public void CreateOrder(Order order) { if (order.orderID == 0) { List <Order> allOrders = GetOrders(); Order lastOrder = allOrders.Aggregate((o1, o2) => (int)o1.orderID > (int)o2.orderID ? o1 : o2); order.orderID = lastOrder.orderID + 1; } _repo.CreateOrder(order); }