Re: Firing an event with an [out, retval] parameter
"Ignacio Burgue?o" <blabla@blabla.com> wrote in message
news:ezR2q%23rfHHA.1008@TK2MSFTNGP05.phx.gbl
I'd like to fire events to get data from my client applications (from
a COM server, out of process).
So, I defined an event like:
HRESULT QueryApplicationStatus([out, retval] VARIANT_BOOL* result)
Is that your actual IDL declaraion? Is your event interface a
dispinterface? I don't believe a dispinterface method can have both a
non-void return type and an [out, retval] parameter. Don't you get any
warnings or errors from MIDL compiler?
Note that dispinterface methods (unlike most other COM methods) don't
have to, and generally shouldn't, return an HRESULT. What appears in the
IDL as the return value is actually returned via pVarResult parameter to
IDispatch::Invoke. IDispatch::Invoke can also return an HRESULT - this
is implied and is not reflected in the IDL declaration.
Source interfaces should use [in, out] parameters instead. Make sure to
assign some initial value that would result in a reasonable behavior in
case the event is not handled.
dispinterface _DMyEvents {
properties:
methods:
[id(1)] void QueryApplicationStatus([in, out] VARIANT_BOOL* result);
};
I had to write some custom code because the code generated by the
wizard (VC 6) does not work.
Check that your code is sufficiently similar to this:
http://vcfaq.mvps.org/com/3.htm
I tested the event with clients written in Visual Basic and in Lua
(using LuaCom, a binding to use COM objects). So far, so good. The
problem is when I try to sink that event in C++.
My sink derives from IDispEventSimpleImpl.
My callback is defined like this:
HRESULT __stdcall OnQueryClientStatus(VARIANT_BOOL* result);
This is the _ATL_FUNC_INFO structure.
_ATL_FUNC_INFO CInteractionManager::EvtQueryClientStatus =
{CC_STDCALL, VT_I4, VT_BOOL | VT_BYREF};
I wonder if that actually matches the method definition in the type
library. Your IDL declaration for the method is nonsensical; I suspect
the TLB description does not look like what you expect. My guess would
be, VARIANT_BOOL is used as return type, HRESULT is ignored, and there
are no parameters. Try
VARIANT_BOOL __stdcall OnQueryClientStatus();
_ATL_FUNC_INFO CInteractionManager::EvtQueryClientStatus =
{CC_STDCALL, VT_BOOL, {}};
Better still, fix IDL declaration the way I've shown.
The code is called but ATL asserts with "Invalid callee" or something
like that after making the call.
That's because TLB says there's no parameter, but your method believes
it takes one parameter and pushes the same off the stack when returning.
So you end up corrupting the stack.
--
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