static void M1()
{
  var list = new List<int>() { 1,2,3,4 };
  IEnumerator<int> iterator = list.GetEnumerator();
  while (iterator.MoveNext())
  {
     Console.Write(iterator.Current);
  }
  iterator.Reset();
  while (iterator.MoveNext())
  {
      Console.Write(iterator.Current);
  }
}
static void M2()
{
  var list = new List<int>() { 1,4 };
  //Here the iterator will be List<int>.Enumerator (which is a struct)
  var iterator = list.GetEnumerator();
  while (iterator.MoveNext())
  {
     Console.Write(iterator.Current);
  }
  //This will not work,as Reset method was implemented explicitly
  //iterator.Reset();

  //So casting it to IEnumerator is required
  //which will lead to Boxing and other issues of struct and interface
  ((IEnumerator<int>)iterator).Reset();

  //Following loop will NOT work
  while (iterator.MoveNext())
  {
    Console.Write(iterator.Current);
  }
}

有几个问题清楚地解释了这个行为,你可以检查他们here,here和here.

我还有以下两个疑问

>为什么List.Enumerator不会为重置抛出“NotSupportedException”？
>为什么重置被明确地实现,而不是隐含地像MoveNext和Current？