How to share an interface between multiple managed C++ assemblies.

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C++

managed

I've got a managed C++ base assembly (BaseAssembly) that defines an interface:

// In MyInterface.h
namespace BaseAssembly
{
    public interface class IMyInterface
    {
        //...
    };
}

Two other managed C++ assemblies each contain a class that implements this interface:

// MyDerived1.h in managed C++ project MyDerived1Lib

#include "MyInterface.h";
namespace DerivedAssembly1
{
    public ref class MyDerived1 : public BaseAssembly::IMyInterface
    {
        //...
    };
}

// MyDerived2.h in managed C++ project MyDerived2Lib

#include "MyInterface.h";
namespace DerivedAssembly2
{
    public ref class MyDerived2 : public BaseAssembly::IMyInterface
    {
        //...
    };
}

When I try to create an instance of IMyInterfaces in a separate C# app, I get error CS0433: The type BaseAssembly.IMyInterface exists in both MyDerived1.dll and MyDerived2.dll. Im not sure how to proceed.
Heres a snippet of the offending C# code:

// In MyClientApp.cs
namespace MyApp
{
    public class MyClientApp
    {
        public void Initialize()
        {
            IMyInterface impl = new MyDerived1();
        }
    }
}

The C# project references both derived assemblies. I've tried leaving the base assembly as a bare header file (i.e. not compiling it to a DLL), and compiling it to a DLL and referencing it from the C# project. Nothing seems to work. Thanks in advance for your help.