汉字与英文字节不一样导致直接截取到的长度不一样

#region 字符串截取函数
    public static string CutString(string inputString, int len)
    {

        System.Text.ASCIIEncoding ascii = new ASCIIEncoding();
        int tempLen = 0;
        string tempString = "";
        byte[] s = ascii.GetBytes(inputString);
        for (int i = 0; i < s.Length; i++)
        {
            if ((int)s[i] == 63)
            {
                tempLen += 2;
            }
            else
            {
                tempLen += 1;
            }

            try
            {
                tempString += inputString.Substring(i, 1);
            }
            catch
            {
                break;
            }

            if (tempLen > len)
                break;
        }
        //如果截过则加上半个省略号
        byte[] mybyte = System.Text.Encoding.Default.GetBytes(inputString);
        if (mybyte.Length > len)
            tempString += "…";

        return tempString;
    }
    #endregion

前台直接调用该方法