in Program try Test.IServer server = new Test.Server(); perhaps then you can call your MethodImpl like server.MethodImpl(params) because Interface(!) is providing your method in COM. Like calling COM from VBA - you don't care about CoClass, because you reference both COM and Class from typelib, last provides just interface for connection from which you call Method. I don't know COM providing classes. CoClass is being compiled with server, not client - you should not care about it in client.

p.s. but, frankly speaking, I have the same problem with NetSxS example with NET.Core 5.0 Reg-free COM