4.5 IDL Correlation Example for iid_is
iid_is is an IDL language extension that allows clients or servers to specify how an interface passed as a parameter is marshaled at run time, as opposed to being statically defined at compile time. See the following example.
HRESULT CreateInstance([in]REFIID riid, [out, iid_is(riid)]Iunknown** ppUnknown);
In this example, the client calls the method, passing the interface to be invoked via the riid parameter. When returning a successful result, the server is required to marshal the ppUnknown parameter as a MInterfacePointer containing an OBJREF instance with its iid field set to the value of riid.