Dispatcher

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WPF controls can only be used from the thread of their dispatcher, the UI thread. After running work on a thread-pool thread, await dispatcher.SwitchToDispatcherThread() continues the method on the thread of the dispatcher, without wrapping the rest of the method in a delegate:

// Do the slow work on a thread-pool thread
var lines = await Task.Run(() => Enumerable.Range(1, 1000).Select(i => $"Line {i}").ToList());

// Continue on the thread of the dispatcher, usually Application.Current.Dispatcher, to update the controls
await dispatcher.SwitchToDispatcherThread();
Assert.True(dispatcher.CheckAccess());

When the code already runs on the thread of the dispatcher, the await continues immediately.
Otherwise, the rest of the method is queued to the dispatcher with BeginInvoke, at the default priority.
When the dispatcher shuts down before running the continuation, for instance because the application is closing, the method still continues instead of waiting forever, but the controls can no longer be used. Check dispatcher.HasShutdownStarted before using them if the application can close while the work runs.
SwitchToDispatcherThread is an extension method of Dispatcher: use Application.Current.Dispatcher, or the Dispatcher property of a control.