[BindProperty(BinderType = typeof(SimpleTypeModelBinder)] no longer works

[dougbu]

Though we've added new constructors to most model binders in a backward-compatible fashion, the change actually breaks a scenario. For example, [BindProperty(BinderType = typeof(SimpleTypeModelBinder)] and [ModelBinder(BinderType = typeof(SimpleTypeModelBinder)] now result in

An unhandled exception occurred while processing the request.
InvalidOperationException: Multiple constructors accepting all given argument types have been found in type 'Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder'. There should only be one applicable constructor.
Microsoft.Extensions.DependencyInjection.ActivatorUtilities.FindApplicableConstructor(Type instanceType, Type[] argumentTypes, out ConstructorInfo matchingConstructor, out Nullable<int>[] parameterMap)

Stack Query Cookies Headers
InvalidOperationException: Multiple constructors accepting all given argument types have been found in type 'Microsoft.AspNetCore.Mvc.ModelBinding.Binders.SimpleTypeModelBinder'. There should only be one applicable constructor.
Microsoft.Extensions.DependencyInjection.ActivatorUtilities.FindApplicableConstructor(Type instanceType, Type[] argumentTypes, out ConstructorInfo matchingConstructor, out Nullable<int>[] parameterMap)
Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateFactory(Type instanceType, Type[] argumentTypes)
Microsoft.AspNetCore.Mvc.ModelBinding.Binders.BinderTypeModelBinder..ctor(Type binderType)
Microsoft.AspNetCore.Mvc.ModelBinding.Binders.BinderTypeModelBinderProvider.GetBinder(ModelBinderProviderContext context)
...

[mkArtakMSFT]

This is an effect of ActivatorUtilities/multiple constructors issue. This specific use-case is not common, so this is a low priority issue.

[mkArtakMSFT]

The real fix will be handled by a fix we consider for #7330

[pranavkm]

Going back all the way to 1.0 - SimpleTypeModelBinder expects a type parameter to be passed in that BinderTypeModelBinder - https://github.com/aspnet/Mvc/blob/release/1.0/src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/BinderTypeModelBinder.cs#L40 never passed in.

The reason you're seeing the error at all is because the two constructors are treated as equivalent since neither can be resolved. Should this be closed as invalid?

[dougbu]

@pranavkm I believe I was updating a project I'd used in previous validation rounds when I hit this. But, I don't think I have that test system anymore.

In any case, I'll retry the scenario with typeof(BodyTypeModelBinder) and see if I hid an ActivatorUtilities issue with that type too.

[pranavkm]

A bit of a tangent, but the current solution for disambiguating ctors in ActivatorUtilities is to annotate the preferred one with an attribute. If we run in to this sort of scenario frequently enough, we could consider adding a negative preference to ctors with Obsolete attribute. That said, so far most of the ctors that we've added generally add additional parameters that come from DI which lends itself to the disambiguation algorithm.

[mkArtakMSFT]

Closing this as it seems the underlying issue has been fixed already: dotnet/aspnetcore#2871