When declaring inherited COM interfaces in .NET you must declare all inherited interfaces members, recursively. So for example, if you have this as the base interface:

[Guid("2cd90691-12e2-11dc-9fed-001143a055f9"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IBase
{
    int Blabla();
}

Then an IDerived definition would be something like this:

[Guid("65019f75-8da2-497c-b32c-dfa34e48ede6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IDerived
{
    // IBase methods
    int Blabla();

    // IDerived methods
    ...
}

Or better, like this, if you want to keep inheritance hierarchy in .NET:

[Guid("65019f75-8da2-497c-b32c-dfa34e48ede6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IDerived : IBase
{
    // IBase methods
    new int Blabla();

    // IDerived methods
    ...
}