public static class StringPlus
    {
        public static string Reverse(this string s)
        {
            return ReverseOlder(s);
        }

        public static string ReverseOlder(string s)
        {
            string ret = "";
            for (int i = s.Length - 1; i >= 0; i--)
            {
                ret += s[i];
            }
            return ret;
        }
    }


    class Program
    {
        static void Main(string[] args)
        {
            int a = 123;
            //C# 3.0 扩展方法的写法
              Console.WriteLine(a.ToString().Reverse());
            //C# 2.0 普通的写法
              Console.WriteLine(StringPlus.ReverseOlder(a.ToString()));
            Console.ReadKey();
        }
    }