Argthtjtr

I'm trying to parse birthdays in the format of d/M without any year specified.

Using DateTime.TryParseExact(birthday, "d/M", CultureInfo.InvariantCulture, DateTimeStyles.None, out DateTime birthdayDate) works most of the time, except when the birthday is at leap year day (aka 29/2), and the parsing never succeeds because it is default to current year. Using DatTimeStyles.NoCurrentDateDefault does not work either since it defaults to year = 1, which is not a leap year.

How do I do the parsing so it does not involve a hack (I don't want to parse it manually or manually add an arbitrary year to parse it as a full date either, they are all ugly and potentially fragile) and still work across all possible dates, including leap day? Existing questions does not help at all since nobody bothers to check if they work for leap day at all. I tried all of them and none of them worked.

A day and a month does not make a date - therefore they can't be parsed as a DateTime value without assuming a year. The .NET Framework assumes a year that is either the current year or year 1 - exactly because 2/29 is valid only on leap years - and this is a very reasonable assumption.

The .Net framework does not provide a built in way to store Day/Month values, but Noda Time does - Take a look at AnnualDate - It stores a day and a month but no year.

However, it doesn't have Parse or TryParse methods - so for that you still need to manually manipulate the input string and add a year (that is a leap year like 2016) in order to use the DateTime's TryParseExact method.

Update

As Matt Johnson wrote in his comment, Noda Time does provide a way for parsing text as an AnnualDate, using the AnnualDatePattern class.

The documentation has a page called Patterns for AnnualDate values that lists the supported patterns.

This question lacks the appropriate details, however

If you just want to parse a birth date and month including leap year dates, then just added a leap year to the end of the date.

I am not sure what you expect to do here, however you could try this

birthday = $"{birthday}/2016"; // leap year



DateTime.TryParseExact(birthday, "d/M/yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out DateTime birthdayDate);

Update

my question is how to not make TryParseExact assume the year
automatically by manually overriding it in some way

To be technical here, no you have to specify the leap year in the string if you are parsing a leap year month and day exclusively as in your example

There is a lot of checks and balances the TryParseExact method does, however here are the important bits.

In short, it uses the current year or year 1, and there is no way to tell it to choose a leap year specifically

private static bool CheckDefaultDateTime(ref DateTimeResult result, ref Calendar cal, DateTimeStyles styles)

{

  if ((result.flags & ParseFlags.CaptureOffset) != (ParseFlags) 0 && (result.Month != -1 || result.Day != -1) && ((result.Year == -1 || (result.flags & ParseFlags.YearDefault) != (ParseFlags) 0) && (result.flags & ParseFlags.TimeZoneUsed) != (ParseFlags) 0))

  {

    result.SetFailure(ParseFailureKind.Format, "Format_MissingIncompleteDate", (object) null);

    return false;

  }

  if (result.Year == -1 || result.Month == -1 || result.Day == -1)

  {

    DateTime dateTimeNow = DateTimeParse.GetDateTimeNow(ref result, ref styles);

    if (result.Month == -1 && result.Day == -1)

    {

      if (result.Year == -1)

      {

        if ((styles & DateTimeStyles.NoCurrentDateDefault) != DateTimeStyles.None)

        {

          cal = GregorianCalendar.GetDefaultInstance();

          result.Year = result.Month = result.Day = 1;

        }

        else

        {

          result.Year = cal.GetYear(dateTimeNow);

          result.Month = cal.GetMonth(dateTimeNow);

          result.Day = cal.GetDayOfMonth(dateTimeNow);

        }

      }

      else

      {

        result.Month = 1;

        result.Day = 1;

      }

    }

    else

    {

      if (result.Year == -1)

        result.Year = cal.GetYear(dateTimeNow);

      if (result.Month == -1)

        result.Month = 1;

      if (result.Day == -1)

        result.Day = 1;

    }

  }

  if (result.Hour == -1)

    result.Hour = 0;

  if (result.Minute == -1)

    result.Minute = 0;

  if (result.Second == -1)

    result.Second = 0;

  if (result.era == -1)

    result.era = 0;

  return true;

}