Memory management for .NET components used by unmanaged code

Question

When working in the unmanaged world, we have to make sure that we clean up after ourselves if we have allocated memory on the heap (e.g. by using the new keyword in C++); we also have to make sure that we AddRef COM components that are created using CreateInstance and to Release it later; perhaps something like:

SomeNameSapce::IObjPtr obj;
HRESULT hr = obj.CreateInstance(L"SomeObject");
if (hr == S_OK)
{
 obj->AddRef();
 m_anotherObj= obj->GetObj();
 obj->Release();
}

Obviously we could use smart pointers and other such things (in C++) but that's besides the point...

Do we also have to AddRef/Release for objects that are grabbed from COM components (like m_anotherObj in the example above)?

To make things more confusing, what happens if this particular component that is actually a .NET component which is being exposed to unmanaged code via a COM interface? Does the garbage collector know to clear stuff up or does it all have to be done manually in the unmanaged world?

Answer 1

CreateInstance will give you back an object with a reference count of 1, so you do not need to AddRef it. (The smart pointer you have used will Release the object when it is destroyed.) Similarly, objects you receive from methods should have the reference count already incremented, so you do not need to AddRef them again - but you do need to Release them, unless you are using a smart pointer.

COM components exposed by .NET are no different from COM components written by any other technology. The garbage collector will not collect any .NET objects referenced from COM references.