不阻止 windows 关闭 OnCloseQuery

Question

I have an application that hides itself on closing by the red cross. User can exit it by right-clicking tray icon and choosing Exit. But it would apparently block windows from shutting down, so I made a procedure to respond to a WM_QUERYENDSESSION to enable closing, this is the relevant code:

procedure TMainForm.OnWindowsEnd(var Msg: TMessage); // responds to message WM_QUERYENDSESSION;
begin
  AllowClose:=true;
  Close;
end;

procedure TMainForm.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
begin
  CanClose:=AllowClose;
  if NOt AllowClose then
    Hide;
end;

But weird thing keeps happening. When I issue a shutdown, this application closes nicely. But that is all. When I issue a second shutdown, system quits fine. (I'm testing this in WinXP).

What can be the cause? Thank you

---

ANSWER Code should look like this

procedure TMainForm.OnWindowsEnd(var Msg: TMessage); // responds to message WM_ENDSESSION;
begin
  // Possible checking for flags, see http://msdn.microsoft.com/en-us/library/aa376889%28v=vs.85%29.aspx
  AllowClose:=true;
  Msg.Result:=1;
end;

procedure TMainForm.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
begin
  CanClose:=AllowClose;
  if NOt AllowClose then
    Hide;
end;

Answer 1

WM_QUERYENDSESSION is a "query", not a shutdown command: Windows asks you if you're OK with shutting down, doesn't ask you to shut down. You shouldn't call Close !

Secondly Windows expects you to return TRUE when processing that message, so it knows you're OK with a potential Shut Down. I assume you're not setting the result to something TRUE, so Windows aborts the first Shut Down request.