or sets an indication that the model binding system will make another binding attempt (e.g. fall back
-        /// to the empty prefix) after this one.
-        /// </summary>
-        /// <remarks>
-        /// Not passed into the model binding system but instead set by the top-level binder. With built-in binders,
-        /// <c>true</c> only in binders called directly from a
-        /// <c>Microsoft.AspNet.Mvc.ModelBinding.CompositeModelBinder</c> that was passed a
-        /// <see cref="ModelBindingContext"/> with <see cref="FallbackToEmptyPrefix"/> <c>true</c>. <c>false</c>
-        /// otherwise.
-        /// </remarks>
-        public bool IsFirstChanceBinding { get; set; }
-
         /// <summary>
         /// Gets or sets the <see cref="IValueProvider"/> associated with this context.
         /// </summary>
@@ -191,4 +175,4 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         /// </summary>
         public Func<ModelBindingContext, string, bool> PropertyFilter { get; set; }
     }
-}
+}

src/Microsoft.AspNet.Mvc.Abstractions/ModelBinding/ModelBindingResult.cs

@@ -15,7 +15,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         public ModelBindingResult(string key)
         {
             Key = key;
-            IsFatalError = true;
         }
 
         /// <summary>
@@ -62,12 +61,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         /// </summary>
         public string Key { get; }
 
-        /// <summary>
-        /// Gets a value indicating the caller should not attempt binding again. This attempt encountered a fatal
-        /// error.
-        /// </summary>
-        public bool IsFatalError { get; }
-
         /// <summary>
         /// <para>
         /// Gets a value indicating whether or not the <see cref="Model"/> value has been set.

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

@@ -29,9 +29,9 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var model = bindingContext.Model;
             if (!bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName))
             {
-                // If this is the fallback case and we failed to find data for a top-level model, then generate a
+                // If we failed to find data for a top-level model, then generate a
                 // default 'empty' model (or use existing Model) and return it.
-                if (!bindingContext.IsFirstChanceBinding && bindingContext.IsTopLevelObject)
+                if (bindingContext.IsTopLevelObject)
                 {
                     if (model == null)
                     {

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

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.Core;
@@ -35,29 +34,14 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
         public virtual async Task<ModelBindingResult> BindModelAsync([NotNull] ModelBindingContext bindingContext)
         {
-            // Will there be a last chance (fallback) binding attempt?
+            var newBindingContext = CreateNewBindingContext(bindingContext);
-            var isFirstChanceBinding = bindingContext.FallbackToEmptyPrefix &&
-                !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 RunModelBinders(newBindingContext);
-            var modelBindingResult = await TryBind(newBindingContext);
-
-            if (modelBindingResult == null && isFirstChanceBinding)
-            {
-                // 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)
             {
                 // Unable to bind or something went wrong.
@@ -99,7 +83,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 modelBindingResult.ValidationNode);
         }
 
-        private async Task<ModelBindingResult> TryBind(ModelBindingContext bindingContext)
+        private async Task<ModelBindingResult> RunModelBinders(ModelBindingContext bindingContext)
         {
             RuntimeHelpers.EnsureSufficientExecutionStack();
 
@@ -108,18 +92,11 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 var result = await binder.BindModelAsync(bindingContext);
                 if (result != null)
                 {
-                    // Use returned ModelBindingResult if it indicates the model was set, indicates the binder
+                    // This condition is necessary because the ModelState entry would never be validated if
-                    // encountered a fatal error, or is related to a ModelState entry.
-                    //
-                    // The second condition is necessary because the BodyModelBinder unconditionally binds during the
-                    // first attempt and does not always create ModelState values using ModelName.
-                    //
-                    // The third condition is necessary because the ModelState entry would never be validated if
                     // caller fell back to the empty prefix, leading to an possibly-incorrect !IsValid. In most
                     // (hopefully all) cases, the ModelState entry exists because some binders add errors before
                     // returning a result with !IsModelSet. Those binders often cannot run twice anyhow.
-                    if (result.IsFatalError ||
+                    if (result.IsModelSet ||
-                        result.IsModelSet ||
                         bindingContext.ModelState.ContainsKey(bindingContext.ModelName))
                     {
                         return result;
@@ -136,26 +113,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             return null;
         }
 
