private void ProductListTreeView_Selected(object sender, RoutedEventArgs e)
 {
     if ((e.OriginalSource as TreeViewItem).HasHeader)
     {
         CityViewModel cv = (e.OriginalSource as TreeViewItem).Header as CityViewModel;
         if (cv != null)
         {
             cv.LoadDetails(this);
             _selectedSymbol = cv.Name;
         }
     }
 }