The SelectionChanged event is designed to handle controls that allow multiple selections,
so it can be a little confusing for a single-selection selector such as ComboBox. The
SelectionChangedEventArgs type passed to event handlers has two properties of type
IList: AddedItems and RemovedItems. AddedItems contains the new selection and
RemovedItems contains the previous selection.

e.g.
void ComboBoSelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (e.AddedItems.Count > 0)
object newSelection = e.AddedItems[0];
}
Like this code, never assume that there’s a selected item! Besides the fact that ComboBox’s
selection can be cleared programmatically, it can get cleared by the user when IsEditable is
true and IsReadOnly is false. In this case, if the user changes the selection box text to
something that doesn’t match any item, the SelectionChanged event is raised with an
empty AddedItems collection.