-        private static ModelBindingContext CreateNewBindingContext(
+        private static ModelBindingContext CreateNewBindingContext(ModelBindingContext oldBindingContext)
-            ModelBindingContext oldBindingContext,
-            string modelName)
         {
-            var newBindingContext = new ModelBindingContext
-            {
-                Model = oldBindingContext.Model,
-                ModelMetadata = oldBindingContext.ModelMetadata,
-                ModelName = modelName,
-                FieldName = oldBindingContext.FieldName,
-                ModelState = oldBindingContext.ModelState,
-                ValueProvider = oldBindingContext.ValueProvider,
-                OperationBindingContext = oldBindingContext.OperationBindingContext,
-                PropertyFilter = oldBindingContext.PropertyFilter,
-                BinderModelName = oldBindingContext.BinderModelName,
-                BindingSource = oldBindingContext.BindingSource,
-                BinderType = oldBindingContext.BinderType,
-                IsTopLevelObject = oldBindingContext.IsTopLevelObject,
-            };
-
             // If the property has a specified data binding sources, we need to filter the set of value providers
             // to just those that match. We can skip filtering when IsGreedy == true, because that can't use
             // value providers.
@@ -174,15 +133,15 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // public IActionResult UpdatePerson([FromForm] Person person) { }
             //
             // In this example, [FromQuery] overrides the ambient data source (form).
+            IValueProvider valueProvider = oldBindingContext.ValueProvider;
             var bindingSource = oldBindingContext.BindingSource;
             if (bindingSource != null && !bindingSource.IsGreedy)
             {
-                var valueProvider =
+                var bindingSourceValueProvider = valueProvider as IBindingSourceValueProvider;
-                    oldBindingContext.OperationBindingContext.ValueProvider as IBindingSourceValueProvider;
+                if (bindingSourceValueProvider != null)
-                if (valueProvider != null)
                 {
-                    newBindingContext.ValueProvider = valueProvider.Filter(bindingSource);
+                    valueProvider = bindingSourceValueProvider.Filter(bindingSource);
-                    if (newBindingContext.ValueProvider == null)
+                    if (valueProvider == null)
                     {
                         // Unable to find a value provider for this binding source.
                         return null;
@@ -190,6 +149,36 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 }
             }
 
+            var newBindingContext = new ModelBindingContext
+            {
+                Model = oldBindingContext.Model,
+                ModelMetadata = oldBindingContext.ModelMetadata,
+                FieldName = oldBindingContext.FieldName,
+                ModelState = oldBindingContext.ModelState,
+                ValueProvider = valueProvider,
+                OperationBindingContext = oldBindingContext.OperationBindingContext,
+                PropertyFilter = oldBindingContext.PropertyFilter,
+                BinderModelName = oldBindingContext.BinderModelName,
+                BindingSource = oldBindingContext.BindingSource,
+                BinderType = oldBindingContext.BinderType,
+                IsTopLevelObject = oldBindingContext.IsTopLevelObject,
+            };
+
+            if (bindingSource != null && bindingSource.IsGreedy)
+            {
+                newBindingContext.ModelName = oldBindingContext.ModelName;
+            }
+            else if (
+                !oldBindingContext.FallbackToEmptyPrefix ||
+                newBindingContext.ValueProvider.ContainsPrefix(oldBindingContext.ModelName))
+            {
+                newBindingContext.ModelName = oldBindingContext.ModelName;
+            }
+            else
+            {
+                newBindingContext.ModelName = string.Empty;
+            }
+
             return newBindingContext;
         }
     }

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

@@ -69,6 +69,13 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             {
                 var form = await request.ReadFormAsync();
 
+                // If we're at the top level, then use the FieldName (paramter or property name).
+                // This handles the fact that there will be nothing in the ValueProviders for this parameter
+                // and so we'll do the right thing even though we 'fell-back' to the empty prefix.
+                var modelName = bindingContext.IsTopLevelObject
+                    ? bindingContext.FieldName
+                    : bindingContext.ModelName;
+
                 foreach (var file in form.Files)
                 {
                     ContentDispositionHeaderValue parsedContentDisposition;
@@ -82,8 +89,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                         continue;
                     }
 
-                    var modelName = HeaderUtilities.RemoveQuotes(parsedContentDisposition.Name);
+                    var fileName = HeaderUtilities.RemoveQuotes(parsedContentDisposition.Name);
-                    if (modelName.Equals(bindingContext.ModelName, StringComparison.OrdinalIgnoreCase))
+                    if (fileName.Equals(modelName, StringComparison.OrdinalIgnoreCase))
                     {
                         postedFiles.Add(file);
                     }

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

@@ -62,9 +62,9 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             }
             else
             {
-                // If this is the fallback case and we failed to find data for a top-level model, then generate a
+                // If we failed to find data for a top-level model, then generate a
                 // default 'empty' model and return it.
-                if (!bindingContext.IsFirstChanceBinding && bindingContext.IsTopLevelObject)
+                if (bindingContext.IsTopLevelObject)
                 {
                     var model = new KeyValuePair<TKey, TValue>();
 

