截取字符串，区分中英字母长度

  /// <summary>
        ///  截取字符串
        /// </summary>
        /// <param name="str"></param>
        /// <param name="maxByteCount"></param>
        /// <returns></returns>
        private static string TrancateString(string str, int maxByteCount)
        {
            if (string.IsNullOrWhiteSpace(str))
            {
                return "";
            }
            var count = Encoding.Default.GetByteCount(str);
            if (count > maxByteCount)
            {
                var result = "";
                var charnum = 0;
                var array = str.ToCharArray();
                for (int i = 0; i < array.Length; i++)
                {
                    var one = str[i];
                    if (one >= 0x4e00 && one <= 0x9fa5)
                    {
                        charnum += 2;
                    }
                    else
                        charnum += 1;
                    if (maxByteCount >= charnum)
                        result += one;
                    else
                    {
                        return result;
                    }
                }
                return result;
            }
            else
            {
                return str;
            }
        }