按長度截取字符串

      /// <summary>
        /// 截取字段長度 
        /// </summary>
        /// <param name="str">需要截取的字符串</param>
        /// <param name="Len">最大長度限制</param>
        /// <returns></returns>
        public string CutStringByte(string str, int Len)
        {
            if (string.IsNullOrEmpty(str))
            {
                return "";
            }
            if (System.Text.Encoding.Default.GetByteCount(str) <= Len)
            {
                return str;
            }
            int i = 0;//字節數
            int j = 0;//實際截取長度
            foreach (char newChar in str)
            {
                if ((int)newChar > 127)
                {
                    //漢字占兩個字符
                    i += 2;
                }
                else
                {
                    i++;
                }

                if (i <= Len)
                    j++;
                else
                    break;
            }
            str = str.Substring(0, j);
            return str;
        }