Update `FormFileModelBinder` to re-add prefix `ParameterBinder` removed incorrectly

- #7562 part 2
- add `OriginalModelName` to `ModelBindingContext`

nit: take VS suggestions, mostly to inline collection initialization in `FormFileModelBinderTest`

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

@@ -67,6 +67,12 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding
         /// </summary>
         public abstract string ModelName { get; set; }
 
+        /// <summary>
+        /// Gets or sets the name of the top-level model. This is not reset to <see cref="string.Empty"/> when value
+        /// providers have no match for that model.
+        /// </summary>
+        public string OriginalModelName { get; protected set; }
+
         /// <summary>
         /// Gets or sets the <see cref="ModelStateDictionary"/> used to capture <see cref="ModelStateEntry"/> values
         /// for properties in the object graph of the model when binding.

src/Microsoft.AspNetCore.Mvc.Core/Internal/DefaultModelBindingContext.cs

@@ -237,6 +237,7 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding
                 // Because this is the top-level context, FieldName and ModelName should be the same.
                 FieldName = binderModelName ?? modelName,
                 ModelName = binderModelName ?? modelName,
+                OriginalModelName = binderModelName ?? modelName,
 
                 IsTopLevelObject = true,
                 ModelMetadata = metadata,

src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/FormFileModelBinder.cs

@@ -48,7 +48,7 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
 
             _logger = loggerFactory.CreateLogger<FormFileModelBinder>();
         }
-        
+
         /// <inheritdoc />
         public async Task BindModelAsync(ModelBindingContext bindingContext)
         {
@@ -85,6 +85,19 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
 
             await GetFormFilesAsync(modelName, bindingContext, postedFiles);
 
+            // If ParameterBinder incorrectly overrode ModelName, fall back to OriginalModelName prefix. Comparisons
+            // are tedious because e.g. top-level parameter or property is named Blah and it contains a BlahBlah
+            // property. OriginalModelName may be null in tests.
+            if (postedFiles.Count == 0 &&
+                bindingContext.OriginalModelName != null &&
+                !string.Equals(modelName, bindingContext.OriginalModelName, StringComparison.Ordinal) &&
+                !modelName.StartsWith(bindingContext.OriginalModelName + "[", StringComparison.Ordinal) &&
+                !modelName.StartsWith(bindingContext.OriginalModelName + ".", StringComparison.Ordinal))
+            {
+                modelName = ModelNames.CreatePropertyModelName(bindingContext.OriginalModelName, modelName);
+                await GetFormFilesAsync(modelName, bindingContext, postedFiles);
+            }
+
             object value;
             if (bindingContext.ModelType == typeof(IFormFile))
             {

test/Microsoft.AspNetCore.Mvc.Core.Test/ModelBinding/Binders/FormFileModelBinderTest.cs

@@ -20,8 +20,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
         public async Task FormFileModelBinder_SingleFile_BindSuccessful()
         {
             // Arrange
-            var formFiles = new FormFileCollection();
+            var formFiles = new FormFileCollection
-            formFiles.Add(GetMockFormFile("file", "file1.txt"));
+            {
+                GetMockFormFile("file", "file1.txt")
+            };
             var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
             var bindingContext = GetBindingContext(typeof(IEnumerable<IFormFile>), httpContext);
             var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
@@ -38,6 +40,192 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
             Assert.Null(entry.Metadata);
         }
 
