It looks like Silverlight 5's ComboBox is more restrictive. In Silverlight 4 I could change the ItemsSource of a ComboBox that had a SelectedItem != null, while in Silverlight 5 I have to set SelectedItem to null before I assign a new ItemsSource. Why even mention this? Isn't this obvious? Well, the Silverlight 4 ComboBox simply ignored the SelectedItem and did not complain about the new ItemsSource, so moving to Silverlight 5 may cause some (unexpected) pain. But the real reason for this post is...