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COM and DCOM
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.
COM and DCOM
- 1. COM and DCOM Sisimon Soman
- 2. COM • Binary standardby 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)