Significance of declaring a WPF event handler as 'async' in C# 5

You may find my async/await intro helpful.

An async method is re-written by the compiler to support the await operator. Every async method starts out synchronous (in this case, on the UI thread) until it awaits some operation (that is not already completed).

By default, the context is saved, and when the operation completes, the rest of the method is scheduled to execute in that context. The "context" here is SynchronizationContext.Current unless it is null, in which case it is TaskScheduler.Current. As Drew pointed out, WPF provides a DispatcherSynchronizationContext which is tied to the WPF Dispatcher.

Regarding error handling:

When you await a Task inside a WPF async void event handler, the error handling goes like this:

• The Task completes with an error. The exception is wrapped into an AggregateException, like all Task errors.
• The await operator sees that the Task completed with an error. It unwraps the original exception and re-throws it, preserving the original stack trace.
• The async void method builder catches the exception escaping from an async void method and passes it to the SynchronizationContext that was active when the async void method started executing (in this case, the same WPF context).
• The exception is raised (with the original stack trace, and without any annoying AggregateException wrapping) on the Dispatcher.

This is rather convoluted, but the intent is to have exceptions raised from async event handlers be practically the same as exceptions raised from regular event handlers.

A partial answer. From MSDN:

An async method that has a void return type can’t be awaited, and the caller of a void-returning method can't catch any exceptions that the method throws.

So any errors would only be available via the TaskScheduler.

Also, there's nothing XAML-specific going on with the event handler registration. It could have been done in code:

this.button.Click += OnButtonClick;

Or even as an async lambda:

this.button.Click += async (s,e) => { ... };

As for safety of UI updates after an await, it seems that the continuation is executed within SynchronisationContext.Current, which is set per thread. In WPF, this is a DispatcherSynchronisationContext that's coupled to the WPF Dispatcher that pumped the event in the first place.