Preserve `ViewDataDictionary.ModelType` for `Nullable<T>` properties when `Model` is non-`null`

- #2539
- reuse `ModelMetadata` and occasionally `ModelExplorer` when `ModelType` is `Nullable<T>`

src/Microsoft.AspNet.Mvc.ViewFeatures/ViewDataDictionary.cs

@@ -186,7 +186,9 @@ namespace Microsoft.AspNet.Mvc
         {
             // This is the core constructor called when Model is known.
             var modelType = GetModelType(model);
-            if (modelType == source.ModelMetadata.ModelType && model == source.ModelExplorer.Model)
+            var metadataModelType =
+                Nullable.GetUnderlyingType(source.ModelMetadata.ModelType) ?? source.ModelMetadata.ModelType;
+            if (modelType == metadataModelType && model == source.ModelExplorer.Model)
             {
                 // Preserve any customizations made to source.ModelExplorer.ModelMetadata if the Type
                 // that will be calculated in SetModel() and source.Model match new instance's values.
@@ -384,7 +386,13 @@ namespace Microsoft.AspNet.Mvc
             // null. When called from the Model setter, ModelMetadata will (temporarily) be null. When called from
             // a constructor, current ModelMetadata may already be set to preserve customizations made in parent scope.
             var modelType = GetModelType(value);
-            if (ModelExplorer?.Metadata.ModelType != modelType)
+            Type metadataModelType = null;
+            if (ModelExplorer != null)
+            {
+                metadataModelType = Nullable.GetUnderlyingType(ModelMetadata.ModelType) ?? ModelMetadata.ModelType;
+            }
+
+            if (metadataModelType != modelType)
             {
                 ModelExplorer = _metadataProvider.GetModelExplorerForType(modelType, value);
             }

test/Microsoft.AspNet.Mvc.ViewFeatures.Test/ViewDataDictionaryTest.cs

@@ -133,7 +133,7 @@ namespace Microsoft.AspNet.Mvc.Core
         {
             // Arrange
             var vdd = new ViewDataDictionary(new EmptyModelMetadataProvider());
-            
+
             // Act
             vdd.Model = model;
 
@@ -257,12 +257,15 @@ namespace Microsoft.AspNet.Mvc.Core
         {
             get
             {
-                // Instances in this data set must have exactly the same type as the corresponding Type. Otherwise
-                // the copy constructor ignores the source ModelMetadata.
+                // Instances in this data set must have exactly the same type as the corresponding Type or be null.
+                // Otherwise the copy constructor ignores the source ModelMetadata.
                 return new TheoryData<Type, object>
                 {
                     { typeof(int), 23 },
+                    { typeof(ulong?), 24ul },
+                    { typeof(ushort?), null },
                     { typeof(string), "hello" },
+                    { typeof(string), null },
                     { typeof(List<string>), new List<string>() },
                     { typeof(string[]), new string[0] },
                     { typeof(Dictionary<string, object>), new Dictionary<string, object>() },
@@ -359,6 +362,33 @@ namespace Microsoft.AspNet.Mvc.Core
             Assert.NotSame(originalExplorer, viewData.ModelExplorer);
         }
 
+        [Fact]
+        public void ModelSetter_SetNullableNonNull_UpdatesModelExplorer()
+        {
+            // Arrange
+            var metadataProvider = new EmptyModelMetadataProvider();
+            var metadata = metadataProvider.GetMetadataForType(typeof(bool?));
+            var explorer = new ModelExplorer(metadataProvider, metadata, model: null);
+            var viewData = new ViewDataDictionary(metadataProvider)
+            {
+                ModelExplorer = explorer,
+            };
+
+            // Act
+            viewData.Model = true;
+
+            // Assert
+            Assert.NotNull(viewData.ModelMetadata);
+            Assert.NotNull(viewData.ModelExplorer);
+            Assert.Same(metadata, viewData.ModelMetadata);
+            Assert.Same(metadata.ModelType, explorer.ModelType);
+            Assert.NotSame(explorer, viewData.ModelExplorer);
+            Assert.Equal(viewData.Model, viewData.ModelExplorer.Model);
+
+            var model = Assert.IsType<bool>(viewData.Model);
+            Assert.True(model);
+        }
+
         [Fact]
         public void ModelSetter_SameType_BoxedValueTypeUpdatesModelExplorer()
         {