Windows is shutting down

How to process WM_ENDSESSION etc

Contributor: HALLVARD VASSBOTN



{

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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