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

@@ -96,8 +96,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             }
 
             // Create the object if:
-            // 1. It is a top level model and no later fallback (to empty prefix) will occur.
+            // 1. It is a top level model.
-            if (isTopLevelObject && !bindingContext.IsFirstChanceBinding)
+            if (isTopLevelObject)
             {
                 return true;
             }

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

@@ -37,33 +37,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
         }
 
         [Fact]
-        public async Task ArrayModelBinder_DoesNotCreateCollection_IfIsTopLevelObjectAndIsFirstChanceBinding()
+        public async Task ArrayModelBinder_CreatesEmptyCollection_IfIsTopLevelObject()
-        {
-            // Arrange
-            var binder = new ArrayModelBinder<string>();
-
-            var context = CreateContext();
-            context.IsFirstChanceBinding = true;
-            context.IsTopLevelObject = true;
-
-            // Explicit prefix and empty model name both ignored.
-            context.BinderModelName = "prefix";
-            context.ModelName = string.Empty;
-
-            var metadataProvider = context.OperationBindingContext.MetadataProvider;
-            context.ModelMetadata = metadataProvider.GetMetadataForType(typeof(string[]));
-
-            context.ValueProvider = new TestValueProvider(new Dictionary<string, object>());
-
-            // Act
-            var result = await binder.BindModelAsync(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        [Fact]
-        public async Task ArrayModelBinder_CreatesEmptyCollection_IfIsTopLevelObjectAndNotIsFirstChanceBinding()
         {
             // Arrange
             var binder = new ArrayModelBinder<string>();

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

@@ -76,7 +76,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Returns non-null because it understands the metadata type.
             Assert.NotNull(binderResult);
-            Assert.True(binderResult.IsFatalError);
             Assert.False(binderResult.IsModelSet);
             Assert.Null(binderResult.ValidationNode);
             Assert.Null(binderResult.Model);
@@ -103,7 +102,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             Assert.NotNull(binderResult);
-            Assert.True(binderResult.IsFatalError);
             Assert.False(binderResult.IsModelSet);
             Assert.Null(binderResult.ValidationNode);
         }
@@ -172,7 +170,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Returns non-null because it understands the metadata type.
             Assert.NotNull(binderResult);
-            Assert.True(binderResult.IsFatalError);
             Assert.False(binderResult.IsModelSet);
             Assert.Null(binderResult.ValidationNode);
             Assert.Null(binderResult.Model);
@@ -209,7 +206,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Returns non-null result because it understands the metadata type.
             Assert.NotNull(binderResult);
-            Assert.True(binderResult.IsFatalError);
             Assert.False(binderResult.IsModelSet);
             Assert.Null(binderResult.Model);
             Assert.Null(binderResult.ValidationNode);

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

@@ -227,33 +227,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
         }
 
         [Fact]
-        public async Task CollectionModelBinder_DoesNotCreateCollection_IfIsTopLevelObjectAndIsFirstChanceBinding()
+        public async Task CollectionModelBinder_CreatesEmptyCollection_IfIsTopLevelObject()
-        {
-            // Arrange
-            var binder = new CollectionModelBinder<string>();
-
-            var context = CreateContext();
-            context.IsTopLevelObject = true;
-            context.IsFirstChanceBinding = true;
-
-            // Explicit prefix and empty model name both ignored.
-            context.BinderModelName = "prefix";
-            context.ModelName = string.Empty;
-
-            var metadataProvider = context.OperationBindingContext.MetadataProvider;
-            context.ModelMetadata = metadataProvider.GetMetadataForType(typeof(List<string>));
-
-            context.ValueProvider = new TestValueProvider(new Dictionary<string, object>());
-
-            // Act
-            var result = await binder.BindModelAsync(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        [Fact]
-        public async Task CollectionModelBinder_CreatesEmptyCollection_IfIsTopLevelObjectAndNotIsFirstChanceBinding()
         {
             // Arrange
             var binder = new CollectionModelBinder<string>();
@@ -284,7 +258,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             Assert.Same(result.ValidationNode.ModelMetadata, context.ModelMetadata);
         }
 
-        // Setup like CollectionModelBinder_CreatesEmptyCollection_IfIsTopLevelObjectAndNotIsFirstChanceBinding  except
+        // Setup like CollectionModelBinder_CreatesEmptyCollection_IfIsTopLevelObject  except
         // Model already has a value.
         [Fact]
         public async Task CollectionModelBinder_DoesNotCreateEmptyCollection_IfModelNonNull()

