用函数方法实现迭代器

     private static IEnumerable<T> Sequence<T>(Func<T, T> getNext, T startValue, Func<T, bool> endReached)
        {
            if (getNext==null)   
            {
                yield break;
            }
            yield return startValue;
            T val = startValue;
            while (endReached==null||!endReached(val))
            {
                val = getNext(val);
                yield return val;
            }
        } 

　　var s = Sequence(x => x + 2, 1,x=>x>=19);
            foreach (var i in s)
            {
                Console.WriteLine(i);
            }