+        [Fact]
+        public async Task FormFileModelBinder_SingleFileAtTopLevel_BindSuccessfully_WithEmptyModelName()
+        {
+            // Arrange
+            var formFiles = new FormFileCollection
+            {
+                GetMockFormFile("file", "file1.txt")
+            };
+
+            var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
+            var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
+
+            // Mimic ParameterBinder overwriting ModelName on top level model. In this top-level binding case,
+            // FormFileModelBinder uses FieldName from the get-go. (OriginalModelName will be checked but ignored.)
+            var bindingContext = DefaultModelBindingContext.CreateBindingContext(
+                new ActionContext { HttpContext = httpContext },
+                Mock.Of<IValueProvider>(),
+                new EmptyModelMetadataProvider().GetMetadataForType(typeof(IFormFile)),
+                bindingInfo: null,
+                modelName: "file");
+            bindingContext.ModelName = string.Empty;
+
+            // Act
+            await binder.BindModelAsync(bindingContext);
+
+            // Assert
+            Assert.True(bindingContext.Result.IsModelSet);
+
+            var entry = bindingContext.ValidationState[bindingContext.Result.Model];
+            Assert.False(entry.SuppressValidation);
+            Assert.Equal("file", entry.Key);
+            Assert.Null(entry.Metadata);
+        }
+
+        [Fact]
+        public async Task FormFileModelBinder_SingleFileWithinTopLevelPoco_BindSuccessfully()
+        {
+            // Arrange
+            const string propertyName = nameof(NestedFormFiles.Files);
+            var formFiles = new FormFileCollection
+            {
+                GetMockFormFile($"{propertyName}", "file1.txt")
+            };
+
+            var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
+            var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
+
+            // In this non-top-level binding case, FormFileModelBinder tries ModelName and succeeds.
+            var propertyInfo = typeof(NestedFormFiles).GetProperty(propertyName);
+            var metadata = new EmptyModelMetadataProvider().GetMetadataForProperty(
+                propertyInfo,
+                propertyInfo.PropertyType);
+            var bindingContext = DefaultModelBindingContext.CreateBindingContext(
+                new ActionContext { HttpContext = httpContext },
+                Mock.Of<IValueProvider>(),
+                metadata,
+                bindingInfo: null,
+                modelName: "FileList");
+            bindingContext.IsTopLevelObject = false;
+            bindingContext.Model = new FileList();
+            bindingContext.ModelName = propertyName;
+
+            // Act
+            await binder.BindModelAsync(bindingContext);
+
+            // Assert
+            Assert.True(bindingContext.Result.IsModelSet);
+
+            var entry = bindingContext.ValidationState[bindingContext.Result.Model];
+            Assert.False(entry.SuppressValidation);
+            Assert.Equal($"{propertyName}", entry.Key);
+            Assert.Null(entry.Metadata);
+        }
+
+        [Fact]
+        public async Task FormFileModelBinder_SingleFileWithinTopLevelPoco_BindSuccessfully_WithShortenedModelName()
+        {
+            // Arrange
+            const string propertyName = nameof(NestedFormFiles.Files);
+            var formFiles = new FormFileCollection
+            {
+                GetMockFormFile($"FileList.{propertyName}", "file1.txt")
+            };
+
+            var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
+            var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
+
+            // Mimic ParameterBinder overwriting ModelName on top level model then ComplexTypeModelBinder entering a
+            // nested context for the NestedFormFiles property. In this non-top-level binding case, FormFileModelBinder
+            // tries ModelName then falls back to add an (OriginalModelName + ".") prefix.
+            var propertyInfo = typeof(NestedFormFiles).GetProperty(propertyName);
+            var metadata = new EmptyModelMetadataProvider().GetMetadataForProperty(
+                propertyInfo,
+                propertyInfo.PropertyType);
+            var bindingContext = DefaultModelBindingContext.CreateBindingContext(
+                new ActionContext { HttpContext = httpContext },
+                Mock.Of<IValueProvider>(),
+                metadata,
+                bindingInfo: null,
+                modelName: "FileList");
+            bindingContext.IsTopLevelObject = false;
+            bindingContext.Model = new FileList();
+            bindingContext.ModelName = propertyName;
+
+            // Act
+            await binder.BindModelAsync(bindingContext);
+
+            // Assert
+            Assert.True(bindingContext.Result.IsModelSet);
+
+            var entry = bindingContext.ValidationState[bindingContext.Result.Model];
+            Assert.False(entry.SuppressValidation);
+            Assert.Equal($"FileList.{propertyName}", entry.Key);
+            Assert.Null(entry.Metadata);
+        }
+
+        [Fact]
+        public async Task FormFileModelBinder_SingleFileWithinTopLevelDictionary_BindSuccessfully()
+        {
+            // Arrange
+            var formFiles = new FormFileCollection
+            {
+                GetMockFormFile("[myFile]", "file1.txt")
+            };
+
+            var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
+            var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
+
+            // In this non-top-level binding case, FormFileModelBinder tries ModelName and succeeds.
+            var bindingContext = DefaultModelBindingContext.CreateBindingContext(
+                new ActionContext { HttpContext = httpContext },
+                Mock.Of<IValueProvider>(),
+                new EmptyModelMetadataProvider().GetMetadataForType(typeof(IFormFile)),
+                bindingInfo: null,
+                modelName: "FileDictionary");
+            bindingContext.IsTopLevelObject = false;
+            bindingContext.ModelName = "[myFile]";
+
+            // Act
+            await binder.BindModelAsync(bindingContext);
+
+            // Assert
+            Assert.True(bindingContext.Result.IsModelSet);
+
+            var entry = bindingContext.ValidationState[bindingContext.Result.Model];
+            Assert.False(entry.SuppressValidation);
+            Assert.Equal("[myFile]", entry.Key);
+            Assert.Null(entry.Metadata);
+        }
+
+        [Fact]
+        public async Task FormFileModelBinder_SingleFileWithinTopLevelDictionary_BindSuccessfully_WithShortenedModelName()
+        {
+            // Arrange
+            var formFiles = new FormFileCollection
+            {
+                GetMockFormFile("FileDictionary[myFile]", "file1.txt")
+            };
+
+            var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
+            var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
+
+            // Mimic ParameterBinder overwriting ModelName on top level model then DictionaryModelBinder entering a
+            // nested context for the KeyValuePair.Value property. In this non-top-level binding case,
+            // FormFileModelBinder tries ModelName then falls back to add an OriginalModelName prefix.
+            var bindingContext = DefaultModelBindingContext.CreateBindingContext(
+                new ActionContext { HttpContext = httpContext },
+                Mock.Of<IValueProvider>(),
+                new EmptyModelMetadataProvider().GetMetadataForType(typeof(IFormFile)),
+                bindingInfo: null,
+                modelName: "FileDictionary");
+            bindingContext.IsTopLevelObject = false;
+            bindingContext.ModelName = "[myFile]";
+
+            // Act
+            await binder.BindModelAsync(bindingContext);
+
+            // Assert
+            Assert.True(bindingContext.Result.IsModelSet);
+
+            var entry = bindingContext.ValidationState[bindingContext.Result.Model];
+            Assert.False(entry.SuppressValidation);
+            Assert.Equal("FileDictionary[myFile]", entry.Key);
+            Assert.Null(entry.Metadata);
+        }
+
         [Fact]
         public async Task FormFileModelBinder_ExpectMultipleFiles_BindSuccessful()
         {
@@ -127,8 +315,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
         public async Task FormFileModelBinder_ReturnsFailedResult_WhenNamesDoNotMatch()
         {
             // Arrange
-            var formFiles = new FormFileCollection();
+            var formFiles = new FormFileCollection
-            formFiles.Add(GetMockFormFile("different name", "file1.txt"));
+            {
+                GetMockFormFile("different name", "file1.txt")
+            };
             var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
             var bindingContext = GetBindingContext(typeof(IFormFile), httpContext);
             var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
@@ -147,9 +337,11 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
         public async Task FormFileModelBinder_UsesFieldNameForTopLevelObject(bool isTopLevel, string expected)
         {
             // Arrange
-            var formFiles = new FormFileCollection();
+            var formFiles = new FormFileCollection
-            formFiles.Add(GetMockFormFile("FieldName", "file1.txt"));
+            {
-            formFiles.Add(GetMockFormFile("ModelName", "file1.txt"));
+                GetMockFormFile("FieldName", "file1.txt"),
+                GetMockFormFile("ModelName", "file1.txt")
+            };
             var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
 