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

@@ -6,7 +6,6 @@ using System;
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Threading.Tasks;
-using Microsoft.AspNet.Mvc.ModelBinding.Validation;
 using Moq;
 using Xunit;
 
@@ -65,6 +64,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             var bindingContext = new ModelBindingContext
             {
                 FallbackToEmptyPrefix = true,
+                IsTopLevelObject = true,
                 ModelMetadata = new EmptyModelMetadataProvider().GetMetadataForType(typeof(List<int>)),
                 ModelName = "someName",
                 ModelState = new ModelStateDictionary(),
@@ -136,49 +136,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             Assert.Null(result);
         }
 
-        [Fact]
-        public async Task ModelBinder_FallsBackToEmpty_IfBinderMatchesButDoesNotSetModel()
-        {
-            // Arrange
-            var bindingContext = new ModelBindingContext
-            {
-                FallbackToEmptyPrefix = true,
-                ModelMetadata = new EmptyModelMetadataProvider().GetMetadataForType(typeof(List<int>)),
-                ModelName = "someName",
-                ModelState = new ModelStateDictionary(),
-                OperationBindingContext = new OperationBindingContext(),
-                ValueProvider = new SimpleValueProvider
-                {
-                    { "someOtherName", "dummyValue" }
-                },
-            };
-
-            var count = 0;
-            var modelBinder = new Mock<IModelBinder>();
-            modelBinder
-                .Setup(mb => mb.BindModelAsync(It.IsAny<ModelBindingContext>()))
-                .Callback<ModelBindingContext>(context =>
-                {
-                    // Expect two calls; the second with empty ModelName.
-                    Assert.InRange(count, 0, 1);
-                    count++;
-                    if (count == 1)
-                    {
-                        Assert.Equal("someName", context.ModelName);
-                    }
-                    else
-                    {
-                        Assert.Empty(context.ModelName);
-                    }
-                })
-                .Returns(Task.FromResult(new ModelBindingResult(model: null, key: "someName", isModelSet: false)));
-
-            var composite = CreateCompositeBinder(modelBinder.Object);
-
-            // Act & Assert
-            var result = await composite.BindModelAsync(bindingContext);
-        }
-
         [Fact]
         public async Task ModelBinder_DoesNotFallBackToEmpty_IfFallbackToEmptyPrefixFalse()
         {
@@ -322,6 +279,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
                 ModelMetadata = new EmptyModelMetadataProvider().GetMetadataForType(typeof(int)),
                 ModelState = new ModelStateDictionary(),
                 OperationBindingContext = new OperationBindingContext(),
+                ValueProvider = new SimpleValueProvider(),
             };
 
             // Act
@@ -493,6 +451,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             var bindingContext = new ModelBindingContext
             {
                 FallbackToEmptyPrefix = true,
+                IsTopLevelObject = true,
                 ModelMetadata = metadataProvider.GetMetadataForType(type),
                 ModelName = "parameter",
                 ModelState = new ModelStateDictionary(),

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

@@ -133,7 +133,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
 
             // Assert
             Assert.NotNull(result);
-            Assert.False(result.IsFatalError);
             Assert.True(result.IsModelSet);
             Assert.Equal(modelName, result.Key);
             Assert.NotNull(result.ValidationNode);
@@ -171,7 +170,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
 
             // Assert
             Assert.NotNull(result);
-            Assert.False(result.IsFatalError);
             Assert.True(result.IsModelSet);
             Assert.Equal("prefix", result.Key);
             Assert.NotNull(result.ValidationNode);
@@ -223,7 +221,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
 
             // Assert
             Assert.NotNull(result);
-            Assert.False(result.IsFatalError);
             Assert.True(result.IsModelSet);
             Assert.Equal("prefix", result.Key);
             Assert.NotNull(result.ValidationNode);
@@ -265,7 +262,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
 
             // Assert
             Assert.NotNull(result);
-            Assert.False(result.IsFatalError);
             Assert.True(result.IsModelSet);
             Assert.Equal("prefix", result.Key);
             Assert.NotNull(result.ValidationNode);
@@ -300,7 +296,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
 
             // Assert
             Assert.NotNull(result);
-            Assert.False(result.IsFatalError);
             Assert.True(result.IsModelSet);
             Assert.Equal(modelName, result.Key);
             Assert.NotNull(result.ValidationNode);
@@ -310,33 +305,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
         }
 
