GetXpsPart method retrieves several objects that make up an XPS document.
HRESULT GetXpsPart( IUnknown **ppIXpsPart );
The XPS part. This part is the IUnknown interface of an object that is an XPS part. If ppIXpsPart is NULL, there are no more XPS parts to consume and the filter is ready to finish processing.
GetXpsPart returns an HRESULT.
Use QueryInterface on the value that the GetXpsPart method returns to determine the type of object that it retrieved.