Re: Passing pointers using a VARIANT

"Igor Tandetnik" <>
Fri, 28 Nov 2008 20:32:15 -0500
<> wrote in message

I have an in-proc COM server (i.e. a COM DLL) that I need to use. I
want to pass a pointer to a class I defined to a method defined in one
of the DLL's interfaces.

You don't pass classes in COM - you pass interface pointers.

Did you implement this DLL? Why do you believe it needs to be a COM DLL?
Why not just make it a regular DLL exporting a set of functions and
perhaps classes? You are trying to do decidedly non-COM-friendly
things - I wonder what benefits of COM do you expect to gain.

So, my COM DLL has a method like Foo( [in] VARIANT ).

So far, the only way I can get that to work is if I cast the pointer
to a long and snaeak it in through the lVal member of the VARIANT. My
guess is that the minute I use something like VT_VOID | VT_BYREF, some
marshaling is attempted, and this fails.

What's the threading model of the component? What apartment does the
thread join that creates this component? Is the class whose pointer you
are trying to sneak through thread-safe?

1. Is my guess correct?


Why is an inproc call even

Because the caller and the callee have incompatible threading models.
For more details, see

2. How can I use the VAIRIANT to pass the pointer correctly?

You define an interface, have your class implement this interface, and
pack the interface pointer into VARIANT of type VT_UNKNOWN or

Should I
be using something else?

You should be using COM. You are not.

For those of you appalled that I would even try something like this:
this COM DLL is legacy code, soon to be replaced by a regular C++

So you don't have control of the callee side's source code? Then you
have no choice - you have to set up the VARIANT exactly the way the
method expects it.
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

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