         [Fact]
-        public async Task DictionaryModelBinder_DoesNotCreateCollection_IfIsTopLevelObjectAndIsFirstChanceBinding()
+        public async Task DictionaryModelBinder_CreatesEmptyCollection_IfIsTopLevelObject()
-        {
-            // Arrange
-            var binder = new DictionaryModelBinder<string, string>();
-
-            var context = CreateContext();
-            context.IsTopLevelObject = true;
-            context.IsFirstChanceBinding = true;
-
-            // Explicit prefix and empty model name both ignored.
-            context.BinderModelName = "prefix";
-            context.ModelName = string.Empty;
-
-            var metadataProvider = context.OperationBindingContext.MetadataProvider;
-            context.ModelMetadata = metadataProvider.GetMetadataForType(typeof(Dictionary<string, string>));
-
-            context.ValueProvider = new TestValueProvider(new Dictionary<string, object>());
-
-            // Act
-            var result = await binder.BindModelAsync(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        [Fact]
-        public async Task DictionaryModelBinder_CreatesEmptyCollection_IfIsTopLevelObjectAndNotIsFirstChanceBinding()
         {
             // Arrange
             var binder = new DictionaryModelBinder<string, string>();

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

@@ -9,6 +9,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Http.Internal;
+using Microsoft.Net.Http.Headers;
 using Moq;
 using Xunit;
 
@@ -116,6 +117,37 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             Assert.Null(result.Model);
         }
 
+        [Theory]
+        [InlineData(true, "FieldName")]
+        [InlineData(false, "ModelName")]
+        public async Task FormFileModelBinder_UsesFieldNameForTopLevelObject(bool isTopLevel, string expected)
+        {
+            // Arrange
+            var formFiles = new FormFileCollection();
+            formFiles.Add(GetMockFormFile("FieldName", "file1.txt"));
+            formFiles.Add(GetMockFormFile("ModelName", "file1.txt"));
+            var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
+
+            var bindingContext = GetBindingContext(typeof(IFormFile), httpContext);
+            bindingContext.IsTopLevelObject = isTopLevel;
+            bindingContext.FieldName = "FieldName";
+            bindingContext.ModelName = "ModelName";
+
+            var binder = new FormFileModelBinder();
+
+            // Act
+            var result = await binder.BindModelAsync(bindingContext);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.True(result.IsModelSet);
+            var file = Assert.IsAssignableFrom<IFormFile>(result.Model);
+            
+            ContentDispositionHeaderValue contentDisposition;
+            ContentDispositionHeaderValue.TryParse(file.ContentDisposition, out contentDisposition);
+            Assert.Equal(expected, contentDisposition.Name);
+        }
+
         [Fact]
         public async Task FormFileModelBinder_ReturnsNull_WithEmptyContentDisposition()
         {

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

@@ -148,33 +148,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
         }
 
         [Fact]
-        public async Task KeyValuePairModelBinder_DoesNotCreateCollection_IfIsTopLevelObjectAndIsFirstChanceBinding()
+        public async Task KeyValuePairModelBinder_CreatesEmptyCollection_IfIsTopLevelObject()
-        {
-            // Arrange
-            var binder = new KeyValuePairModelBinder<string, string>();
-
-            var context = CreateContext();
-            context.IsTopLevelObject = true;
-            context.IsFirstChanceBinding = true;
-
-            // Explicit prefix and empty model name both ignored.
-            context.BinderModelName = "prefix";
-            context.ModelName = string.Empty;
-
-            var metadataProvider = context.OperationBindingContext.MetadataProvider;
-            context.ModelMetadata = metadataProvider.GetMetadataForType(typeof(KeyValuePair<string, string>));
-
-            context.ValueProvider = new TestValueProvider(new Dictionary<string, object>());
-
-            // Act
-            var result = await binder.BindModelAsync(context);
-
-            // Assert
-            Assert.Null(result);
-        }
-
-        [Fact]
-        public async Task KeyValuePairModelBinder_CreatesEmptyCollection_IfIsTopLevelObjectAndNotIsFirstChanceBinding()
         {
             // Arrange
             var binder = new KeyValuePairModelBinder<string, string>();

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

@@ -46,7 +46,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             Assert.Same(newModelMetadata.BindingSource, newBindingContext.BindingSource);
             Assert.False(newBindingContext.FallbackToEmptyPrefix);
             Assert.Equal("fieldName", newBindingContext.FieldName);
-            Assert.False(newBindingContext.IsFirstChanceBinding);
             Assert.False(newBindingContext.IsTopLevelObject);
             Assert.Null(newBindingContext.Model);
             Assert.Same(newModelMetadata, newBindingContext.ModelMetadata);

