字符串截取

/**//// <summary>
        /// 截取字符串
        /// </summary>
        /// <param name="input">输入字符</param>
        /// <param name="length">截取长度</param>        
        /// <param name="isChar">是否是单字节\r\nTrue:单字节长度.  False:双字节长度</param>
        /// <returns></returns>
        public static string Truncate(string input, int length,bool isChar)
        {
            int tmpLenght = GetBytesLength(input);
            int trunLength=0;
            if (!isChar)
            {
                trunLength = length;
                length *= 2;
            }
            else
            {
                trunLength = length / 2;
            }
            if (length >= tmpLenght)
                return input;
            tmpLenght = GetBytesLength(input.Substring(0, trunLength));
            int tempT = 0;
            bool isTrue = true;
            while (isTrue)
            {
                tempT = (length - tmpLenght) / 2 + 1;

                tmpLenght += GetBytesLength(input.Substring(trunLength, tempT));
                if (tmpLenght > length)
                {

                    isTrue = false;
                    tempT--;

                    trunLength += tempT;
                }
                else
                    trunLength += tempT;


            }
            return input.Substring(0, trunLength);
        }
        /**//// <summary>
        /// 得到字符串的字节长度
        /// </summary>
        /// <param name="input"></param>
        /// <returns></returns>
        public static int GetBytesLength(string input)
        {
            return Encoding.Default.GetByteCount(input.ToCharArray());
        }