DLL2 should implement interface defined in DLL1 - how?

Hi list,

I have this VERY annoying thing at my hands. I don't know COM too
much, so please forgive me if that's something every noob should
know.

So.

I have two DLL's with COM components. DLL one defines an interface
IMyInterface. I know want to have a 2nd DLL to have a class which
implements that interface, too.

The IDL of the second DLL looks like this:

        [
            object,
            uuid(982F014C-579C-464E-BA9E-584419041811),
            dual,
            nonextensible,
            helpstring("IMyOtherInterface-Schnittstelle"),
            pointer_default(unique)
        ]
        interface IMyOtherInterface : IDispatch{
        };

        [
            uuid(A10B957D-305F-401A-B12F-58EE7D192F66),
            version(1.0),
        ]
        library MySecondDLLLibrary
        {
            importlib("stdole2.tlb");
            importlib("../myfirstproject/Debug/firstDLL.tlb");

            [
                uuid(0AC402BF-7310-4C56-8E29-48EAF9D5BA76),
                helpstring("MyOtherInterface Class")
            ]
            coclass MyOtherInterface
            {
                [default] interface IMyOtherInterface;
                interface IMyInterface;
            };
        };

Now that compiles just fine. I also checked with some MS DLLs in the
Windows directory, they look just the same.

The problem is now that I don't know how to do this on C++ level. What
I did until now was:

  - I added the IDL files from the IMyInterface to the 2nd project
  - I compiled the 2nd project without any modifications - works.
  - I checked the code, the code looks like this:

        // ADDED BY ME
        #include "IMyInterfaceProject.idl"
        // THE REST IS WIZARD CODE

        // CMyOtherInterface

        class ATL_NO_VTABLE CMyOtherInterface :
            public CComObjectRootEx<CComSingleThreadModel>,
            // TRIED TO ADD THE NEXT LINE
            // public IMyInterface,
            public CComCoClass<CMyOtherInterface,
&CLSID_MyOtherInterface>,
            public IDispatchImpl<IMyOtherInterface,
&IID_IMyOtherInterface, &LIBID_Icw_Sdk_Module_Configuration_C, /
*wMajor =*/ 1, /*wMinor =*/ 0>
        {
        public:
            CMyOtherInterface()
            {
            }

        DECLARE_REGISTRY_RESOURCEID(IDR_MyOtherInterface)

        DECLARE_NOT_AGGREGATABLE(CMyOtherInterface)

        BEGIN_COM_MAP(CMyOtherInterface)
            COM_INTERFACE_ENTRY(IMyOtherInterface)
            // TRIED TO ADD THE NEXT LINE
            // COM_INTERFACE_ENTRY(IMyInterface)
            COM_INTERFACE_ENTRY(IDispatch)
        END_COM_MAP()

            DECLARE_PROTECT_FINAL_CONSTRUCT()

            HRESULT FinalConstruct()
            {
                return S_OK;
            }

            void FinalRelease()
            {
            }

        public:

        };

        OBJECT_ENTRY_AUTO(__uuidof(MyOtherInterface),
CMyOtherInterface)

Well. This is all fine, VS-wizard-generated code. But I missed the
references to IMyInterface, so I tried including them into the header
(the commented out lines). I also found an article in the internet
saying exactly that: http://vcfaq.mvps.org/com/7.htm .

Alas as soon as I include references to IMyInterface in the
inheritance chain, I get the following compiler error:
c:\.....\myotherinterface.h (36) : error C2594: 'static_cast' :
ambiguous conversions from 'CMyOtherInterface::_ComMapClass *' to
'IDispatch *'

I CAN import the IDL files into the 2nd project, but if I do that I
cannot use both at the same time from a third one - both will export
the IMyInterface definition, and the stub compiler - rightfully -
complains about redefinition, and I guess this is not the right way to
go. A look with OLEView at for example SRCHADM.DLL and MSSITLB.IDL
shows that this HAS to be possible :-) . But I only don't know
how ...

Any ideas? Anyone? :-) ? I hope this was enough information to get the
idea across.

Greetings & thanks,
Axel.