[RESOLVED] CallWindowProc

[Jenova]

Hello,

I am trying to get my head around subclassing at the moment using SetWindowLong and CallWindowProc. So far it is a disaster. I am just trying to make a simple example.

Option Explicit
 
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 Const GWL_WNDPROC = (-4)
 
Private Const WM_MOUSEMOVE = &H200
 
Private OldWindowProc       As Long
 
Public Sub SubClass(frm As Form)
 
    OldWindowProc = SetWindowLong(frm.hWnd, GWL_WNDPROC, AddressOf NewWindowProc)
 
End Sub
 
Public Sub UnSubClass(frm As Form)
 
    Call SetWindowLong(frm.hWnd, GWL_WNDPROC, OldWindowProc)
    
End Sub
 
Public Function NewWindowProc(ByVal hWnd As Long, ByVal Msg As Long, ByVal wParam As Long, lParam As Long) As Long
    
    If Msg = WM_MOUSEMOVE Then
        
        Form1.Caption = "Mouse is moving..."
    
    End If
    
    NewWindowProc = CallWindowProc(OldWindowProc, hWnd, Msg, wParam, lParam)
    
End Function

The idea behind it is that i am trying to process the WM_MOUSEMOVE message once the mouse moves on the form, then it will display a message in the form caption to say that the mouse is being moved.

The best that it does at the moment is force VB to close and i get a GPF.

Any ideas on how i can make this work?