字母递增

　　　　 /// <summary>
        /// 过滤，取最大值
        /// </summary>
        /// <returns>Id</returns>
        public string FilteringId()
        {
            // 英字4桁のIdからMAX値取得
            var regex = new Regex(@"^[a-z]{4}$");
            string[] idList = this.XXRepo.All().Select(m => m.Id).ToArray();
            string maxId = idList.Where(m => regex.IsMatch(m) && m != "zzzz").Max();
            if (string.Empty.Equals(maxId))
            {
                return "aaaa";
            }
            return this.NumberingForId(maxId);
        }
//------------------------------cut-line---------------------------------------------------------
　　　　 /// <summary>
        /// 进位
        /// </summary>
        /// <param name="maxId">maxId</param>
        /// <returns>Id</returns>
        public string NumberingForId(string maxId)
        {
            int length = maxId.Length;
            char[] ch = new char[length];
            for (int i = 0; i < length; i++)
            {
                ch[i] = maxId[i];
            }
            // 进位
            bool carry = true;
            int p = length;
            while (carry)
            {
                p--;
                ch[p] = this.LetterIncrement(ch[p]);
                if (ch[p] == 'a')
                {
                    carry = true;
                }
                else
                {
                    carry = false;
                }
            }
            return new string(ch);
        }
//-------------------cut-line-------------------------------------------------
　　　　 /// <summary>
        /// 进位方法
        /// </summary>
        /// <param name="ch">ch</param>
        /// <returns>ch</returns>
        public char LetterIncrement(char ch)
        {
            if (ch == 122)
            {
                ch = (char)97;
            }
            else
            {
                ch++;
            }
            return ch;
        }