How To Limit a Window's Minimum and Maximum Size

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Article ID: 185733 - View products that this article applies to.
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SUMMARY

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.

MORE INFORMATION

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:
168795 : How To Hook Into a Window's Messages Using AddressOf

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.

REFERENCES

For additional information, please see the following article(s) in the Microsoft Knowledge Base:
168795 : How To Hook Into a Window's Messages Using AddressOf

Properties

Article ID: 185733 - Last Review: July 1, 2004 - Revision: 3.1
APPLIES TO
  • Microsoft Visual Basic 6.0 Learning Edition
  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual Basic 6.0 Enterprise Edition
  • Microsoft Visual Basic 5.0 Control Creation Edition
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
Keywords: 
kbhowto kbgrpdsvb KB185733

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