public static Order AddOrder(this OrderData orderData, Customer customer, DateTime orderDate) { int orderNumber = orderData.Any() ? orderData.Max(m => m.OrderNumber) + 1 : OrderData.OrderNumberSeed; Order order = new Order() { CustomerNumber = customer.CustomerNumber, OrderNumber = orderNumber, OrderDate = orderDate, OrderDetails = new List <string>() }; orderData.Add(order); return(order); }