This article was previously published under Q231465
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SYMPTOMS
After adding a User control to a Form, the Visual Basic design environment (IDE) ignores the WM_MOUSEWHEEL message when you roll the wheel of the mouse. This problem does not occur on Windows NT or Windows 2000.
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.
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