Programming .NET Components, 2nd Edition

Traditional Memory De-allocation Schemas

De-allocation of memory and the destruction of objects are also different in .NET, as compared with raw C++ or COM. In C++, the object destructor is called when a stack-based object goes out of scope:

    {//beginning of a C++ scope
        MyClass object;
        //use object;
    }//end of scope, C++ calls the object destructor

The object destructor is also called in C++ when the delete operator is used:

    //in C++:
    MyClass* pObject = new MyClass;
    //using pObject, then de-allocating it
    delete pObject;

COM uses reference counting, and it’s up to the client to increment and decrement the counter associated with each object. Clients that share an object have to call AddRef() to increment the counter. New COM objects are created with a reference count of one. When a client is done with an object, it calls Release() to decrement the counter:

    //COM pseudo-code:
    IMyInterface* pObject = NULL;
    ::CoCreateInstance(CLSID_MyClass,IID_IMyInterface,&pObject);
    //using pObject, then releasing it
    pObject->Release();

When the reference count reaches zero, the object destroys itself:

    //COM implementation of IUnknown::Release()
    ULONG MyClass::Release()
    {
       //m_Counter is this class counter
       m_Counter--;
       if(m_Counter == 0)
       {
          delete this;
          return 0;
       }
       //Should return the counter:
       return m_Counter;
    }