/// <summary>
        /// 每隔n个字符插入一个字符
        /// </summary>
        /// <param name="input">源字符串</param>
        /// <param name="interval">间隔字符数</param>
        /// <param name="value">待插入值</param>
        /// <returns>返回新生成字符串</returns>
        public static string InsertFormat(string input, int interval, string value)
        {
            for (int i = interval; i < input.Length; i += interval + 1)
                input = input.Insert(i, value);
            return input;
        }

 

  //用例

  public static void Main()
        {
            string stest= "EMKEMEMM3335368CKGE43MI3";
            Console.WriteLine( InsertFormat(stest,4,"-"));  // "EMKE-MEMM-3335-368C-KGE4-3MI3"
            Console.ReadKey();
        }