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

Thank you everyone. I 'upped' all of you and settled on a brute force implementation that doesn't use a Dictionary/HashSet and doesn't convert chars to strings:

private const string DateTimeFormatCharacters = "yYmMdDhHsS";
private static bool IsDateTimeFormatString(string input)
{
    foreach (char c in input)
        if (DateTimeFormatCharacters.IndexOf(c) < 0)
            return false;
    return true;
}