C#中的yield关键字

 public class CityCollection : IEnumerable<string>
  {
      string[] _Items = new string[] { "黑龙江", "吉林", "辽宁", "山东", "山西", "陕西", "河北", "河南", "湖南", "湖北", "四川", "广西", "云南", "其他" };
      IEnumerator<string> IEnumerable<string>.GetEnumerator()
      {
          for (int i = 0; i < _Items.Length; i++)
          {
              yield return _Items[i];
              yield return string.Format("Index:{0}", i);
         }
     }
     IEnumerator IEnumerable.GetEnumerator()
     {
         for (int i = 0; i < _Items.Length; i++)
         {
             yield return _Items[i];
         }
     }
 }