Why can't DateTime.ParseExact() parse "9/1/2009" using "M/d/yyyy"

Question

I have a string that looks like this: "9/1/2009". I want to convert it to a DateTime object (using C#).

This works:

DateTime.Parse("9/1/2009", new CultureInfo("en-US"));

But I don't understand why this doesn't work:

DateTime.ParseExact("9/1/2009", "M/d/yyyy", null);

There's no word in the date (like "September"), and I know the specific format, so I'd rather use ParseExact (and I don't see why CultureInfo would be needed). But I keep getting the dreaded "String was not recognized as a valid DateTime" exception.

Thanks

A little follow up. Here are 3 approaches that work:

DateTime.ParseExact("9/1/2009", "M'/'d'/'yyyy", null);
DateTime.ParseExact("9/1/2009", "M/d/yyyy", CultureInfo.InvariantCulture);
DateTime.Parse("9/1/2009", new CultureInfo("en-US"));

And here are 3 that don't work:

DateTime.ParseExact("9/1/2009", "M/d/yyyy", CultureInfo.CurrentCulture);
DateTime.ParseExact("9/1/2009", "M/d/yyyy", new CultureInfo("en-US"));
DateTime.ParseExact("9/1/2009", "M/d/yyyy", null);

So, Parse() works with "en-US", but not ParseExact... Unexpected?

Answer 1

I suspect the problem is the slashes in the format string versus the ones in the data. That's a culture-sensitive date separator character in the format string, and the final argument being null means "use the current culture". If you either escape the slashes ("M'/'d'/'yyyy") or you specify CultureInfo.InvariantCulture, it will be okay.

If anyone's interested in reproducing this:

// Works
DateTime dt = DateTime.ParseExact("9/1/2009", "M'/'d'/'yyyy", 
                                  new CultureInfo("de-DE"));

// Works
DateTime dt = DateTime.ParseExact("9/1/2009", "M/d/yyyy", 
                                  new CultureInfo("en-US"));

// Works
DateTime dt = DateTime.ParseExact("9/1/2009", "M/d/yyyy", 
                                  CultureInfo.InvariantCulture);

// Fails
DateTime dt = DateTime.ParseExact("9/1/2009", "M/d/yyyy", 
                                  new CultureInfo("de-DE"));