C＃Dictionary of Dictionary在匹配引用时匹配select中的null值

Question

下面的Console项目代码示例类似于我的代码; irl有一个更复杂，但我所遇到的问题得到充分证明。我无法理解为什么demo方法中的linq select返回null，当where <>子句似乎与foreach循环中使用的表达式匹配时，填充where <>子句的另一侧。

我尝试了相同代码的不同排列，但无论我做什么，我正在使用null thisOrder;我似乎无法超越它能够使用任何东西。

我错过了什么;为什么我的where子句不返回订单字典的内部字典中的订单？

        using System;
        using System.Collections.Generic;
        using System.Linq;

namespace ShowTest
{
    public class Depot
    {
        public string Name { get; set; }
        public Dictionary<Order, Dictionary<int, OrderLine>> Orders { get; set; } = new Dictionary<Order, Dictionary<int, OrderLine>>(new OrderEqualityComparer());
    }
    public class Order
    {
        public string Reference { get; set; }
        public DateTime DepotDate { get; set; }
    }
    #region comparer
    public class OrderEqualityComparer : IEqualityComparer<Order>
    {
        public bool Equals(Order x, Order y)
        {
            return (x.Reference == y.Reference);
        }

        public int GetHashCode(Order obj)
        {
            return (obj.Reference.GetHashCode());
        }
    }
    #endregion
    public class OrderLine
    {
        public int Qty { get; set; }
        public string ExternalRef { get; set; }
    }
    public class Demo
    {
        public static void Main()
        {
            #region Setting up values
            Order order = new Order { DepotDate = DateTime.Parse("15/01/2010"), Reference = "myOrderRef" };
            OrderLine orderLine1 = new OrderLine { ExternalRef = "Foo", Qty = 4 };
            OrderLine orderLine2 = new OrderLine { ExternalRef = "Bar", Qty = 8 };

            var orderLines = new Dictionary<int, OrderLine>();
            orderLines.Add(1, orderLine1);
            orderLines.Add(2, orderLine2);

            var orders = new Dictionary<Order, Dictionary<int, OrderLine>>();
            orders.Add(order, orderLines);

            Depot myDepot = new Depot { Name = "Funhouse", Orders = orders };
            #endregion

            foreach (string oRef in myDepot.Orders.Select(l => l.Key.Reference))
            {
                //for each order reference, there is an order containing many order lines. So, first we get the relevant order
                var thisOrder = (from l in myDepot.Orders
                                 where l.Key.Reference == oRef
                                 select l.Value.Values) as Dictionary<int, OrderLine>;

                if (thisOrder == null) { Console.WriteLine("Why is thisOrder null when the search criterion was retrieved from the order itself?"); }
                else { Console.WriteLine("Hooray, the damnable thing has contents!"); }
            }
        }
    }
}

Answer 1

因为您将thisOrder解释为错误的类型：

as Dictionary<int, OrderLine>

由于表达式的结果不是Dictionary<int, OrderLine>，因此尝试将其转换为一个的结果是null。结果是一个集合，甚至不是Dictionary<int, OrderLine>对象，而是Dictionary<int, OrderLine>.ValueCollection对象。 （包含一个元素的集合，但仍然是一个集合。）

您可以从该集合中进行选择。例如，如果你想要的是Dictionary<int, OrderLine>：

var thisOrder = (from l in myDepot.Orders
                 where l.Key.Reference == oRef
                 select l.Value).FirstOrDefault();

总的来说，主要的一点是你的查询工作正常，但你告诉系统使用错误的类型。