Re: DLL2 should implement interface defined in DLL1 - how?
"Axel Bock" <axel.bock.news@googlemail.com> wrote in message
news:1184067568.329089.135600@d55g2000hsg.googlegroups.com
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.
See #import
// ADDED BY ME
#include "IMyInterfaceProject.idl"
// THE REST IS WIZARD CODE
You are trying to get C++ compiler to compile IDL. Of course IDL syntax
is not valid C++, I expect you get tons of errors.
Either use #import, or include the .h file that MIDL compiler generated
when compiling the IDL file.
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 *'
The error message suggests that both interfaces you are trying to
implement are dual - that is, both derive from IDispatch. So now you
have two distinct implementations of IDispatch in your coclass. Think
about it - when a client queries for IDispatch, which one should it
get? Which methods should it be able to access via late binding?
Moreover, which IDispatch implementation the client gets depends on the
exact sequence of QueryInterface calls. This is illegal in COM.
You should avoid implementing two dual interfaces on the same object. If
you have to, it's a sign of a design, shall we say, not very well
thought through. If, for some reason, you really really need to
implement two dual interfaces, you would have to provide a custom
IDispatch implementation that enables late binding access to both sets
of methods.
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925