C# exposing to COM - interface inheritance

This is not a .NET problem, it is a consequence of the way COM works. It doesn't support inheritance. You would fix this at the client side, it needs to call QueryInterface() with the IID for IBaseClass to get an interface pointer to the IBaseClass interface so it can call GetA(). .NET interop automatically provides a QI implementation that makes this work. However, it is not very user-friendly in this case, design your C# side code to make it easy for the client to use your class instead of the other way around. You'd typically need a one-liner override method that delegates to the base C# class implementation.

Note that there's a problem with your method signatures and the use of the [PreserveSig] attribute. They are not callable through IDispatch nor can they be auto-marshaled. That requires a method with a HRESULT as the return value type. It is automatic when you remove the attribute.

This appears to be very similar to this question: Interface inheritance in ComVisible classes in C#

As you noted in your edit, this is a limitation on the .net to COM hop, that the COM interface does not include any inherited interfaces.

In some languages you could have solved this by inculding a block of code in all relevant interfaces, but as far as I am able to see this is not possible in C#.