Inheritance: System.NotifyPropertyChangedBase
 private void DeleteOrder(Order order)
 {
     Orders.Remove (order);
 }
 private bool CanAddLineItem(Order order)
 {
     return order != null;
 }
 private bool CanDeleteOrder(Order order)
 {
     return order != null;
 }
 private void AddLineItem(Order order)
 {
     order.LineItems.Add (new LineItem { Index = order.LineItems.Any () ? order.LineItems.Select (o => o.Index).Max () + 1 : 1, Order = order });
 }