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

@@ -20,27 +20,10 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
     public class MutableObjectModelBinderTest
     {
         [Theory]
-        [InlineData(true, true, "", "", false)]
+        [InlineData(true, true)]
-        [InlineData(true, false, "", "", true)]
+        [InlineData(false, false)]
-        [InlineData(false, true, "", "", false)] // !isTopLevelObject && isFirstChanceBinding cases are unexpected
+        public void CanCreateModel_ReturnsTrue_IfIsTopLevelObject(
-        [InlineData(false, false, "", "", false)]
-        [InlineData(true, true, "prefix", "", false)]
-        [InlineData(true, false, "prefix", "", true)]
-        [InlineData(false, true, "prefix", "", false)]
-        [InlineData(false, false, "prefix", "", false)]
-        [InlineData(true, true, "", "dummyModelName", false)]
-        [InlineData(true, false, "", "dummyModelName", true)]
-        [InlineData(false, true, "", "dummyModelName", false)]
-        [InlineData(false, false, "", "dummyModelName", false)]
-        [InlineData(true, true, "prefix", "dummyModelName", false)]
-        [InlineData(true, false, "prefix", "dummyModelName", true)]
-        [InlineData(false, true, "prefix", "dummyModelName", false)]
-        [InlineData(false, false, "prefix", "dummyModelName", false)]
-        public void CanCreateModel_ReturnsTrue_IfIsTopLevelObjectAndNotIsFirstChanceBinding(
             bool isTopLevelObject,
-            bool isFirstChanceBinding,
-            string binderModelName,
-            string modelName,
             bool expectedCanCreate)
         {
             var mockValueProvider = new Mock<IValueProvider>();
@@ -54,7 +37,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 ModelBindingContext = new ModelBindingContext
                 {
                     IsTopLevelObject = isTopLevelObject,
-                    IsFirstChanceBinding = isFirstChanceBinding,
 
                     // Random type.
                     ModelMetadata = metadataProvider.GetMetadataForType(typeof(Person)),
@@ -65,10 +47,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                         MetadataProvider = metadataProvider,
                         ValidatorProvider = Mock.Of<IModelValidatorProvider>(),
                     },
-
-                    // CanCreateModel() ignores the BinderModelName and ModelName properties.
-                    BinderModelName = binderModelName,
-                    ModelName = modelName,
                 },
             };
 
@@ -467,7 +445,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
 
         [Fact]
-        public async Task BindModel_InitsInstance_IfIsTopLevelObjectAndNotIsFirstChanceBinding()
+        public async Task BindModel_InitsInstance_IfIsTopLevelObject()
         {
             // Arrange
             var mockValueProvider = new Mock<IValueProvider>();

test/Microsoft.AspNet.Mvc.Core.Test/ModelBindingHelperTest.cs

@@ -25,7 +25,7 @@ namespace Microsoft.AspNet.Mvc.Test
                 return new TheoryData<ModelBindingResult>
                 {
                     null,
-                    new ModelBindingResult("someKey"), // IsFatalError true as well as IsModelSet false.
+                    new ModelBindingResult("someKey"), // IsModelSet false.
                     new ModelBindingResult(model: null, key: "someKey", isModelSet: false),
                 };
             }

test/Microsoft.AspNet.Mvc.FunctionalTests/ModelBindingTest.cs

@@ -734,10 +734,14 @@ namespace Microsoft.AspNet.Mvc.FunctionalTests
             // Assert
             var json = JsonConvert.DeserializeObject<Dictionary<string, string>>(response);
             Assert.Equal(3, json.Count);
+
+            // The model prefix 'model' is used in the modelstate keys because the key 'model' is present in the 
+            // query string. This causes modelbinding to commit to using the prefix.
+            //
             // Mono issue - https://github.com/aspnet/External/issues/19
-            Assert.Equal(PlatformNormalizer.NormalizeContent("The Field1 field is required."), json["Field1"]);
+            Assert.Equal(PlatformNormalizer.NormalizeContent("The Field1 field is required."), json["model.Field1"]);
-            Assert.Equal(PlatformNormalizer.NormalizeContent("The Field2 field is required."), json["Field2"]);
+            Assert.Equal(PlatformNormalizer.NormalizeContent("The Field2 field is required."), json["model.Field2"]);
-            Assert.Equal(PlatformNormalizer.NormalizeContent("The Field3 field is required."), json["Field3"]);
+            Assert.Equal(PlatformNormalizer.NormalizeContent("The Field3 field is required."), json["model.Field3"]);
         }
 
         [Fact]

test/Microsoft.AspNet.Mvc.IntegrationTests/BinderTypeBasedModelBinderIntegrationTest.cs

