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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

Thread Timers 

The System.Threading.Timer class is useful for periodically running a task on a separate thread. For example, you could use a thread timer to check the status and integrity of a database or to back up critical files.

Thread Timer Example

The following example starts a task every two seconds and uses a flag to initiate the Dispose method that stops the timer. This example posts status to the output window, so you should make this window visible by pressing CTRL+ALT+O before you test the code.

Class StateObjClass
    ' Used to hold parameters for calls to TimerTask
    Public SomeValue As Integer
    Public TimerReference As System.Threading.Timer
    Public TimerCanceled As Boolean
End Class

Sub RunTimer()
    Dim StateObj As New StateObjClass
    StateObj.TimerCanceled = False
    StateObj.SomeValue = 1
    Dim TimerDelegate As New Threading.TimerCallback(AddressOf TimerTask)
    ' Create a timer that calls a procedure every 2 seconds.
    ' Note: There is no Start method; the timer starts running as soon as 
    ' the instance is created.
    Dim TimerItem As New System.Threading.Timer(TimerDelegate, StateObj, _
                                                2000, 2000)
    StateObj.TimerReference = TimerItem  ' Save a reference for Dispose.

    While StateObj.SomeValue < 10 ' Run for ten loops.
        System.Threading.Thread.Sleep(1000)  ' Wait one second.
    End While

    StateObj.TimerCanceled = True  ' Request Dispose of the timer object.
End Sub

Sub TimerTask(ByVal StateObj As Object)
    Dim State As StateObjClass = CType(StateObj, StateObjClass)
    ' Use the interlocked class to increment the counter variable.
    System.Threading.Interlocked.Increment(State.SomeValue)
    System.Diagnostics.Debug.WriteLine("Launched new thread  " & Now)
    If State.TimerCanceled Then    ' Dispose Requested.
        State.TimerReference.Dispose()
        System.Diagnostics.Debug.WriteLine("Done  " & Now)
    End If
End Sub

Thread timers are particularly useful when the System.Windows.Forms.Timer object is unavailable, such as when you are developing console applications.

See Also

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