public static extern int PropVariantClear(ref tagPROPVARIANT pvar);
public static extern int FreePropVariantArray(uint cVariants, ref tagPROPVARIANT rgvars);
public static extern int PropVariantCopy(ref tagPROPVARIANT pvarDest, ref tagPROPVARIANT pvarSrc);