BUG: No WM_MOUSEWHEEL Message is Received after Adding a UserControl to a Form (231465)

MORE INFORMATION

Steps to Reproduce Behavior

WARNING: Failure to unhook a window before its imminent destruction may result in application errors, Invalid Page Faults, and data loss. This is due the fact that the new WindowProc function being pointed to no longer exists, but the window has not been notified of the change. Always unhook the sub-classed window upon unloading the sub-classed form or exiting the application. This is especially important while debugging an application that uses this technique within the Microsoft Visual Basic Development Environment (IDE). Pressing the End button or selecting End from the Run menu without unhooking may cause an Invalid Page Fault and close Microsoft Visual Basic. Changes to the active project will be lost.

1. Start a new Visual Basic Standard EXE project. Form1 is created by default.
2. Add the following code to the General Declaration section of Form1:

   Option Explicit
   
   Private Sub Form_Load()
      Me.Show
      Call SubClassHookForm
   End Sub
   
   Private Sub Form_Unload(Cancel As Integer)
      Call SubClassUnHookForm
   End Sub
   					

3. On the Project menu, select the Add Module menu option.
4. Add the following code to the General Declaration section of Module1:

   Option Explicit
   
   Private MSWHEEL_ROLLMSG     As Long
   Private m_PrevWndProc       As Long
   Private Const GWL_WNDPROC = (-4)
   
   Private Declare Function CallWindowProc Lib "user32" Alias _
      "CallWindowProcA" (ByVal lpPrevWndFunc As Long, ByVal hwnd As Long, _
      ByVal msg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
   Private Declare Function SetWindowLong Lib "user32" Alias _
      "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, _
      ByVal dwNewLong As Long) As Long
   Private Declare Function RegisterWindowMessage Lib "user32" _
      Alias "RegisterWindowMessageA" (ByVal lpString As String) As Long
   
   Public Sub SubClassHookForm()
      MSWHEEL_ROLLMSG = RegisterWindowMessage("MSWHEEL_ROLLMSG")
      ' On Windows NT 4.0, Windows 98, and Windows Me, change the above line to
      ' MSWHEEL_ROLLMSG = &H20A
      m_PrevWndProc = SetWindowLong(Form1.hwnd, GWL_WNDPROC, _
                                    AddressOf WindowProc)
   End Sub
   Public Sub SubClassUnHookForm()
      Call SetWindowLong(Form1.hwnd, GWL_WNDPROC, m_PrevWndProc)
   End Sub
   
   Public Function WindowProc(ByVal hwnd As Long, ByVal msg As Long, _
      ByVal wParam As Long, ByVal lParam As Long) As Long
   
      If msg = MSWHEEL_ROLLMSG Then
         Debug.Print "Receive MSWHEEL_ROLLMSG"
      End If
      WindowProc = CallWindowProc(m_PrevWndProc, hwnd, msg, wParam, lParam)
   End Function
   					

5. Press the F5 key to run the program and roll the wheel button while the mouse pointer is positioned over Form1.
6. You will see "Receive MSWHEEL_ROLLMSG" displayed in the Immediate Window.
7. Stop the program. Do NOT use the END button on the toolbar per the warning note at the beginning of this section.
8. On the File menu, click Add Project... Select ActiveX Control in the Add Project dialog and click OK. UserControl1 is created by default.
9. Close the Project2 window and add a UserControl1 to Form1.
10. Press the F5 key to start the program and roll the wheel button while pointing the mouse on Form1.
11. The string "Receive MSWHEEL_ROLLMSG" no longer occurs in the Immediate Window. If you use Spy++ to detect all the Windows messages received by Form1, you will see that WM_MOUSEMOVE is received instead of WM_MOUSEWHEEL when you roll the wheel of the mouse after the UserControl is added.