{

     public TimerTesting()

     {

     InitializeComponent();

     System.Threading.TimerCallback tc = new System.Threading.TimerCallback(this.OnTimerCallback);

     System.Threading.Timer objTimer = new System.Threading.Timer(tc);

     objTimer.Change(0, 500);

     }

     private void OnTimerCallback(Object obj)

     {

          Random r = new Random();

          this

     }

}

When we wanted to set the background of the window in the Callback method this exception fires. "The calling thread cannot access this object because a different thread owns it.". This problem is due to UI thread in WPF is different from the thread that owns our timer. In fact, the Dispatcher object manage the UI (in this case the window). Therefore updating the UI should handle with Dispatcher object that owns it. So we should change our code in the CallBack method to this way:

private

{

     this.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal,

     (System.Threading.ThreadStart)delegate()

     {

     Random r = new Random();

     this.Background = new SolidColorBrush(Color.FromRgb((byte)(r.Next(0, 255)), (byte)(r.Next(0, 255)), (byte)(r.Next(0, 255))));

     });

}

In WPF the only thread that created the DispatcherObject may access that object. Therefore we should first find the Dispatcher object of the window then tell it to update the data on the window. Moreover, The BeginInvoke method of the DispatcherObject executes the delegate asynchronously on the thread that the Dispatcher is associated with which in our code is the DispatcherObject of the window.

In the Case of Timer, we can use three kinds of Timers in WPF which two of them were available before WPF; As we dicussed above System.Threading.Timer and System.Timers.Timer. But the third one is made especially for WPF; System.Windows.Threading.DispatcherTimer. The difference between this timer with the two others is that it belongs to the same thread as WPF UI thread. By that I mean the DispatcherObject owns it. Better say, in the case of our example above the thread of the window and the timer are the same so we can Update the UI data without any problem.

public

You can download the complete source code here.