protected static extern void PropVariantClear([In, MarshalAs(UnmanagedType.LPStruct)] PropVariant pvar);
protected static extern void PropVariantCopy( [Out, MarshalAs(UnmanagedType.LPStruct)] PropVariant pvarDest, [In, MarshalAs(UnmanagedType.LPStruct)] ConstPropVariant pvarSource );