Order.cs

 public class Order
  {
      public OrderId OrderId { get; }
      public CustomerId CustomerId { get; }
      public IReadOnlyCollection<OrderLine> OrderLines { get; }
      public IPaymentMethod PaymentMethod {get;}
      public DiscountCode DiscountCode {get;}


      public Order(OrderId orderId,
                   CustomerId customer,
                   IReadOnlyCollection<OrderLine> orderLines,
                   IPaymentMethod paymentMethodId,
                   DiscountCode discountCode
                  )
      {
          OrderId = orderId;
          Customer = customer;
          OrderLines = orderLines;
          PaymentMethodId = paymentMethodId;
          DiscountCode = discountCode;
      }
   }