Differences between QueryService and QueryInterface

Although it might seem logical to call QueryInterface to find the interface you want to use, services are a more efficient means of getting the same capabilities.

When related interfaces are grouped into a service, a single call to IServiceProvider returns a pointer to the requested interface, regardless of the object it's implemented on.
The container need not delegate calls to individual interfaces, as is done with QueryInterface. Instead, the ActiveX designer can invoke them directly.
The interfaces need not be implemented by the object that is queried; they may be implemented as part of any object that the container is aware of.

The key difference between the QueryService and QueryInterface methods is that QueryInterface on an object returns an interface on the same object, whereas QueryService makes no such guarantee. QueryService on an object may return an interface on the same object or on a different object.

Because services need not be implemented on the site object, calling QueryInterface on the site to look for interfaces in a service is not guaranteed to succeed.

In addition, calling QueryInterface on an object you receive from QueryService is valid only for interfaces that are part of the service. Querying for any other interfaces results in an error status.