C#将时间格式由yyyyMMdd转化成yyyy-MM-dd

        string str = DateTime.Now.ToString("yyyyMMdd");
        string[] format = {"yyyyMMdd"};
        DateTime date;
        if (DateTime.TryParseExact(str, 
                                   format, 
                                   System.Globalization.CultureInfo.InvariantCulture,
                                   System.Globalization.DateTimeStyles.None, 
                                   out date))
        {
             ....
        }
        else
             ....