Relax CS0030 to not error on conversion of IDynamicInterfaceCastable

[elinor-fung]

Version Used:
.NET Core 5.0.100-preview.7.20316.2

Steps to Reproduce:

For .NET 5, the IDynamicInterfaceCastable (dotnet/runtime#36654) was added (dotnet/runtime#37042) to allow participation in a cast at run time where the implementing class is cast to an interface that is not in its class metadata.

As a result, an explicit cast the compiler recognizes as impossible could actually be possible if the object implements IDynamicInterfaceCastable.

interface ITest { void Method(); }

[System.Runtime.InteropServices.DynamicInterfaceCastableImplementationAttribute]
interface ITestImpl : ITest
{
    void ITest.Method() { }
}

sealed class Castable : System.Runtime.InteropServices.IDynamicInterfaceCastable
{
    public RuntimeTypeHandle GetInterfaceImplementation(RuntimeTypeHandle interfaceType)
    {
        return Type.GetTypeFromHandle(interfaceType) == typeof(ITest)
            ? typeof(ITestImpl).TypeHandle
            : default(RuntimeTypeHandle);
    }

    public bool IsInterfaceImplemented(RuntimeTypeHandle interfaceType, bool throwIfNotImplemented)
    {
        return Type.GetTypeFromHandle(interfaceType) == typeof(ITest);
    }
}

class Program
{
    static void Main(string[] args)
    {
        Castable obj = new Castable();
        ITest t = (ITest)obj; // error CS0030: Cannot convert type 'Castable' to 'ITest'
        t.Method();
    }
}

Expected Behavior:
No error if the type implements IDynamicInterfaceCastable.

Actual Behavior:
error CS0030: Cannot convert type 'testApp.Castable' to 'testApp.ITest'

The user needs to first cast to object, then the desired interface:

ITest t = (ITest)(object)obj;

cc @AaronRobinsonMSFT @jkoritzinsky

[CyrusNajmabadi]

This sounds like a language change request afaict. Would you like me to move this to dotnet/csharplang? I think we'd have to spec this as how the language works in order for the compielr to know that it shoudl allow this sort of thing.

[elinor-fung]

Ah, okay - yes, please do move this where it belongs.

[CyrusNajmabadi]

Done :) Are you the write dotnet/runtime representative to communicate on this with for the LDM?

[YairHalberstadt]

No error if the type implements IDynamicInterfaceCastable

Given that the compiler cannot guarantee this cast is valid, should there be a warning?

[AaronRobinsonMSFT]

Given that the compiler cannot guarantee this cast is valid, should there be a warning?

I don't think so. The compiler check is merely convenience because it can detect the case with the type data. In this case it is no different than if the object was typed as object. I would instead like to see the compiler treat casts from objects typed as object or implementing IDynamicInterfaceCastable the same - the compiler just can't determine success at compile time.