Re: Passing pointers using a VARIANT

"Igor Tandetnik" <>
Mon, 1 Dec 2008 18:33:54 -0500
<eiyqH1AVJHA.5508@TK2MSFTNGP03.phx.gbl> wrote:

What's the threading model of the component? What apartment does the
thread join that creates this component?

Why would this be relevant to objects that are not COM objects?

What do you mean, not COM objects? Isn't Foo( [in] VARIANT ) a method on
some COM interface? Didn't you obtain a pointer to this interface with
CoCreateInstance call?

are just an address. Isn't this address visible between threads,
regardless of the threading model?

You create an instance of a COM object with incompatible threading
model. What you get back is a pointer to proxy, not a pointer
implemented directly by the object. When you call Foo on a proxy, it
needs to marshal its parameters to the real object.

Now, this proxy only knows how to marshal automation-compatible
variants. A variant of type VT_VOID is not automation-compatible. The
proxy looks at the type, and errors out. The call never reaches the
actual object.

The reason it works with integers is that VT_UI4 is an automation type.

Threading models, as far as I
know, describe the way COM handles incoming calls.

Outgoing calls, too. Have you read the article I referred you to?

They do not impose
any special limitations on memory boundaries.

Memory boundaries have nothing to do with the problem.

So, this implies that the
marshaller examines the VARIANT type, and tries to do pointer
marshalling for certain types.

.... and outright rejects certain other types.

This, of course, makes perfect sense,
and is what I was getting at to begin with - why would there be

IUnknown or IDispatch pointer needs to be marshalled, since the object
behind this pointer lives in the calling thread, and has a threading
model incompatible with the thread the call is going to be executed on.

This has been answered - different models. I was under
the impression that inproc servers don't deal with models

You were mistaken.

Apparently this is not the case?

This is not the case.

So, my question was - is there a way to pass the pointer in a VARIANT
without turning it into a COM object (which is unnecessary), but also
without sneaking the pointer in?

No, not really.
With best wishes,
    Igor Tandetnik

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