Services, Interfaces, and Object Identity

QueryService identifies an object that supports the requested service, with the returned pointer already set to the specified interface.

It's important to keep in mind the following:

Some of the interfaces in the service may be optional. The returned object may not support every interface defined in the service description.
The returned object may have additional interfaces that are not part of the definition of the service.
Two services may be implemented on the same object. As a result, passing different SIDs to QueryService does not necessarily mean that you'll receive different objects in return.
An interface with the same name and IID may be part of two different services.

Two different services (SID_SIdaho and SID_SMontana, for example) can both specify the use of a single interface (for example, IDispatch), even though the implementation of the interface may have nothing in common between the two services. This works because the following two hypothetical calls can return different objects:

hr = QueryService (SID_SMontana, IID_IDispatch, &pDisp) 
hr = QueryService (SID_SIdaho, IID_IDispatch, &pDisp)

Remember that object identity is not assumed if you ask for a different service identifier.