Linq ToList奇怪的行为

Question

为什么要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

如果你有这条线:

from b in a.DefaultIfEmpty()

您明确地说,如果该列表中的项目为空,则应使用默认值.引用类型(这些匿名对象是)的默认值是null,并且您取消引用该值,因此您获得NRE.