无法将字符串识别为有效的DateTime格式异常

Question

I am trying to do a simple parsing using following statements:

//In actual code the date time value comes from db
var dateTime = new DateTime(2018, 04, 26);

var dtExtact = DateTime.ParseExact(dateTime.ToString(), "dd MMM yyyy HH:mm:ss:fff",null);

Now when I try doing this getting error,

System.FormatException: 'String was not recognized as a valid DateTime.'

I did have a look at the MSDN sample, but it does not provide any pointers on whats wrong with my date time.

Answer 1

If you would like to parse a date-time string in a specific format, make sure that your formatted date/time string matches the format of your parser.

In your example "round-tripping" a date/time is easy to achieve by reusing the same format string for formatting the date and for parsing:

var dateTime = new DateTime(2018, 04, 26);
const string dateFormat = "dd MMM yyyy HH:mm:ss:fff";
var dtExact = DateTime.ParseExact(
    dateTime.ToString(dateFormat)
,   dateFormat
,   null
);
Console.WriteLine("{0} {1}", dateTime, dtExact);

Demo.

Answer 2

Both ToString() and Parse() are designed to automagically extract the Region setting from Windows. It is rarely a good idea to override this behavior. And apparently your pattern is not a valid one. For all we know, ':' is not actually the proper cultural seperator for Time elements

Overall this seems rather suspect - you turn a DateTime to string, only to parse it again right there. I can only guess it is for testing. But testing for what is the question.

I have 3 general rules when dealing with DateTimes:

• Always store, retrieve and transmit the UTC value. You do not want ot add timezones to your issues. That way lies madness . There are rare exceptions, but then you are pretty much on your own to deal with that mess
• Avoid storing, retreival or transmission as Text. keep it in proper DataTypes as long as possible
• If you can follow the 2nd rule (using XML or some other serialsiation), at least pick a fixed Culture Format, Format String and string encoding at all endpoints. You do not want to add those issues to your worries

Following those rules, I rarely had any issues.