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Thread Timers

The Threading.Timer class is useful to periodically run 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. 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 Control+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)
   Dim x As Integer
   ' Use the interlocked class to increment the counter variable.
   System.Threading.Interlocked.Increment(State.SomeValue)
   Debug.WriteLine("Launched new thread  " & Now)
   If State.TimerCanceled Then    ' Dispose Requested.
      State.TimerReference.Dispose()
      Debug.WriteLine("Done  " & Now)
   End If
End Sub

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

See Also

Multithreaded Applications | Thread Pooling | Thread Synchronization | Parameters and Return Values for Multithreaded Procedures | Multithreading with Forms and Controls | System.Threading Namespace | SyncLock Statement

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