DateTime.ParseExact失败取决于解析的字符串的长度

Question

This is very weird. I have the following code:

Assert.AreEqual(new DateTime(2000, 1, 1), DateTime.ParseExact("2000", "yyyy", CultureInfo.InvariantCulture));
Assert.AreEqual(new DateTime(2000, 1, 1), DateTime.ParseExact("20000705", "yyyy", CultureInfo.InvariantCulture));

The first line passes, the second one fails with " System.FormatException: String was not recognized as a valid DateTime ".

I cannot shorten the string to be parsed to match the length of the format - it would work in this particular case, but this is part of a more generic method, and it would fail somewhere else. Any idea on why the second call fails?

[Edit] Ok, I was hoping it was parsing just as much of the input string as needed to satisfy the format.

I get a lot of strings in a lot of formats, and I have one method that accepts both and, after a bit of processing (I only get dates, so I replace "m" with "M" and "Y" with "y" and so on), I call DateTime.ParseExact . The reason I cannot call DateTime.Parse is because it doesn't allow for a format argument... I can get ddMMyyyy in one part of the program and yyyyddMM in another, there's no way for it to correctly figure it out.

[Edit 2] I guess it's my fault... I'll have to truncate the input when that issue arises. Fortunately I generally have all parts of the date in the format, this (just the year) is a rare occurence. Thanks for the help.

Answer 1

It's absolutely right to fail - you've told it to expect exactly 4 characters, and then given it 8!

Options:

• Provide more format strings (eg "yyyyMMdd") to the call (you can specify multiple valid formats)
• Only pass in the piece of the string you expect to match the format
• Parse the string yourself

You claim that truncating the string "would fail somewhere else" - could you give more details? If you're specifying "yyyy" then presumably you know you need exactly 4 characters... do you know exactly where they'll be within the string? If so, why can't you perform the truncation?

Answer 2

It failed because DateTime.ParseExact requires the input string to match the specified format exactly (err, I mean almost exactly , per your comment below), and "20000705" does not match "yyyy".

To parse "20000705", you'll need to use a different technique - either ParseExact with an appropriate format string (like "yyyyMMdd"), or DateTime.Parse , which is a bit more forgiving (though also less accurate).

It looks like you're trying to extract the year from the date string - no parse method will do that for you.

Instead you should get the actual DateTime represented by the string - July 5th 2000 or May 7th 2000, depending on your format - and then extract the year using DateTime.Year .

Answer 3

The highest date that the DateTime can represent is December 31st 9999. You're asking it to understand a year greater than 9999. You either need to specify a format matching the value you're giving it, so that it understands that just the first 4 digits is the year. "yyyymmdd", for example.