DispatchTimer - Prevent the tick event to be triggered if the previous tick is still running
DispatcherTimer dt = new DispatcherTimer();
dt.Interval = new TimeSpan(0, 0, 0, 0, 500); // 500 Milliseconds
dt.Tick += new EventHandler(dt_Tick);
dt.Start();

if(!busy) {
  busy = true;

  // some code which could take longer than 500 ms

  busy = false;
}