yield 递归

简单例子：

public static System.Collections.Generic .IEnumerable<int> Power(int number, int exponent)
        {
            int result = 1;
            for (int i = 0; i < exponent; i++)
            {
                result = result * number;
                yield return result;
            }
        }

 var abc = Power(2, 8);

            foreach (int i in Power(2, 8))
            {
                Console.Write("{0} ", i);
            }

abc,输出结果是

2 4 8 16 32 64 128 256

http://msdn.microsoft.com/zh-cn/library/9k7k7cf0.aspx

http://www.cnblogs.com/ldp615/archive/2011/10/07/can-you-find-the-error-in-foreach-extension-method.html