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    {
    In a previous mailed with the subject "[Delphi] Serios bug when closing
    windows???" bimmer@ibm.net(Per Bakkendorff) writes:
    >When you have a delphi application running, and you are shutting down windows,
    >(don't close your app first), NONE of your destructors are called!!!!

    At first I thought the problem was programmer's code, but then I realized
    that it really is a bug or at least a very lazy "feature".

    The problem seems to be that when you close down Windows, it will first
    send a WM_QueryEndSession message to all running top-level windows. This
    is handled and processed correctly by the TForm object in VCL.

    Then assuming that all applications indicated that it was ok to close down,
    Windows will send WM_EndSession messages to all windows. This message is
    not handled by VCL. The application is simply brought down with a bang.
    No windows are closed, no destructor called and no exit procedures called.

    The solution is to handle the WM_EndSession message yourself. There are
    several ways of handling messages in Delphi, but the only reliable way
    of handling the WM_ENDSESSION is to use the HookWindow method of
    Application.

    In the message handler, check to see if the message is a WM_ENDSESSION.
    If so, we should close down the application. We could have called the
    Close method of the main window, but the Windows API documentation states
    that the system might go down anytime after return from the WM_ENDSESSION,
    and a posted WM_QUIT message might never arrive to the application.

    The solution is to simply call Halt instead. This will call all registred
    exit procedures, including the ones in Controls and DB units. These will
    free the application and screen objects and take the BDE down correctly.

    A simple example follows:
    }

    unit Tst2u;

    interface

    uses
      SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
      Forms, Dialogs, Grids, DBGrids, DB, DBTables;

    type
      TForm1 = class(TForm)
        DataSource1: TDataSource;
        Table1: TTable;
        DBGrid1: TDBGrid;
        procedure FormCreate(Sender: TObject);
      private
        { Private declarations }
        function HookProc(var Message: TMessage): boolean;
      public
        { Public declarations }
      end;

    var
      Form1: TForm1;

    implementation

    {$R *.DFM}

    const
      FlagFileName = 'C:\Flag.Fil';

    procedure CreateFlagFile;
    var
      F: System.Text;
    begin
      System.Assign(F, FlagFileName);
      System.Rewrite(F);
      Writeln(F, 'This is a dummy flag file');
      System.Close(F);
    end;

    procedure KillFlagFile;
    var
      F: File;
    begin
      System.Assign(F, FlagFileName);
      System.Erase(F);
    end;

    procedure MyExitProc; far;
    begin
      KillFlagFile;
    end;

    procedure TForm1.FormCreate(Sender: TObject);
    begin
      Application.HookMainWindow(HookProc);
    end;

    function TForm1.HookProc(var Message: TMessage): boolean;
    begin
      Result := false;
      if Message.Msg = WM_EndSession then
      begin
        if WordBool(Message.wParam) then
        begin
          { Windows is closing down - clean up!! }

          { This should execute all ExitProcs, close windows and call destructors... }
           Halt; { This works! }

          { This should close things down properly,
            but do we have enough time to handle any posted messages before Windows
            is already down??  This will result in a PostQuitMessage that might
            never arrive!}
    {      Close;} { This doesn't always work - avoid it }
        end;
      end;
    end;

    initialization
      CreateFlagFile;
      AddExitProc(MyExitProc);
    end.

    This unit demonstrates that the exit procedures are called when closing
    the app normally and when closing down Windows and using HookMainWindow.
    Without the HookMainWindow call, the exit proc will not be called. This
    is specially important for DB applications. Without the Halt, LCK files
    will not be deleted, buffers might not be flushed, changes posted and
    so on.

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