string msg = "liyifeng";         //判断这个字符串中出现i的次数，和每次出现的索引位置             int count = 0;             string keywords = "i";           int index = 0;             while ((index = msg.IndexOf(keywords, index)) != -1) {                 count++;                 Console.WriteLine("第{0}次出现【{1}】，索引是：{2}", count, keywords, index);                  //下次从这个词末的位置开始查找，所以要跳过本次该词的长度个字符位置继续查找                 index = index + keywords.Length;            }             Console.WriteLine("==========================================");            Console.WriteLine("【{0}】一次一共出现了{1}次。", keywords, count);            Console.ReadLine();