int SizeOfIstream = ByteSizeOfIPStreamInit(pPersistStream) * 2;  // two bytes per value -- See my OleSaveToStream (ole32)              

    private static extern void OleSaveToStream(IPersistStreamInit pPStm, IStream pStm);

              OleSaveToStream(pPersistStream,IPtrStream);

    private static extern void OleSaveToStream(IPersistStreamInit pPStm, IStream pStm);

              OleSaveToStream(pPersistStream,IPtrStream);