按字节数截取字符串

/// <summary>
        /// 截取字符串,取固定字节数,多余的用...替代,为空或者null的返回空字符串
        /// </summary>
        /// <param name="s">要截取的字符串</param>
        /// <param name="len">要截取的字节数</param>
        /// <param name="emptyReturn">当字符串为null或者空时,返回的字符串,比如"暂未内容"之类的</param>
        /// <returns>截取后的字符串</returns>
        public static string CutString(string s, int len,string emptyReturn)
        {
            if (string.IsNullOrEmpty(s)) return emptyReturn;
            if (System.Text.Encoding.Default.GetByteCount(s) <= len)//如果长度比需要的长度n小,返回原字符串
            {
                return s;
            }
            int t = 0 ,i = 0 ,tlen = s.Length;
            char[] cs = s.ToCharArray();
            for (i = 0; i < tlen && t < len; i++)
            {
                if ((int)cs[i] >= 0x4e00 && (int)cs[i] <= 0x9fa5)//是否汉字
                {
                    t += 2;
                }
                else
                {
                    t++;
                }
            }
            return s.Substring(0, i) + "...";
        }