The previous behaviour was to raise an exception unless Execute.Dispatcher
had been explicitly defined. This was to detect cases where the user hadn't
set up Execute correctly, and treat them as errors rather than simply going
ahead with some possibly-unexpected behaviour.
However, since BootstrapperBase sets Execute.Dispatcher automatically, it's
highly unlikely that itwould ever not be set when it needed to be. Exceptions
are design mode and unit tests, both of which want (or can cope with) a
synchronous dispatcher.
Hence the behaviour change. Execute.Dispatcher is a synchronous dispatcher
by default, but is overridden to one that uses Application.Current.Dispatcher
by the bootstrapper if we're in a real application.
This is because PropertyChanged events can be async - WPF handles the
marshalling. However if CollectionChanged events are async, then the
thing listening to the event can still be iterating the collection while
the user's editing it, leading to irritating little exceptions
Antony Male