按指定长度截取字符串(以字节计算长度)

using System.Text.RegularExpressions;

        /// <summary>
        /// 按指定长度截取字符串(以字节计算长度)
        /// </summary>
        /// <param name="stringToSub"></param>
        /// <param name="length"></param>
        /// <returns></returns>
        public static string GetFirstString(string stringToSub, int length)
        {
            if (System.Text.Encoding.Default.GetByteCount(stringToSub) <= length)
                return stringToSub;

            Regex regex = new Regex("[\u4e00-\u9fa5]+", RegexOptions.Compiled);
            char[] stringChar = stringToSub.ToCharArray();
            StringBuilder sb = new StringBuilder();
            int nLength = 0;
            bool isCut = false;
            for (int i = 0; i < stringChar.Length; i++)
            {
                if (regex.IsMatch((stringChar[i]).ToString()))
                {
                    sb.Append(stringChar[i]);
                    nLength += 2;
                }
                else
                {
                    sb.Append(stringChar[i]);
                    nLength = nLength + 1;
                }

                if (nLength > length)
                {
                    isCut = true;
                    break;
                }
            }
            if (isCut)
                return sb.ToString() + ""; //可加上省略号
            else
                return sb.ToString();
        }