Programming WPF, 2nd Edition

The Dispatcher

Each thread that creates user interface objects needs a Dispatcher object. This effectively owns the thread, running a loop that dispatches input messages to the appropriate handlers. (It performs a similar role to a message pump in Win32.) As well as handling input, the dispatcher enables us to get calls directed through to the right thread.

Tip

The Dispatcher class lives in the System.Windows.Threading namespace along with all other WPF-specific threading classes, including DispatcherObject.

Obtaining a Dispatcher

Recall that all WPF objects with thread affinity derive from the DispatcherObject base class. This class defines a Dispatcher property, which returns the Dispatcher object for the thread to which the object belongs.

You can also retrieve the Dispatcher for the current thread by using the Dispatcher.CurrentDispatcher static property.

Getting Onto the Right Thread with a Dispatcher

If you need to update the user interface after doing some work on a worker thread, you must make sure the update is done on the UI thread. The Dispatcher provides methods that let you invoke the code of your choice on the dispatcher's thread.

You can use either Invoke or BeginInvoke. Both of these accept any delegate and an optional list of parameters. They both invoke the delegate's target method on the dispatcher's thread, regardless of which thread you call them from. Invoke does not return until the method has been executed, whereas BeginInvoke queues the request to invoke the method, ...