ComboBox ItemsSource changed => SelectedItem is ruined

Question

Ok, this has been bugging me for a while now. And I wonder how others handle the following case:

Answer 1

After loosing half of my head hairs and smashing my keyboard several times, i think that for the combobox control, it is preferable not to write the selectedItem,Selectedindex and ItemsSource binding expression in the XAML as we cannot check whether the ItemsSource has changed, when using ItemsSource property of course.

In the window or user control constructor i set the ItemsSource property of the Combobox then in the loaded event handler of the window or user control, i set the binding expression and it work perfectly. If i would set ItemsSource binding expression in the XAML without the "selectedItem" one, i wouldn't find any event handler to set the SelectedItem binding expression while preventing the combobox to update source with a null reference (selectedIndex = -1).