Author: Lou Adler

I'd like to know if there's some way to prevent a user from closing Windows, or 
closing an application while it is currently performing a process. Do you know of 
way?

Answer:

There are two ways to approach this problem. One is strictly Delphi; the other 
involves creating a message handler for the WM_CLOSE message in Windows. Let's look 
at the former method first.

The easiest way to decide whether to allow or disallow a user from closing a 
program is with the OnCloseQuery method of the main form. If you open that method, 
you'll see the following:
1   
2   procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
3   begin
4   
5   end;


The most important part of that method is the CanClose formal parameter. If False, 
the window will not be allowed to close. If true, the program will close. Here's an 
example. Let's say that while some background processing is occuring, you have a 
Boolean variable called "ProcessRunning" set to true during the course of the 
process. If the user tries to close the program during the run you could do 
something like this:
6   
7   procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
8   begin
9     if ProcessRunning then
10      CanClose := False
11    else
12      CanClose := True;
13  end;


The point to this is to check certain conditions within the program. If the 
criteria for closing is correct, you can close the window. Otherwise, leave it up.

You'd do the same thing with a custom message handler for the WM_CLOSE message. The 
only difference is, you have to create the handler. To do this, look at the 
following section of code taken out of a program that I wrote that is set up to 
handle WM_CLOSE:

14  private
15  
16  procedure WMClose(var Msg: TWMClose); message WM_CLOSE;
17  public
18  
19  end;
20  
21  var
22    Form1: TForm1;
23    CloseDown: Boolean;
24  
25  implementation
26  
27  {$R *.DFM}
28  
29  procedure TForm1.WMClose(var Msg: TWMClose);
30  begin
31    if not CloseDown then
32      Msg.Result := 0
33    else
34      inherited;
35  end;


If you're new to creating custom Windows message handlers, while not a trivial 
thing, it's not that hard to do. It just requires some work. I won't go into a long 
discussion about Windows message handlers. WM_CLOSE happens to be a fairly simple 
message to handle, but there are more complex ones out there that take much more 
space than a simple e- mail reply. So I'll just tell you how to go about handling 
the message, then let you play with the code:

First, in the private section of the form's type declaration, you must declare the 
handler. In this case, I've declared it as:
36  
37  procedure WMClose(var Msg: TWMClose); message WM_CLOSE;


You can name the procedure anything you like.

As far as the implementation is concerned, to deny closure, we have to set the 
message's result to 0. To allow closure, we just call the inherited WM_CLOSE 
handler.

38  procedure TForm1.WMClose(var Msg: TWMClose);
39  begin
40    if not CloseDown then
41      Msg.Result := 0
42    else
43      inherited;
44  end;


As you can see, the principle is exactly the same as using the OnCloseQuery method of the form. But you can also see that to accomplish the same task, a bit more work had to be done. So a good "rule of thumb" is to use Delphi's built-in capabilities as much as possible, and to use only custom message handlers for events that aren't handled by default in Delphi.