// 将当前的购物车中的项目转换为订单项
 public OrderItem ConvertToOrderItem()
 {
     OrderItem orderItem = new OrderItem();
     orderItem.Id = Guid.NewGuid(); // 为每个SalesLine设置一个不同的ID,以便EF的Context能够识别不同的OrderItem
     orderItem.Product = this.Product;
     orderItem.Quantity = this.Quantity;
     return orderItem;
 }
 // 将当前的购物车中的项目转换为订单项
 public OrderItem ConvertToOrderItem()
 {
     var orderItem = new OrderItem
     {
         Id = Guid.NewGuid(), 
         Product = this.Product, 
         Quantity = this.Quantity
     };
     return orderItem;
 }