@@ -226,7 +226,7 @@ namespace Microsoft.AspNet.Mvc.IntegrationTests
 
             // ModelBindingResult
             Assert.NotNull(modelBindingResult);
-            Assert.Equal("Parameter1", modelBindingResult.Key);
+            Assert.Equal(string.Empty, modelBindingResult.Key);
 
             // Model
             var boundPerson = Assert.IsType<Person>(modelBindingResult.Model);
@@ -237,7 +237,7 @@ namespace Microsoft.AspNet.Mvc.IntegrationTests
             Assert.True(modelState.IsValid);
 
             Assert.Equal(1, modelState.Keys.Count);
-            var key = Assert.Single(modelState.Keys, k => k == "Parameter1.Address.Street");
+            var key = Assert.Single(modelState.Keys, k => k == "Address.Street");
             Assert.Equal(ModelValidationState.Valid, modelState[key].ValidationState);
             Assert.NotNull(modelState[key].RawValue); // Value is set by test model binder, no need to validate it.
         }

test/Microsoft.AspNet.Mvc.IntegrationTests/ModelPrefixSelectionIntegrationTest.cs

@@ -0,0 +1,256 @@
+// 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.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Http;
+using Microsoft.AspNet.Http.Internal;
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.Framework.Primitives;
+using Xunit;
+
+namespace Microsoft.AspNet.Mvc.IntegrationTests
+{
+    // Integration tests for the decision logic about how a model-name/prefix is selected at the top-level
+    // of ModelBinding.
+    public class ModelPrefixSelectionIntegrationTest
+    {
+        private class Person1
+        {
+            [FromForm]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public async Task ComplexModel_PrefixSelected_ByValueProvider()
+        {
+            // Arrange
+            var argumentBinder = ModelBindingTestHelper.GetArgumentBinder();
+            var parameter = new ParameterDescriptor()
+            {
+                Name = "parameter",
+                ParameterType = typeof(Person1),
+            };
+
+            var operationContext = ModelBindingTestHelper.GetOperationBindingContext(request =>
+            {
+                // This will cause selection of the "parameter" prefix.
+                request.QueryString = new QueryString("?parameter=");
+
+                // This value won't be used, because we select the "parameter" prefix.
+                request.Form = new FormCollection(new Dictionary<string, StringValues>()
+                {
+                    { "Name", "Billy" },
+                });
+            });
+
+            var modelState = new ModelStateDictionary();
+
+            // Act
+            var modelBindingResult = await argumentBinder.BindModelAsync(parameter, modelState, operationContext);
+
+            // Assert
+            Assert.NotNull(modelBindingResult);
+            Assert.True(modelBindingResult.IsModelSet);
+            Assert.Equal("parameter", modelBindingResult.Key);
+
+            var model = Assert.IsType<Person1>(modelBindingResult.Model);
+            Assert.Null(model.Name);
+
+            Assert.Equal(0, modelState.Count);
+            Assert.Equal(0, modelState.ErrorCount);
+            Assert.True(modelState.IsValid);
+        }
+
+        private class Person2
+        {
+            [FromForm]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public async Task ComplexModel_PrefixSelected_ByValueProviderValue_WithFilteredValueProviders()
+        {
+            // Arrange
+            var argumentBinder = ModelBindingTestHelper.GetArgumentBinder();
+            var parameter = new ParameterDescriptor()
+            {
+                Name = "parameter",
+                ParameterType = typeof(Person2),
+                BindingInfo = new BindingInfo()
+                {
+                    BindingSource = BindingSource.Query,
+                },
+            };
+
+            var operationContext = ModelBindingTestHelper.GetOperationBindingContext(request =>
+            {
+                // This will cause selection of the "parameter" prefix.
+                request.QueryString = new QueryString("?parameter=");
+
+                // This value won't be used, because we select the "parameter" prefix.
+                request.Form = new FormCollection(new Dictionary<string, StringValues>()
+                {
+                    { "Name", "Billy" },
+                });
+            });
+
+            var modelState = new ModelStateDictionary();
+
+            // Act
+            var modelBindingResult = await argumentBinder.BindModelAsync(parameter, modelState, operationContext);
+
+            // Assert
+            Assert.NotNull(modelBindingResult);
+            Assert.True(modelBindingResult.IsModelSet);
+            Assert.Equal("parameter", modelBindingResult.Key);
+
+            var model = Assert.IsType<Person2>(modelBindingResult.Model);
+            Assert.Null(model.Name);
+
+            Assert.Equal(0, modelState.Count);
+            Assert.Equal(0, modelState.ErrorCount);
+            Assert.True(modelState.IsValid);
+        }
+
+        private class Person3
+        {
