Application.Idle acting differently

Question

I'm currently working on a from that will logout the user in a certain amount of inactivity time. I declared Application.Idle

Private Sub Application_Idle(sender As Object, e As EventArgs)
    Timer.Interval = My.Settings.LockOutTime
    Timer.Start()
End Sub

Then, call it on form load event

Private Sub ctlManagePw_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    AddHandler System.Windows.Forms.Application.Idle, AddressOf Application_Idle
End Sub

And on the timer

Private Sub Timer_Tick(sender As Object, e As EventArgs) Handles Timer.Tick
    Try
        If My.Settings.TrayIcon = 1 Then
            Me.ParentForm.Controls.Remove(Me)
            control_acPasswords()
            _Main.NotifyIcon.Visible = True
            _Main.NotifyIcon.ShowBalloonTip(1, "WinVault", "You've been locked out due to innactivity", ToolTipIcon.Info)
        End If
        'Stop
        Timer.Stop()
        Timer.Enabled = False
        'Flush memory
        FlushMemory()
    Catch ex As Exception
        'Error is trapped. LOL
        Dim err = ex.Message
    End Try
End Sub

The problem with this is whenever an Idle event is over I'm still getting a notification that I've been lockout again and/or the application has entered an Idle event.

control_acPasswords() is the logout user control

And here is where I free up memory

Declare Function SetProcessWorkingSetSize Lib "kernel32.dll" (ByVal process As IntPtr, ByVal minimumWorkingSetSize As Integer, ByVal maximumWorkingSetSize As Integer) As Integer
Public Sub FlushMemory()
    Try
        GC.Collect()
        GC.WaitForPendingFinalizers()
        If (Environment.OSVersion.Platform = PlatformID.Win32NT) Then
            SetProcessWorkingSetSize(Process.GetCurrentProcess().Handle, -1, -1)
            Dim myProcesses As Process() = Process.GetProcessesByName(Application.ProductName)
            Dim myProcess As Process
            For Each myProcess In myProcesses
                SetProcessWorkingSetSize(myProcess.Handle, -1, -1)
            Next myProcess
        End If
    Catch ex As Exception
        Dim err = ex.Message
    End Try
End Sub

If I put a MsgBox(ex.Message)on Timer_Tick event Exception, I keep getting

Object reference not set to an instance of an object

My expected result is whenever the form enters the Idle event, it will get the interval or the time from My.Settings.LockOutTime which is a minute value and stored as 60000 for 1 minute or 60 seconds and start the timer. Now on Timer_Tick then logout the user if the interval is over.

Is there something wrong how I handle events?

Answer 1

The Application.Idle event fires multiple times. Every time Winforms retrieved all messages from the message queue and emptied it. Problem is, the second and subsequent times it fires, you are starting a timer that was already started. That has no effect, you have to reset it so it starts ticking again for the programmed interval. Easy to do:

Private Sub Application_Idle(sender As Object, e As EventArgs)
    Timer.Interval = My.Settings.LockOutTime
    Timer.Stop()
    Timer.Start()
End Sub

Next problem, possibly the reason for the exception, is that you explicitly have to unsubscribe the event when your form closes. It is not automatic, Application.Idle is a static event. Use the FormClosed event:

Protected Overrides Sub OnFormClosed(ByVal e As System.Windows.Forms.FormClosedEventArgs)
    Timer.Stop()
    RemoveHandler Application.Idle, AddressOf Application_Idle
    MyBase.OnFormClosed(e)
End Sub

Answer 2

In addition to Hans' answer: You don't appear to stop the timer running when the user is no longer idle. That is, the timer starts when I go idle, but if I come back I will still be locked out when the timer ticks.

You need to make sure that you are stopping the timer when the user becomes active again.