In the COM server sample, CHello object is allocated and managed by gc, and the interface pointer is returned to outer clients. How can the gc know if the object is referenced outside?

It is specified that some solutions need to be present when memory allocated by gc is exposed to outer clients, such as adding a root to the gc so gc won¡¯t free the memory.  However I read through all the source files and could not find any code that tells the gc not to free the CHello COM object.