CallWindowProc Issue

[Jenova]

I am in the middle of reading Steve Romans "Win32 API Programming with Visual Basic 6" and i am on to chapter 18 "Windows Subclassing", at the moment i am reading up on the CalWindowProc. I have come with this so far and it does not seem to be working, it is supposed to be a classic example of restricting the size of the form.

' Module Code
Option Explicit
 
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
       
' Allows us to set the window style
' Returns the Procedure address which will
' be stored in 'OldWindowProc'
Declare Function SetWindowLong _
    Lib "user32" _
    Alias "SetWindowLongA" ( _
        ByVal hwnd As Long, _
        ByVal nIndex As Long, _
        ByVal dwNewLong As Long) _
        As Long
 
' Returns the cordinates for the
' mouse
Public Type POINTAPI
        X               As Long
        Y               As Long
End Type
 
' Holds the sizing information for
' the form
Private Type MINMAXINFO
    ptReserved          As POINTAPI
    ptMaxSize           As POINTAPI
    ptMaxPosition       As POINTAPI
    ptMinTrackSize      As POINTAPI
    ptMaxTrackSize      As POINTAPI
End Type
 
Const GWL_WNDPROC = (-4)            ' Sets a new procdure address
Const WM_GETMINMAXINFO = &H24       ' Gets the window size information
 
Public OldWindowProc       As Long  ' Holds thw Old Window Address
 
Public Function NewWindowProc(hwnd As Long, Msg As Long, wParam As Long, lParam As MINMAXINFO) As Long
    
    Dim Point       As POINTAPI
    
    ' Check for any messages
    Select Case Msg
        
        ' If the message is a sizeing message
        Case WM_GETMINMAXINFO
            
            ' Limit the form size to the cursor co-ordinates
            With lParam
                .ptMaxTrackSize.X = Point.X
                .ptMaxTrackSize.Y = Point.Y
                .ptMinTrackSize.X = Point.X
                .ptMinTrackSize.Y = Point.Y
            End With
            
            NewWindowProc = 0
                
        Case Else
            
            ' Call the new Window Procedure
            NewWindowProc = CallWindowProc(OldWindowProc, hwnd, Msg, wParam, lParam)
                
    End Select
End Function
 
' Form Code
 
Option Explicit
 
Private Sub Form_Load()
    OldWindowProc = SetWindowLong(Form1.hwnd, GWL_WNDPROC, AddressOf NewWindowProc)
End Sub
 
 
Private Sub Form_Unload(Cancel As Integer)
    OldWindowProc = SetWindowLong(Form1.hwnd, GWL_WNDPROC, OldWindowProc)
End Sub

However, when i run the code it does not seem to work, i have tried other methods such as setting the form size to a specific height and width but it still does not seem to work. From what i have made out so far it it looks as though my application can not process the message and i am quite confident that this is the issue. Can you guys give me any hints as to where the problem lies and how i might be able to fix it?

Regards,

Jenova

[triggernum5]

your lparam isn't of proper type I believe. it expects a long, but you give it MAXMININFO which totals 8 longs see if it works if you declare SetWindowLong with MINMAXINFO for lparam..

[Jenova]

I did that before but it did not seem to work.

Regards,

Jenova

[triggernum5]

You should put types and constants above function declarations.. I doubt it sees your types..

[Jenova]

I don't think that would cause the issue

[Jenova]

Can anyone point in the right direction at all?

[bushmobile]

the lParam needs to be a Long.

you then use the CopyMemory API to get the MINMAXINFO structure that you can work with - then copy it back when you're done.

Check the example in the API FAQs