             var bindingContext = GetBindingContext(typeof(IFormFile), httpContext);
@@ -173,8 +365,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
         public async Task FormFileModelBinder_ReturnsFailedResult_WithEmptyContentDisposition()
         {
             // Arrange
-            var formFiles = new FormFileCollection();
+            var formFiles = new FormFileCollection
-            formFiles.Add(new Mock<IFormFile>().Object);
+            {
+                new Mock<IFormFile>().Object
+            };
             var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
             var bindingContext = GetBindingContext(typeof(IFormFile), httpContext);
             var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
@@ -191,8 +385,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
         public async Task FormFileModelBinder_ReturnsFailedResult_WithNoFileNameAndZeroLength()
         {
             // Arrange
-            var formFiles = new FormFileCollection();
+            var formFiles = new FormFileCollection
-            formFiles.Add(GetMockFormFile("file", ""));
+            {
+                GetMockFormFile("file", "")
+            };
             var httpContext = GetMockHttpContext(GetMockFormCollection(formFiles));
             var bindingContext = GetBindingContext(typeof(IFormFile), httpContext);
             var binder = new FormFileModelBinder(NullLoggerFactory.Instance);
@@ -323,5 +519,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
         private class FileList : List<IFormFile>
         {
         }
+
+        private class NestedFormFiles
+        {
+            public FileList Files { get; }
+        }
     }
 }