String contains only a given set of characters

Question

I need to know if a given string is a valid DateTime format string because the string may represent other things. I tried DateTime.ParseExact(somedate.ToString(format), format)

Answer 1

I'd just do this:

public static class DateTimeFormatHelper
{
    // using a Dictionary instead of a HashSet
    // since you said you're using .NET 2.0
    private static Dictionary _legalChars;

    static DateTimeFormatHelper()
    {
        _legalChars = new Dictionary();
        foreach (char legalChar in "yYmMdDsShH")
        {
            _legalChars.Add(legalChar, 0);
        }
    }

    public static bool IsPossibleDateTimeFormat(string format)
    {
        if (string.IsNullOrEmpty(format))
            return false; // or whatever makes sense to you

        foreach (char c in format)
        {
            if (!_legalChars.ContainsKey(c))
                return false;
        }

        return true;
    }
}

Of course, this might be an excessively strict definition, as it rules out what most people would consider valid formats such as "yyyy-MM-dd" (since that includes "-" characters).

Determining exactly what characters you wish to allow is your judgment call.