Linq ToList奇怪的行为

Question

为什么List<Component> l = second.ToList(); 导致

System.NullReferenceException”发生在ConsoleApplication1.exe中

在select new { id = b.id, Name = b.Name };

如何解决这个问题？

namespace ConsoleApplication1
{
class Program
{
    static void Main(string[] args)
    {
        List<Component> firstlist = new List<Component>();
        List<Component> secondlist = new List<Component>();
        List<Component> thirdlist = new List<Component>();

        firstlist.Add(new Component { id = 1, Name = "Jhon" });
        secondlist.Add(new Component { id = 2, Name = "Jhon" });
        thirdlist.Add(new Component { id = 3, Name = "Jhon" });

        var first = from d in firstlist
                                 join i in secondlist
                                 on d.id equals i.id
                                 into a
                                 from b in a.DefaultIfEmpty()
                                 select new { id = b.id, Name = b.Name };

        var second = from f in first
                     join s in thirdlist
                     on f.id equals s.id
                     into a
                     from b in a.DefaultIfEmpty()
                     select new Component { id = b.id, Name = b.Name };

        List<Component> l = second.ToList();

    }

    public class Component
    {
        public int id { get; set; }

        public string Name { get; set; }
    }
}
}

Answer 1

当您有一行时：

from b in a.DefaultIfEmpty()

您是专门说如果列表中的项目为空，则应使用默认值。 引用类型（那些匿名对象是）的默认值是null ，并且您正在取消引用该值，因此得到一个NRE。

Answer 2

第一列表和第二列表之间没有相等关系，联接无法工作;）