COM Question - QueryInterface?

Chris Tavares tavares at
Fri Jun 11 03:43:59 CEST 1999

Toby Dickenson wrote:

> (posted and cced to Chris)
> Chris Tavares <tavares at> wrote:
> >Thanks. The reason that the interface in question isn't a dual is because my
> >object design depends quite heavily on having multiple interfaces. If you have
> >multiple duals, you end up with lots of problems with IDispatch, since you can't
> >safely implement more than one version of the same interface on a single object
> >(and lots of people have tried, REALLY HARD).
> This is a fallacy about IDispatch that I've heard from many sources, but it
> certainly is not true.
> One true statement is that you can only have one interface known as
> IID_IDispatch. This causes a problem for scripting languages that can not
> QueryInterface (or, worse, which QueryInterface behind your back).[

I would disagree that this is a fallacy. It DOES cause problems for scripting languages
that won't do QI, but there's another client that can do a QI behind your back - the
COM remoting layer. You've got the possibility of some really wierd bugs that don't
show up in inproc that jump up and bite you when you go oop or remote.

> If you need to support this kind of client (VBScript, JScript, what else?
> definitely not Python) then you need to implement IID_IDispatch with the union
> of the methods from all your other interfaces. The page Chris mentions (below)
> has a great summary of some solutions to this.
> >So what I'm doing is having multiple oleautomation compatible interfaces, and then
> >using a typelib-driven IDispatch implementation that I got from
> > This automagically merges
> >all my separate interfaces into a pseudo-object model. Really neat stuff, and
> >saves a TON of work.
> Follow that advice if you need to support those lesser scripting clients, or
> don't if you don't.
> In any case, there is still no reason not to make all your other interfaces dual
> too. This should be an easy step. I am assuming you already have some IDL for
> these interfaces? Change the oleautomation line to dual, and hook up the same
> typelib-driven IDispatch implementation you are already using for IID_Dispatch.
> If you are using ATL for this implementation then it all comes for free. The
> advantages? You get easier access from Python, and in VB you get compile time
> type safety.

I understand the argument you're making. To be honest, I don't expect anyone other than
me to call these objects from Python - heck, my coworkers are still doing database apps
in C++!

I don't want to make the interfaces dual. Just chalk it up to stubborness, I guess. <G>
And in VB I've already got type safety.

> In your specific example, the type library would say your collection object's
> Item method returns an ITavaresDrive (or whatever) and python would use an
> appropriate makepy-generated wrapper.
> >Having tried that, I find that I now have access to IDispatch::Invoke. Well, what
> >I really wanted was win32com.client.dynamic.Dispatch. How do I get to the wrapped
> >IDispatch rather than the raw one?
> drive = drives.Item("c:")._oleobj_.QueryInterface(pythoncom.IID_IDispatch)
> win32com.client.Dispatch(drive)
> (from memory)

Thanks, I think this will do the trick!

> I hope this helps,
> Toby Dickenson

Sure does!


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