Re: Doubts on mixed threading (FTM -Both ) objects

"Brian Muth" <>
Tue, 17 Oct 2006 13:09:38 -0700
"buddie84" <> wrote in message

Thanks Brian ,
I am novice in this area and trying to understand the concepts ..
Kindly please correct if I am wrong .

1) a COM Object can be obtained from (I am not sure the usage of term
"COM Object" here is correct or not )


-only if we have CoClass implementation source
-we will be getting the CoClass object pointer

If we have 2 Objects of CoClass1 and CoClass2 implemented Interface1
and Interface2
If both are created in the same process and apartment , Is that legal
to implement communications between these classes using member
functions(member of the class but not defined by Interface1 or
interface 2) ?

If both are being created using CComObject<CoClass>::CreateInstance() then
you can freely invoke any method from any thread. There are no proxies

b) CoCreateInstance
In this case a class will be created and I 'll be getting the requested
interface pointer(pInter) .

And that pointer may point directly to the interface of the object, or it
may point to a proxy, depending on the apartment-modeling.

Can I cast pInter to CoClass pointer ?

Absolutely not! Two reasons: the pointer may be pointing to a proxy, and not
your object. Secondly, even if it points to the class object, because of the
nature of multiple inheritance the address of the pointer may not match the
address of the object. (C++ experts will know how to deal with this, but
this is generally dangerous practise and should be avoided).

2) we have this class registered as -BOTH in registry

Do you know what this means? It's a promise that your COM object can run
correctly in either an STA or an MTA. You need to implement accordingly. If
you want it only to run in an MTA, change it to "free".


 CreateThread(this); //???



Member Variable m_Var;

static MyThread(void *p)


CoClass *pCoClass= (CoClass*) p;

p->Function(); // {call 1} ?

p->InterfaceMethod2(); // {call 2} ?

p->m_Var; //{call 3} ?

Note that COM is not playing a role here. You are simply calling member
functions of the class object. So of course all of these are permitted, just
as long as neither of the member functions are making any COM calls.

You might find enlightening.


where 'Function()' is just a member function which is not the part of
any interface .
and 'InterfaceMethod1 & InterfaceMethod2' is defined in 'Interface1'

For example when a client create the above object on a single thread

Interface1 *pInter =CoCreateInstance(CLSID_CoClass, IID_Interface1)

In the above scenario {call 1} and {call 2} becomes illegal ... right

No. InterfaceMethod1 is creating a class object and passing it to the
thread. COM is not involved.


To resolve this we could pass a marshaled pointer and legally perform
{call 2} , But there might be situations where we need to perform
something like {call 1} and {call3}
(which are not the part of interface)
how could we resolve it ?

The question doesn't apply because of my last answer.

When we talked about marshalling ..Interface can be marshalled using
GIT etc can I marshal a CoClass object pointer ?

The GIT only understands interfaces. The question really makes no sense.


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