Tip 54: Preventing a Window Form from Being Resized

Created: April 10, 1995

Abstract

In many Windows®-based applications, the user can resize a window by dragging one of the window's borders to a new position. This article will tell you how to prevent a form from being resized. You can also modify the example program so that it will allow the user to resize the form only to a certain size.

Using Get/SetWindowPlacement to Size a Form

In a Visual Basic® application, you can use the Windows® application programming interface (API) GetWindowPlacement and SetWindowPlacement functions to restrict a form's size. Each time Visual Basic loads a form or the form is resized by the user, the Resize event is triggered. By including code in the Resize event, you can prevent a form from being resized smaller, larger, or at all.

The GetWindowPlacement function returns the specified window's current location, visibility status, and its minimized and maximized positions. The SetWindowPlacement function sets the specified window's current location, visibility status, and its minimized and maximized positions. Both functions use the same type structures and arguments.

To declare these two functions within your program, include the following Declare statements in the Global Module or General Declarations section of your application's form (note that each Declare statement must be typed as one single line of text):

Declare Sub GetWindowPlacement Lib "User" (ByVal hWnd As Integer, lpWnd
   As WINDOWPLACEMENT)

Declare Function SetWindowPlacement Lib "User" (ByVal hWnd As Integer, lpWndPl 
   As WINDOWPLACEMENT) As Integer

The GetWindowPlacement or SetWindowPlacement function must be called with two arguments: The hWnd argument must contain the handle of the window that you want to retrieve information for; the lpWndPl argument is a WINDOWPLACEMENT structure that will hold the window's information.

The WINDOWPLACEMENT structure uses the RECT and POINTAPI structures as well, and is defined as follows:

Type WINDOWPLACEMENT    '22 bytes
  Length As Integer
  Flags As Integer
  ShowCmd As Integer
  PtMinPosition As POINTAPI
  PtMaxPosition As POINTAPI
  RcNormalPosition As RECT
End Type

The RECT structure, which describes the coordinates of a rectangle, is defined as follows:

Type RECT       '8 bytes
  Left As Integer
  Top As Integer
  Right As Integer
  Bottom As Integer
End Type

The values in the RECT structure represent the position of the window, in screen coordinates.

And, finally, the POINTAPI structure is defined as follows:

Type POINTAPI   '4 bytes
  X As Integer
  Y As Integer
End Type

The following table describes each field in the WINDOWPLACEMENT structure.

After we call the GetWindowPlacement function, we can retrieve the window's current size and save these values in variables for later use in our application.

Because we want to prevent the user from resizing a form, we insert code in the form's resize event. This code uses the GetWindowPlacement function to retrieve the form's current size. If the window's size is not the same as it was when our program was first executed, we call SetWindowPlacement, which sets the window's size back to its original coordinates. This function has the same arguments as the GetWindowPlacement function.

Example Program

The example program below uses the GetWindowPlacement and SetWindowPlacement functions to prevent the form from being resized. You can optionally add code to allow the window to be resized only to a specific size, if desired.

1. Start a new project in Visual Basic. Form1 is created by default.
   
   
2. Add the following code to the general declarations section of Form1:

   Dim lpRect As RECT
     Dim lpWnd As WINDOWPLACEMENT
     Dim nPoint As POINTAPI
     Dim lpRect2 As RECT
     Dim lpWnd2 As WINDOWPLACEMENT
     Dim nPoint2 As POINTAPI
     Dim lpWndPl As WINDOWPLACEMENT
     Dim X As Integer
     Dim FormWidth As Integer
     Dim FormHeight As Integer
     Dim FormWidth2 As Integer
     Dim FormHeight2 As Integer
   

3. Add the following code to the Form_Load event for Form1:

   Sub Form_Load()
     'Get the form's current size and position.
     lpWnd.Length = 22   'set length of lpWnd first!
     
     Call GetWindowPlacement(Form1.hWnd, lpWnd)
     FormWidth = lpWnd.RcNormalPosition.Right - lpWnd.RcNormalPosition.Left
     FormHeight = lpWnd.RcNormalPosition.Bottom - lpWnd.RcNormalPosition.Top
   End Sub
   

4. Add the following code to the Form_Resize event for Form1:

   Sub Form_Resize()
     lpWnd2.Length = 22   'set length of lpWnd2 first!
     Call GetWindowPlacement(Form1.hWnd, lpWnd2)
     FormWidth2 = lpWnd2.RcNormalPosition.Right - lpWnd2.RcNormalPosition.Left
     FormHeight2 = lpWnd2.RcNormalPosition.Bottom - lpWnd2.RcNormalPosition.Top
     
     If FormWidth <> FormWidth2 Then
         GoTo NoWidthChange
     End If
     If FormHeight <> FormHeight2 Then
         GoTo NoHeightChange
     End If
       
     Exit Sub
   'Do not allow form's width or height to be changed. Reset to original values.
   
   NoWidthChange:
     X = SetWindowPlacement(Form1.hWnd, lpWnd)
     Exit Sub
   NoHeightChange:
     X = SetWindowPlacement(Form1.hWnd, lpWnd)
     
   End Sub
   

5. Add a new BAS module to the project. Module1.Bas is created by default.
   
   
6. Add the following Declare statements and type structures to Module1.Bas (note that each Declare statement should be typed as a single line of text):

   Declare Sub GetWindowPlacement Lib "User" (ByVal hWnd As Integer, lpWnd
      As WINDOWPLACEMENT)
   
   Declare Function SetWindowPlacement Lib "User" (ByVal hWnd As Integer, lpWndPl 
      As WINDOWPLACEMENT) As Integer
   
   Type RECT       '8 bytes
       Left As Integer
       Top As Integer
       Right As Integer
       Bottom As Integer
   End Type
   
   Type POINTAPI   '4 bytes
       X As Integer
       Y As Integer
   End Type
   
   Type WINDOWPLACEMENT    '22 bytes
       Length As Integer
       Flags As Integer
       ShowCmd As Integer
       PtMinPosition As POINTAPI
       PtMaxPosition As POINTAPI
       RcNormalPosition As RECT
   End Type
   

7. Run the program. Try to resize the form by clicking and dragging one of the form's borders. The form will immediately reset to its previous size.