COM allows objects exposed from DLLs to be accessed through well-defined interfaces. A DLL registers an object by implementing the DllGetClassObject() function, which returns an interface pointer when called. The COM subsystem handles object instantiation by looking up the DLL in the registry, loading it, and calling DllGetClassObject(). DCOM extends COM to allow objects to be accessed remotely by starting a DCOM server process.

1. COM and DCOM
Sisimon Soman

2. COM
• Binary standard by M$
• Objects exposed from dll through well defined
interface.
• All dll servers expose common function
(DllGetClassObject()) so that COM subsystem
can get the object.
• One COM object support multiple interfaces, can
query (QueryInterface()) other interfaces from an
interface pointer.
• All interfaces should derive from IUnknown (for
reference counting, QueryInterface()).
• Client calls CoCreateInstance() -> dll name from
registry->LoadLibrary-> DllGetClassObject(IID).

3. Foo.dll
IFoo
Interface IFoo:public IUnknown
{
F1();
}
Class CFoo : public IFoo
{
F1() {….}
};
DllGetCLassObject (IID, pFoo)
{
If(IID == IID_IFoo)
{
pFoo = new CFoo;
return SUCCESS;
}
else
{
return E_FAIL;
}
}

4. COM Subsystem
Object DB
Start DCOM process
2
DCOM Server Process
Request COM Subsystem to
get the Object pointer 3
Interface IFoo:public
Client IUnknown
1 IFoo interface
{
F1();
}
CoCreateInstance(Foo) Class CFoo : public IFoo
{
4 F1() {….}
Register object to Object DB };
CoRegisterClassObjects(pFoo)