多角度让你彻底明白yield语法糖的用法和原理及在C#函数式编程中的作用

可以看到这地方做了一个new ConsoleApp2.Program/'d__1’操作，然后进行了<>3__nums=0，最后再把这个迭代类返回出来，这就解释了为什么你的GetList可以是IEnumerable而不报错。

你可能会说，你说了这么多有啥用？ IL代码我也看不懂，如果能回写成C#代码那就了，还好回写成C#代码不算太难。。。

namespace ConsoleApp2
{
    class GetListEnumerable : IEnumerable, IEnumerator
    {
 private int state;
 private int current;
 private int threadID;
 public int[] nums;
 public int[] s1_nums;
 public int s2;
 public int num53;

 public GetListEnumerable(int state)
 {
     this.state = state;
     this.threadID = Environment.CurrentManagedThreadId;
 }
 public int Current => current;

 public IEnumerator GetEnumerator()
 {
     GetListEnumerable rangeEnumerable;

     if (state == -2 && threadID == Environment.CurrentManagedThreadId)
     {
  state = 0;
  rangeEnumerable = this;
     }
     else
     {
  rangeEnumerable = new GetListEnumerable(0);
     }

     rangeEnumerable.nums = nums;
     return rangeEnumerable;
 }

 public bool MoveNext()
 {
     switch (state)
     {
  case 0:

      state = -1;
      s1_nums = nums;
      s2 = 0;
      num53 = s1_nums[s2];
      current = num53;
      state = 1;
      return true;
  case 1:
      state = -1;
      s2++;

      if (s2 < s1_nums.Length)
      {
   num53 = s1_nums[s2];
   current = num53;
   state = 1;
   return true;
      }

      s1_nums = null;
      return false;
     }
     return false;
 }
 object IEnumerator.Current => Current;
 public void Dispose() { }
 public void Reset() { }
 IEnumerator IEnumerable.GetEnumerator() { return this.GetEnumerator(); }
    }
}

    public static class Program
    {
 public static void Main(string[] args)
 {
     //哈哈，我最后想要HashSet。。。因为我要做高效的集合去重
     var hashSet1 = new HashSet(GetList(new int[] { 1, 2, 3, 4, 5 }));
     var hashSet2 = new HashSet(GetList2(new int[] { 1, 2, 3, 4, 5 }));
 }

 //编码阶段就预先决定了用List承载
 public static List GetList(int[] nums)
 {
     return nums.Where(num => num % 2 == 0).ToList();
 }

 //编码阶段还没想好用什么集合承载，有可能是HashSet,SortList，鬼知道呢？
 public static IEnumerable GetList2(int[] nums)
 {
     foreach (var num in nums)
     {
  if (num % 2 == 0) yield return num;
     }
 }
    }

    public static class Program
    {
 public static void Main(string[] args)
 {
     var nums = M1(true).ToList();
 }

 public static IEnumerable M1(bool desc)
 {
     return desc ? M2(2).OrderByDescending(m => m) : M2(2).OrderBy(m => m);
 }

 public static IEnumerable M2(int mod)
 {
     return M3(0, 10).Where(m => m % mod == 0);
 }

 public static IEnumerable M3(int start, int end)
 {
     var nums = new int[] { 1, 2, 3, 4, 5 };
     return nums.Where(i => i > start && i < end);
 }
    }

 static IEnumerable TakeWhileIterator(IEnumerable source, Func predicate) {
     foreach (TSource element in source) {
  if (!predicate(element)) break;
  yield return element;
     }
 } 

static IEnumerable WhereIterator(IEnumerable source, Func predicate) {
     int index = -1;
     foreach (TSource element in source) {
  checked { index++; }
  if (predicate(element, index)) yield return element;
     }
 }