使用yield return 语句可一次返回一个元素。

   迭代器的声明必须满足以下要求：

• 返回类型必须为 IEnumerable、IEnumerable<T>、IEnumerator 或 IEnumerator<T>。

　返回 IEnumerable 或 IEnumerator 的迭代器的 yield 类型为 object。如果迭代器返回 IEnumerable<T> 或 IEnumerator<T>，则必须将yield return 语句中的表达式  　　类型隐式转换为泛型类型参数。

   你不能在具有以下特点的方法中包含 yield return 或 yield break 语句：

• 匿名方法。 
  

• 包含不安全的块的方法。

public class Student    {        public String Name { get; set; }        public int Age { get; set; }        //代码更简洁        public IEnumerable<Student> Students        {            get            {                yield return new Student { Name = "Tadpole", Age = 400 };                yield return new Student { Name = "Pinwheel", Age = 25 };                yield return new Student { Name = "Milky Way", Age = 0 };                yield return new Student { Name = "Andromeda", Age = 3 };            }        }         public IEnumerable<Student> Students2        {            get            {                List<Student> list = new List<Student>();                list.Add(new Student { Name = "Tadpole", Age = 400 });                list.Add(new Student { Name = "Tadpole", Age = 400 });                list.Add(new Student { Name = "Tadpole", Age = 400 });                list.Add(new Student { Name = "Tadpole", Age = 400 });                list.Add(new Student { Name = "Tadpole", Age = 400 });                return list;            }        }    }

    //将不会返回空        public static IEnumerable<string> Test()        {            yield break;        }