static extern bool IsChild(IntPtr hWndParent, IntPtr hWnd);

      void WindowClosing([MarshalAs(UnmanagedType.VariantBool)] bool IsChildWindow, [In, Out, MarshalAs(UnmanagedType.VariantBool)] ref bool Cancel);

      Sub WindowClosing(<MarshalAs(UnmanagedType.VariantBool)> ByVal IsChildWindow As Boolean, <In, Out, MarshalAs(UnmanagedType.VariantBool)> ByRef Cancel As Boolean)