static void OnItemsSourceChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
MyComboBox box = (MyComboBox)obj;
box.PopulateInternalItems();
box.AddUnknownItem();
}
static void OnSelectedValueChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
MyComboBox box = (MyComboBox)obj;
box.AddUnknownItem();
}