Avoid logging unsupported alternative for complex type binding (dotnet#39353)

Author: captainsafia

src/Mvc/Mvc.Core/src/ModelBinding/Binders/ComplexTypeModelBinder.cs

@@ -484,6 +484,14 @@ protected virtual object CreateModel(ModelBindingContext bindingContext)
             var modelType = bindingContext.ModelType;
             if (modelType.IsAbstract || modelType.GetConstructor(Type.EmptyTypes) == null)
             {
+                // If the model is not a top-level object, we can't examine the defined constructor
+                // to evaluate if the non-null property has been set so we do not provide this as a valid
+                // alternative.
+                if (!bindingContext.IsTopLevelObject)
+                {
+                    throw new InvalidOperationException(Resources.FormatComplexTypeModelBinder_NoParameterlessConstructor_ForType(modelType.FullName));
+                }
+
                 var metadata = bindingContext.ModelMetadata;
                 switch (metadata.MetadataKind)
                 {

src/Mvc/Mvc.Core/test/ModelBinding/Binders/ComplexTypeModelBinderTest.cs

@@ -583,11 +583,8 @@ public void CreateModel_ForStructModelType_AsProperty_ThrowsException()
             string.Format(
                 CultureInfo.CurrentCulture,
                 "Could not create an instance of type '{0}'. Model bound complex types must not be abstract or " +
-                "value types and must have a parameterless constructor. Alternatively, set the '{1}' property to" +
-                " a non-null value in the '{2}' constructor.",
-                typeof(PointStruct).FullName,
-                nameof(Location.Point),
-                typeof(Location).FullName),
+                "value types and must have a parameterless constructor.",
+                typeof(PointStruct).FullName),
             exception.Message);
     }
 
@@ -997,6 +994,32 @@ public async Task BindModelAsync_Success()
         Assert.True(bindingContext.ModelState.IsValid);
     }
 
+    // Validates fix for https://github.com/dotnet/aspnetcore/issues/21916
+    [Fact]
+    public async Task BindModelAsync_PropertyInitializedInNonParameterlessConstructorConstructor()
+    {
+        // Arrange
+        var model = new ModelWithPropertyInitializedInConstructor("TestName");
+        var property = GetMetadataForProperty(model.GetType(), nameof(ModelWithPropertyInitializedInConstructor.NameContainer));
+        var nestedProperty = GetMetadataForProperty(typeof(ClassWithNoParameterlessConstructor), nameof(ClassWithNoParameterlessConstructor.Name));
+        var bindingContext = CreateContext(property);
+        bindingContext.IsTopLevelObject = false;
+        var valueProvider = new Mock<IValueProvider>(MockBehavior.Strict);
+        valueProvider
+            .Setup(provider => provider.ContainsPrefix("theModel.Name"))
+            .Returns(true);
+        bindingContext.ValueProvider = valueProvider.Object;
+        var binder = CreateBinder(bindingContext.ModelMetadata);
+
+        binder.Results[nestedProperty] = ModelBindingResult.Success(null);
+
+        // Act
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(async () => await binder.BindModelAsync(bindingContext));
+        // Assert
+        var unexpectedMessage = "Alternatively, set the 'NameContainer' property to a non-null value in the 'Microsoft.AspNetCore.Mvc.ModelBinding.Binders.ComplexTypeModelBinderTest+ModelWithPropertyInitializedInConstructor' constructor.";
+        Assert.DoesNotContain(exception.Message, unexpectedMessage);
+    }
+
     [Fact]
     public void SetProperty_PropertyHasDefaultValue_DefaultValueAttributeDoesNothing()
     {
@@ -1297,6 +1320,17 @@ public ClassWithNoParameterlessConstructor(string name)
         public string Name { get; set; }
     }
 
+    private class ModelWithPropertyInitializedInConstructor
+    {
+        public ModelWithPropertyInitializedInConstructor(string name)
+        {
+            NameContainer = new ClassWithNoParameterlessConstructor(name);
+        }
+
+        [ValueBinderMetadataAttribute]
+        public ClassWithNoParameterlessConstructor NameContainer { get; set; }
+    }
+
     private class BindingOptionalProperty
     {
         [BindingBehavior(BindingBehavior.Optional)]

src/Mvc/test/Mvc.IntegrationTests/ActionParametersIntegrationTest.cs

@@ -473,11 +473,8 @@ public async Task ActionParameter_UsingComplexTypeModelBinder_ModelPropertyTypeW
             string.Format(
                 CultureInfo.CurrentCulture,
                 "Could not create an instance of type '{0}'. Model bound complex types must not be abstract or " +
-                "value types and must have a parameterless constructor. Alternatively, set the '{1}' property to" +
-                " a non-null value in the '{2}' constructor.",
-                typeof(ClassWithNoDefaultConstructor).FullName,
-                nameof(Class1.Property1),
-                typeof(Class1).FullName),
+                "value types and must have a parameterless constructor.",
+                typeof(ClassWithNoDefaultConstructor).FullName),
             exception.Message);
     }