Author: William Gerbert
How to preventing a user from closing a window except at shutdown or restart
Answer:
We all know how to prevent a window from closing: Simply write event code for the
OnCloseQuery event and set CanClose to False. Once that's done, no matter what a
user presses, the window won't close. Here's some code:
1
2 procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
3 begin
4 CanClose := False;
5 end;
But what if you wanted to be able to close a window only when you reboot the
machine? With the scenario above, that would be impossible. Fortunately though,
there is a solution, and it resides in the windows message WM_QUERYENDSESSION.
WM_QUERYENDSESSION is generated by Windows when the OS is resetting: Either at a
shutdown or a restart. Using this message, we can set a boolean flag variable and
interrogate its value in OnCloseQuery to allow us to close the window and reset the
operating system. Look at the unit code below:
6 unit Unit1;
7
8 interface
9
10 uses
11 Windows, Messages, SysUtils, Classes,
12 Graphics, Controls, Forms, Dialogs;
13
14 type
15 TForm1 = class(TForm)
16 procedure FormCloseQuery(Sender: TObject; var CanClose: Boolean);
17 private
18 procedure WMQueryEndSession(var message: TWMQueryEndSession);
19 message WM_QUERYENDSESSION;
20 public
21 WindowsClosing: Boolean;
22 end;
23
24 var
25 Form1: TForm1;
26
27 implementation
28
29 {$R *.DFM}
30
31 { TForm1 }
32
33 procedure TForm1.FormCreate(Sender: TObject);
34 begin
35 WindowsClosing := False;
36 end;
37
38 procedure TForm1.WMQueryEndSession(var message: TWMQUERYENDSESSION);
39 begin
40 WindowsClosing := True;
41 end;
42
43 procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
44 begin
45 CanClose := WindowsClosing;
46 end;
47
48 end.
As you can see, I've created a public variable called WindowsClosing. When WM_QUERYENDSESSION fires, it sets the variable to True. Then in OnCloseQuery, CanClose is set to the value of WindowsClosing. Notice that I set WindowsClosing to False in OnCreate. This is purely for initialization purposes to make sure that previous attempts to close the window are foiled.
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