Option Explicit

Private Sub SubClass()
  m_wndprcNext = SetWindowLong(hWnd, GWL_WNDPROC, AddressOf WindowProc)
End Sub

Private Sub UnSubClass()
  SetWindowLong hWnd, GWL_WNDPROC, m_wndprcNext
End Sub

Private Sub Form_Load()
  AppendMenu GetSystemMenu(hWnd, 0), MF_SEPARATOR, 0&, 0&
  AppendMenu GetSystemMenu(hWnd, 0), MF_STRING, IDM_INCR, "&Incriment"
  SubClass
End Sub

Private Sub Form_Unload(Cancel As Integer)
  UnSubClass
End Sub