title: WPF DisconnectedItem issue
body: <p>I have a problem concerning the issue of DisconnectedItem in wpf bindings.</p>
  
  <p>I should say I have a List namely ProvinceList and in each Province I have an ObservableCollection wich is bound to the ItemsSource property of a ListBox.</p>
  
  <p>The issue comes when I call ProvinceList.Clear() when the Visual Studio throws and InvalidCastException exception and when I trace it I see that the cast object is a City is returning DisconnectedItem.</p>
  
  <p>I have no idea what the problem is. any help is appreciated.</p>