4.10.3 Dual Interfaces

All interfaces defined within the FDT Specification are implemented as dual interfaces. This decision was made to support C++, Visual Basic, Java and other COM compliant development languages. The functionality of an object is implemented in separate task oriented interfaces, so that only the default interface is accessible via the dispatch interface. This prevents marshaling of the extra interfaces to dispatch-only clients, but the extra interfaces can be made available for a dispatch only-client via a wrapper that holds the other interfaces as properties or merges all methods to a single interface.
Due to the better performance, the developer should use the custom interface. However, in general the dispatch interface can be accepted, because the marshalling time of most of the FDT methods can be neglected compared with the runtime of each method.