DispatcherTimer Constructors

How come in some classes I have to specify this constructor to get the c开发者_StackOverflow中文版allback to be invoked?

_timer = new DispatcherTimer(DispatcherPriority.Normal, Application.Current.Dispatcher);

Instead of just:

_timer = new DispatcherTimer();

The default constructor will use the current Dispatcher instead of the current application's Dispatcher. If this is called on a thread other than the UI thread, the first overload is required in order to get the correct synchronization context.