Use a global interface table to transfer com interfaces in different threads

In MultithreadingProgramUsing COM objects in is really a headache. When you can access an interface pointer, it does not mean that you can call methods on the interface. I have never really understood several thread modes of COM. The problem is that when I tried to call a COM interface method in a thread and this COM interface was created in another thread, it never succeeded. Even worse, there are no error messages.
Passing interfaces in multiple threads requires additional work. The so-called Marshal interface method is introduced in various books, but I have never used it. A simpler method is to use a global interface table (globalinterfacetable ).
Globalinterfacetable allows you to access the COM interface created in any thread anywhere. Globalinterfacetable is a COM Object and implements the iglobalinterfacetable interface. This interface has three methods for registering an interface, obtaining an interface, and canceling an interface. The following example describes how to use globalinterfacetable.
First, when you create a COM interface that needs to be used by other threads, register it with globalinterfacetable.

Ccomptr < Imyinterface > Spmyinterface;
Spmyinterface. cocreateinstance ();
// Register interface in global interface table
Ccomptr < Iglobalinterfacetable > Spgit;
Spgit. cocreateinstance (clsid_stdglobalinterfacetable );
If (Spgit)
{
Spgit->Registerinterfaceinglobal (spmyinterface, iid_imyinterface,&M_dwcookie );
}

A cookie is returned during registration. Remember this cookie and obtain the interface through this cookie when any thread needs to use the previous interface.

Ccomptr < Imyinterface > Spmyinterface;

If (M_dwcookie ! = 0 )
{
Ccomptr < Iglobalinterfacetable > Spgit;
Spgit. cocreateinstance (clsid_stdglobalinterfacetable );
If (Spgit)
{
Spgit->Getinterfacefromglobal (m_dwcookie, iid_imyinterface ,(Void**)&Spmyinterface. P );
}
}

If (Spmyinterface)
{
//Call my interface
}

Finally, as a responsible programmer, close the previously registered interface.

If (M_dwcookie ! = 0 )
{
Ccomptr < Iglobalinterfacetable > Spgit;
Spgit. cocreateinstance (clsid_stdglobalinterfacetable );
If (Spgit)
{
Spgit->Revokeinterfacefromglobal (m_dwcookie );
M_dwcookie= 0;
}
}

Note that every time we create a new globalinterfacetable instance, because it is a COM object and its pointer cannot be passed between different threads.