+            [FromForm]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public async Task ComplexModel_EmptyPrefixSelected_NoMatchingValueProviderValue()
+        {
+            // Arrange
+            var argumentBinder = ModelBindingTestHelper.GetArgumentBinder();
+            var parameter = new ParameterDescriptor()
+            {
+                Name = "parameter",
+                ParameterType = typeof(Person3),
+            };
+
+            var operationContext = ModelBindingTestHelper.GetOperationBindingContext(request =>
+            {
+                // This can't be used because of [FromForm] on the property.
+                request.QueryString = new QueryString("?Name=");
+            });
+
+            var modelState = new ModelStateDictionary();
+
+            // Act
+            var modelBindingResult = await argumentBinder.BindModelAsync(parameter, modelState, operationContext);
+
+            // Assert
+            Assert.NotNull(modelBindingResult);
+            Assert.True(modelBindingResult.IsModelSet);
+            Assert.Equal(string.Empty, modelBindingResult.Key);
+
+            var model = Assert.IsType<Person3>(modelBindingResult.Model);
+            Assert.Null(model.Name);
+
+            Assert.Equal(0, modelState.Count);
+            Assert.Equal(0, modelState.ErrorCount);
+            Assert.True(modelState.IsValid);
+        }
+
+        private class Person4
+        {
+            [FromForm]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public async Task ComplexModel_EmptyPrefixSelected_NoMatchingValueProviderValue_WithFilteredValueProviders()
+        {
+            // Arrange
+            var argumentBinder = ModelBindingTestHelper.GetArgumentBinder();
+            var parameter = new ParameterDescriptor()
+            {
+                Name = "parameter",
+                ParameterType = typeof(Person4),
+                BindingInfo = new BindingInfo()
+                {
+                    BindingSource = BindingSource.Query,
+                },
+            };
+
+            var operationContext = ModelBindingTestHelper.GetOperationBindingContext(request =>
+            {
+                // This will only match empty prefix, but can't be used because of [FromForm] on the property.
+                request.QueryString = new QueryString("?Name=");
+
+                // This value won't be used to select a prefix, because we're only looking at the query string.
+                request.Form = new FormCollection(new Dictionary<string, StringValues>()
+                {
+                    { "parameter", string.Empty },
+                });
+            });
+
+            var modelState = new ModelStateDictionary();
+
+            // Act
+            var modelBindingResult = await argumentBinder.BindModelAsync(parameter, modelState, operationContext);
+
+            // Assert
+            Assert.NotNull(modelBindingResult);
+            Assert.True(modelBindingResult.IsModelSet);
+            Assert.Equal(string.Empty, modelBindingResult.Key);
+
+            var model = Assert.IsType<Person4>(modelBindingResult.Model);
+            Assert.Null(model.Name);
+
+            Assert.Equal(0, modelState.Count);
+            Assert.Equal(0, modelState.ErrorCount);
+            Assert.True(modelState.IsValid);
+        }
+
+        private class Person5
+        {
+            [FromForm]
+            public string Name { get; set; }
+        }
+
+        [Fact]
+        public async Task ComplexModel_EmptyPrefixSelected_NoMatchingValueProviderValue_WithFilteredValueProviders_NoValues()
+        {
+            // Arrange
+            var argumentBinder = ModelBindingTestHelper.GetArgumentBinder();
+            var parameter = new ParameterDescriptor()
+            {
+                Name = "parameter",
+                ParameterType = typeof(Person5),
+                BindingInfo = new BindingInfo()
+                {
+                    BindingSource = BindingSource.Query,
+                },
+            };
+
+            var operationContext = ModelBindingTestHelper.GetOperationBindingContext(request =>
+            {
+                // This value won't be used to select a prefix, because we're only looking at the query string.
+                request.Form = new FormCollection(new Dictionary<string, StringValues>()
+                {
+                    { "parameter", string.Empty },
+                });
+            });
+
+            var modelState = new ModelStateDictionary();
+
+            // Act
+            var modelBindingResult = await argumentBinder.BindModelAsync(parameter, modelState, operationContext);
+
+            // Assert
+            Assert.NotNull(modelBindingResult);
+            Assert.True(modelBindingResult.IsModelSet);
+            Assert.Equal(string.Empty, modelBindingResult.Key);
+
+            var model = Assert.IsType<Person5>(modelBindingResult.Model);
+            Assert.Null(model.Name);
+
+            Assert.Equal(0, modelState.Count);
+            Assert.Equal(0, modelState.ErrorCount);
+            Assert.True(modelState.IsValid);
+        }
+    }
+}