Question

I am trying to implement auto log off feature for my vb.net desktop application. Here is the code:

Private Sub AutoLogOffTimer(ByVal myUIContext As globals)

         If myUIContext.parameters.LogOutTime <> 0 Then
            myTimer = New System.Windows.Forms.Timer()
            myTimer.Enabled = False
            'myTimer.Dispose()
            myTimer.Interval = (myUIContext.parameters.LogOutTime * 60 * 1000) - 20000
            AddHandler myTimer.Tick, AddressOf logOutUser
            myTimer.Start()
            Application.AddMessageFilter(Me)
         ElseIf myUIContext.parameters.LogOutTime = 0 Then
            Application.RemoveMessageFilter(Me)
            RemoveHandler myTimer.Tick, AddressOf logOutUser
            myTimer.Stop()
            myTimer.Enabled = False
         End If
      End Sub

 Public Function PreFilterMessage(ByRef m As Message) As Boolean Implements    IMessageFilter.PreFilterMessage

         ' Monitor message for keyboard and mouse messages
         Dim active As Boolean = m.Msg = &H100 OrElse m.Msg = &H101
         ' WM_KEYDOWN/U
         'active = active OrElse m.Msg = &HA0 ' OrElse m.Msg = &H200
         active = active OrElse m.Msg = &HA3 OrElse m.Msg = &H201 OrElse m.Msg = &H202 OrElse m.Msg = &H1 OrElse m.Msg = &H200 OrElse m.Msg = &H2 OrElse m.Msg = &H208 OrElse m.Msg = &H2A3 OrElse m.Msg = &H2A1
         ' WM_(NC)MOUSEMOVE
         active = active OrElse m.Msg = &H10 OrElse m.Msg = &H3 OrElse m.Msg = &H5
         ' WM_CLOSE, in case dialog closes
         If active Then
            myTimer.Enabled = False
            myTimer = New System.Windows.Forms.Timer()
            If _myUIContext.parameters.LogOutTime <> 0 Then
               myTimer.Interval = (_myUIContext.parameters.LogOutTime * 60 * 1000) - 20000
               Try
                  RemoveHandler myTimer.Tick, AddressOf logOutUser
               Catch ex As Exception
               End Try
               AddHandler myTimer.Tick, AddressOf logOutUser
               myTimer.Start()
               myTimer.Enabled = True
            ElseIf _myUIContext.parameters.LogOutTime = 0 Then
               myTimer = New System.Windows.Forms.Timer()
               myTimer.Stop()
            End If
         End If
         Return False
      End Function

 Private Sub logOutUser(ByVal sender As Object, ByVal e As EventArgs)

         If Not isLogoutWindowOpen Then
            If ComputeCpuUsage() < 1 Then
               LogoutWindow = New LogoutApp(20)
               AddHandler LogoutWindow.logOutCancel, AddressOf logOutCancel
               AddHandler LogoutWindow.logOutProceed, AddressOf logOutProceed

               LogoutWindow.MdiParent = Me
               LogoutWindow.Show()
               myTimer.Stop()

               isLogoutWindowOpen = True
            Else
               myTimer.Enabled = False
               myTimer.Start()
            End If

         End If

      End Sub

  Private Sub logOutCancel()

         myTimer = Nothing
         myTimer = New System.Windows.Forms.Timer()
         myTimer.Enabled = False
         myTimer.Stop()
         If _myUIContext.parameters.LogOutTime <> 0 Then
            myTimer.Interval = (_myUIContext.parameters.LogOutTime * 60 * 1000) - 20000
            myTimer.Start()
         End If
         myTimer.Enabled = True
         isLogoutWindowOpen = False
      End Sub

The problem is when i set the log out time to be lets say 1 minute, it works fine if and gives me the logout window after 1 minute if i do not do anything on my application. But after I cancel my logout window message and start working on my application the log out message box keeps coming up arbitrarily .First I thought my timers were not getting reset properly but myTimer.Enabled=False should have done the trick if no that myTimer.Start() should have definitely done the trick, but that that does not seem to work also. i have checked my pre-filter messages but still no luck.

Any help would be much appreciated.

Thanks.

Answer 1

OK I think I might have figured out your problem.

Sometimes, Windows timers don’t care if they have been null referenced, and continue to fire their events without a parent. I think your problem lies in that in your logOutCancel code you are clearing the reference to the timer, but not first disabling it.

Try a call to myTimer.Stop() before your line where you set myTimer = Nothing.

Alternatively, why do you need to keep creating new Timers at all? Can’t you just stop then restart your original one?