Question

I have a date string that is coming in as what I believe to be Mddyyyy. However, TryParseExact doesn’t seem to be working. Here’s the sample code that fails:

string datestring = "1212012";

DateTime td;
if (DateTime.TryParseExact(datestring, "Mddyyyy", new CultureInfo("en-US"),     DateTimeStyles.None, out td))
{
    Console.WriteLine(td.ToShortDateString());
}
else
{
    Console.WriteLine("Invalid Date String");
}

That same code works if there’s a leading zero, but I would think then that the leading zero would only work with a formatting string of MMddyyyy.

Answer 1

Here I propose an explanation and provide evidence for the proposal.

Proposed Explanation: The parser internally uses the format string to create a regular expression that contains a greedy quantifier (which means, in this case, it prefers to match 2-digit months over 1-digit months). The M in the OP’s format string becomes something like \d{1,2} (though that would match months numbered from 0 to 99!) in the parser’s internal regular expression.

Evidence: If you move the month to the end of both the data and the format string, the greedy quantifier cannot obtain more than 1 digit and so it matches the month as desired:

string datestring = "2120121";

DateTime td;
if (DateTime.TryParseExact(datestring, "ddyyyyM", new CultureInfo("en-US"), DateTimeStyles.None, out td))
{
    Console.WriteLine(td.ToShortDateString());
}
else
{
    Console.WriteLine("Invalid Date String");
}

Bottom Line: Don’t rely on undocumented behavior. Always use unambiguous data, i.e., 2-digit months.