Linq的let语句设置对null

Question

在Linq查询中使用SortedList时，包含null的某些对象星座会引用设置为let的列表。

这是一个最低限度的工作示例。其中大部分内容只是为了确保它能够反映我正在从事的工作;重要的部分是在最后一行，因NullReferenceException为testList["1"].Elements而引发null。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TypecastTest {

  public abstract class ComplexBase { }

  public abstract class ComplexClass : ComplexBase {
    internal SortedList<string, ComplexElement> Elements { get; set; }
  }

  public abstract class ComplexElement : ComplexBase { }

  public class ComplexClassA : ComplexClass {
    internal new SortedList<string, ComplexElementA> Elements { get; set; }

    public ComplexClassA() {
      Elements = new SortedList<string, ComplexElementA>();
    }
  }

  public class ComplexElementA : ComplexElement { }

  public class ComplexClassB : ComplexClass {
    internal new SortedList<string, ComplexElementB> Elements { get; set; }

    public ComplexClassB() {
      Elements = new SortedList<string, ComplexElementB>();
    }
  }

  public class ComplexElementB : ComplexElement { }

  public class TheBigCollection {
    internal SortedList<string, ComplexClass> Classes { get; set; }

    public TheBigCollection() {
      Classes = new SortedList<string, ComplexClass>();
    }
  }

  class Program {
    static void Main(string[] args) {
      ComplexClassA ccA1 = new ComplexClassA();
      ComplexElementA ceA1a = new ComplexElementA();
      ComplexElementA ceA1b = new ComplexElementA();
      ccA1.Elements.Add("A1a", ceA1a);
      ccA1.Elements.Add("A1b", ceA1b);

      ComplexClassA ccA2 = new ComplexClassA();
      ComplexElementA ceA2a = new ComplexElementA();
      ComplexElementA ceA2b = new ComplexElementA();
      ccA2.Elements.Add("A2a", ceA2a);
      ccA2.Elements.Add("A2b", ceA2b);

      ComplexClassB ccB1 = new ComplexClassB();
      ComplexElementB ceB1a = new ComplexElementB();
      ComplexElementB ceB1b = new ComplexElementB();
      ccB1.Elements.Add("B1a", ceB1a);
      ccB1.Elements.Add("B1b", ceB1b);

      ComplexClassB ccB2 = new ComplexClassB();
      ComplexElementB ceB2a = new ComplexElementB();
      ComplexElementB ceB2b = new ComplexElementB();
      ccB2.Elements.Add("B2a", ceB2a);
      ccB2.Elements.Add("B2b", ceB2b);

      TheBigCollection tbc = new TheBigCollection();
      tbc.Classes.Add("1", ccA1);
      tbc.Classes.Add("2", ccA2);
      tbc.Classes.Add("3", ccB1);
      tbc.Classes.Add("4", ccB2);

      var query = from kvp in tbc.Classes
                  let tmp = new {
                    tmpKey = kvp.Key,
                    tmpValue = kvp.Value
                  }
                  select tmp;
      Dictionary<string, ComplexClass> testDictionary = query.ToDictionary(x => x.tmpKey, x => x.tmpValue);
      SortedList<string, ComplexClass> testList = new SortedList<string, ComplexClass>(testDictionary);
      int test = testList["1"].Elements.Count;
    }
  }
}

为什么这是null？在let中生成匿名对象会阻止正确复制引用会发生什么？