Author: Jonas Bilinkevicius

How to centre a MessageBox on a form

Answer:

1   { ... }
2   msgCaption: PChar; {var to hold caption}
3   { ... }
4   
5   procedure pmChangeMessageBox(var Msg: TMessage); message WM_USER + 1024;
6   
7   procedure TForm1.pmChangeMessageBox(var Msg: TMessage);
8   var
9     MBHwnd: THandle;
10    MBRect: TRect;
11    x, y, w, h: integer;
12  begin
13    MBHwnd := FindWindow(MAKEINTRESOURCE(WC_DIALOG), msgCaption);
14    if (MBHwnd <> 0) then
15    begin
16      GetWindowRect(MBHWnd, MBRect);
17      w := MBRect.Right - MBRect.Left;
18      h := MBRect.Bottom - MBRect.Top;
19      {center horizontal}
20      x := Form1.Left + ((Form1.Width - w) div 2);
21      {keep on screen}
22      if x < 0 then
23        x := 0
24      else if x + w > Screen.Width then
25        x := Screen.Width - w;
26      {center vertical}
27      y := Form1.Top + ((Form1.Height - h) div 2);
28      {keep on screen}
29      if y < 0 then
30        y := 0
31      else if y + h > Screen.Height then
32        y := Screen.Height - h;
33      SetWindowPos(MBHWnd, 0, x, y, 0, 0, SWP_NOACTIVATE or SWP_NOSIZE or
34        SWP_NOZORDER);
35    end;
36  end;
37  
38  //Example use:
39  
40  PostMessage(Handle, WM_USER + 1024, 0, 0);
41  msgCaption := 'Confirm';
42  MessageBox(Handle, 'Save changes?', msgCaption, MB_ICONQUESTION or MB_YESNOCANCEL);