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) thinking it would barf on an invalid format, but it doesn’t.

So I’m good with simply testing if the string contains only “yYmMdDsShH” characters. Something like std::string.find_first_not_of would work, but System.String doesn’t have this.

I thought that RegEx might do the trick, but I’m very weak with regular expressions.

Note that Linq is not available for this one (.NET 2.0 only).

Updated

To clarify, I need to know if a given string represents a date time format and not something else like this:

if (input == "some special value")
... // it's a special case value
else if (Environment.GetEnvironmentVariable(input))
... // it's an environment variable name
else if (IsDateTimeFormatString(input))
... // it's a date time format string
else if (input.IndexOfAny(Path.GetInvalidPathChars()) < 0)
... // it's a file path
else
   throw new Exception(); // Not a valid input

I can restrict a DateTime format string to only “yYmMdDsShH”, or I can add a few separator characters into it as well, it’s up to me what to allow or not allow.

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));