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

With .NET2, you need to roll your own check for this. For example, the following method uses a foreach to check:

bool FormatValid(string format)
{
    string allowableLetters = "yYmMdDsShH";

    foreach(char c in format)
    {
         // This is using String.Contains for .NET 2 compat.,
         //   hence the requirement for ToString()
         if (!allowableLetters.Contains(c.ToString()))
              return false;
    }

    return true;
}

If you had the option of using .NET 3.5 and LINQ, you could use Enumerable.Contains to work with characters directly, and Enumerable.All. This would simplify the above to:

bool valid = format.All(c => "yYmMdDsShH".Contains(c));