Parsing the DateTime format to get Format String

Question

I would like to be able to get the format string from a DateTime string.

e.g.

"2012-12-08 15:00:00" => "yyyy-MM

Answer 1

I had the same idea as Jon Skeet and went to implement it:

// Helper method
IEnumerable DateTimeFormatPatterns(DateTimeFormatInfo format)
{
    var accessors = new Func[]
    {
        f => f.FullDateTimePattern,
        f => f.LongDatePattern,
        f => f.LongTimePattern,
        f => f.MonthDayPattern,
        f => f.ShortDatePattern,
        f => f.SortableDateTimePattern,
        f => f.UniversalSortableDateTimePattern,
        f => f.YearMonthPattern,
    };

    return accessors.Select(accessor => accessor(format));
}

// The real function
string DetectDateTimeFormat(string date, CultureInfo culture)
{
    DateTime dummy;
    foreach (var pattern in DateTimeFormatPatterns(culture.DateTimeFormat))
    {
        if (DateTime.TryParseExact(date, pattern, culture,
                                   DateTimeStyles.None, out dummy))
        {
            return pattern;
        }
    }

    return null;
}

There is space for improvement here (e.g. the hardcoded DateTimeStyles.None doesn't help, an overload which assumes the current culture would also be useful), but you can use it like this:

var format = DetectDateTimeFormat("2012-12-08 15:00:00",
                                  CultureInfo.CurrentCulture);