创建新的MessageBox窗口前，先关掉之前已经创建好的

procedure TForm2.Timer1Timer(Sender: TObject);
var
  h:HWND;
begin
  h := FindWindow(nil,PAnsiChar('123'));
  if h > 0 then
  begin
    //PostMessage(h,WM_SYSCOMMAND, SC_CLOASE,0);//现在这样写法不管用
    SendMessage(h,WM_SYSCOMMAND, SC_CLOSE,0);
  end;
  if Application.messageBOX(PChar('是否继续？'), PChar('123'), MB_OKCANCEL + MB_ICONINFORMATION + MB_DEFBUTTON2 + MB_TOPMOST) = IDCANCEL then
  begin
    Exit;
  end
end;