Using [iid_is] gives me stack overflows. What's with that?

10 Jul 2001 09:44 com

The problem here is that you have a piece of IDL like this:

[ object, uuid(...), oleautomation, pointer_default(unique) ]
interface IFoo
{
    HRESULT Bind(IID iid, [iid_is(iid), out] IUnknown **ppObject);
};

…and it doesn’t work. When you try calling an object in the same apartment as the client, everything’s fine. As soon as you call an object in a different apartment (either STA to MTA or vice versa), you get “stack overflow” errors. Some further investigation revealed that the [iid_is] wasn’t being used to marshal the interface, and all that was getting returned was the IUnknown interface.

So, I posted to the DCOM mailing list, and it turns out that [iid_is] and [oleautomation] aren’t compatible. Unfortunately, MIDL doesn’t bother telling you this, it just drops the [iid_is] attribute. This leads to the stack overflow error.

Possible solutions? Try one of these:

  • Lose the [iid_is] parameter, and query the IUnknown once you’ve got it.
  • Specify the exact interface you’re expecting.
  • Stop using TLB marshalling. This requires messing around with proxy-stub DLLs.