Proposal for System.Runtime.CompilerServices.ICastable .NET Core public API

[shrah]

We need the System.Runtime.CompilerServices.ICastable for building the CoreCLR version of System.Private.Interop.

Current Workarounds:

Today we use a hack by publishing the System.Private.ICastable.dll though myget and consume it from CoreRT.

Discussion:

The ideal solution would be to expose it as a public .NET Core API possibly in System.Runtime.InteropServices.dll

[karelz]

@shrah can you provide more info what is the interface for, how is it used, etc.? See example in API review process

[morganbr]

What's the value of ICastable to developers outside of interop? As I understand it right now, it's very dangerous and fragile and only fulfills a very specific purpose. If you just have layering problems for System.Private.Interop, we can probably work out something more scoped than making ICastable public.

[shrah]

ICastable is an interface used by the runtime to support dynamic interface casting. Implementing this interface on a type will allow the type to support interfaces (for the purposes of casting and interface dispatch) that do not appear in its interface map.

An example of ICastable is here __ComObject .

@morganbr that is a good question. I don't have any other use cases for ICastable outside of interop right now.

@jkotas , can you think of any other use cases of ICastable outside of interop?

[jkotas]

we can probably work out something more scoped than making ICastable public.

• Compatibility: We want to be able to maintain same level of compatibility for this API as for any other public API.
• Documentation: It is ok to document this API for use by tools.

can you think of any other use cases of ICastable outside of interop?

Yes, this is primarily to support interop.

[jkotas]

For ProjectN, we have invented CastableObject that fixed some of the issues with ICastable. Would we want to expose the CastableObject instead?

https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/CastableObject.cs

[AaronRobinsonMSFT]

Closing this issue in lieu of #36654