How To Limit a Window's Minimum and Maximum Size

By default, Visual Basic allows a form to be resized to any size you want provided the BorderStyle is set appropriately. At times, it may be desirable or necessary to limit the range of a window's size (such that it cannot be smaller or larger than a predetermined size).

For example, the Windows accessory, Microsoft Paint, prevents its window from being sized below a certain point. This is done to allow the user to always see at least part of the image they are editing.

With the aid of Windows API functions, you can achieve this same functionality within your Visual Basic application.

WARNING: ANY USE BY YOU OF THE SAMPLE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this sample code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

Below are steps to create a sample application that demonstrate the behavior discussed above. To achieve this functionality, a concept known as subclassing must be used to detect when the WM_GETMINMAXINFO message occurs. This message occurs when an attempt is made to resize the window.

For more information on subclassing, please see the following article in the Microsoft Knowledge Base:

Failure to unhook a window before its destruction results in application errors, Invalid Page Faults, and data loss. This problem can occur when the new WindowProc function being pointed to no longer exists, but the window has not been notified of the change. Always unhook the subclassed window upon unloading the subclassed form or exiting the application. This is especially important while debugging an application that uses this technique within the Microsoft Visual Basic Development Environment. Clicking the End button or, on the Run menu, clicking End, without unhooking causes an Invalid Page Fault and closes Microsoft Visual Basic.

Steps to Create a Sample Application

1. Create a new Standard EXE project.
2. Paste the following code into Form1's code module:

         Option Explicit
   
         Private Sub Form_Load()
             'Save handle to the form.
             gHW = Me.hwnd
   
             'Begin subclassing.
             Hook
         End Sub
   
         Private Sub Form_Unload(Cancel As Integer)
             'Stop subclassing.
             Unhook
         End Sub
   
   						

3. Add a standard module to the project.
4. Paste the following code into the module:

         Option Explicit
   
         Private Const GWL_WNDPROC = -4
         Private Const WM_GETMINMAXINFO = &H24
   
         Private Type POINTAPI
             x As Long
             y As Long
         End Type
   
         Private Type MINMAXINFO
             ptReserved As POINTAPI
             ptMaxSize As POINTAPI
             ptMaxPosition As POINTAPI
             ptMinTrackSize As POINTAPI
             ptMaxTrackSize As POINTAPI
         End Type
   
         Global lpPrevWndProc As Long
         Global gHW As Long
   
         Private Declare Function DefWindowProc Lib "user32" Alias _
            "DefWindowProcA" (ByVal hwnd As Long, ByVal wMsg As Long, _
             ByVal wParam As Long, ByVal lParam As Long) As Long
         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 Sub CopyMemoryToMinMaxInfo Lib "KERNEL32" Alias _
            "RtlMoveMemory" (hpvDest As MINMAXINFO, ByVal hpvSource As Long, _
             ByVal cbCopy As Long)
         Private Declare Sub CopyMemoryFromMinMaxInfo Lib "KERNEL32" Alias _
            "RtlMoveMemory" (ByVal hpvDest As Long, hpvSource As MINMAXINFO, _
             ByVal cbCopy As Long)
   
         Public Sub Hook()
             'Start subclassing.
             lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
                AddressOf WindowProc)
         End Sub
   
         Public Sub Unhook()
             Dim temp As Long
   
             'Cease subclassing.
             temp = SetWindowLong(gHW, GWL_WNDPROC, lpPrevWndProc)
         End Sub
   
         Function WindowProc(ByVal hw As Long, ByVal uMsg As Long, _
            ByVal wParam As Long, ByVal lParam As Long) As Long
             Dim MinMax As MINMAXINFO
   
             'Check for request for min/max window sizes.
             If uMsg = WM_GETMINMAXINFO Then
                 'Retrieve default MinMax settings
                 CopyMemoryToMinMaxInfo MinMax, lParam, Len(MinMax)
   
                 'Specify new minimum size for window.
                 MinMax.ptMinTrackSize.x = 200
                 MinMax.ptMinTrackSize.y = 200
   
                 'Specify new maximum size for window.
                 MinMax.ptMaxTrackSize.x = 500
                 MinMax.ptMaxTrackSize.y = 500
   
                 'Copy local structure back.
                 CopyMemoryFromMinMaxInfo lParam, MinMax, Len(MinMax)
   
                 WindowProc = DefWindowProc(hw, uMsg, wParam, lParam)
             Else
                 WindowProc = CallWindowProc(lpPrevWndProc, hw, uMsg, _
                    wParam, lParam)
             End If
         End Function
   
   						

5. Save and run the sample application.
6. Attempt to resize Form1.
   
   RESULT: The form is not allowed to be sized smaller than 200 by 200 pixels or larger than 500 by 500 pixels.

For additional information, please see the following article(s) in the Microsoft Knowledge Base: