With C#, you can implement an IUnknown-derived interface completely in C++ (with `class` etc), and then define it in C#, and it transparently does all the mapping of members for you.

Yes, it's COM. People assume that makes it non-cross-platform, but there's nothing Windows-specific about COM, it's just a slightly higher-level ABI than C. So Mono supports it on Linux in the same manner. Not sure what the story is in .NET Core.