c# - Convert string to datetime with DateTime.ParseExact

Question

I have to convert a string extract from a file to datetime. The problem is that my string doesn't have a unique format. For example my string can be something like : 19-05-2016 1:24:09:560 or 19-05-2016 21:24:09:56 or 19-05-2016 10:24:09:560 or 19-05-2016 10:24:09 and so on. I didn't encounter all the possibilities yet ( these strings are pulled out of an json response from an API call)

This is how my code looks like

public static DateTime ConveDateTime(string a)
{
    DateTime finished;
    try
    {
        finished = DateTime.ParseExact(a, "dd-MM-yyyy HH:mm:ss:fff", CultureInfo.InvariantCulture,
            DateTimeStyles.AssumeUniversal |
            DateTimeStyles.AdjustToUniversal);
    }
    catch (Exception)
    {
        try
        {
            finished = DateTime.ParseExact(a, "dd-MM-yyyy HH:mm:ss:ff", CultureInfo.InvariantCulture,
                DateTimeStyles.AssumeUniversal |
                DateTimeStyles.AdjustToUniversal);
        }
        catch (Exception)
        {
            try
            {
                finished = DateTime.ParseExact(a, "dd-MM-yyyy HH:mm:ss:f", CultureInfo.InvariantCulture,
                    DateTimeStyles.AssumeUniversal |
                    DateTimeStyles.AdjustToUniversal);
            }
            catch (Exception)
            {
                try
                {
                    finished = DateTime.ParseExact(a, "dd-MM-yyyy HH:mm:ss", CultureInfo.InvariantCulture,
                        DateTimeStyles.AssumeUniversal |
                        DateTimeStyles.AdjustToUniversal);
                }
                catch (Exception)
                {
                    try
                    {
                        finished = DateTime.ParseExact(a, "dd-MM-yyyy H:mm:ss:fff", CultureInfo.InvariantCulture,
                            DateTimeStyles.AssumeUniversal |
                            DateTimeStyles.AdjustToUniversal);
                    }
                    catch (Exception)
                    {
                        try
                        {
                            finished = DateTime.ParseExact(a, "dd-MM-yyyy H:mm:ss:ff", CultureInfo.InvariantCulture,
                            DateTimeStyles.AssumeUniversal |
                            DateTimeStyles.AdjustToUniversal);
                        }
                        catch (Exception)
                        {

                            finished = DateTime.ParseExact(a, "dd-MM-yyyy H:mm:ss:f", CultureInfo.InvariantCulture,
                            DateTimeStyles.AssumeUniversal |
                            DateTimeStyles.AdjustToUniversal);
                        }                            
                    }
                }
            }
        }
    }

    return finished;
}

I want to know if there is any better solution in converting a string than the solution I have.

Answer 1

DateTime.ParseExact has an overload that takes a string array of possible formats to use for parsing. Use that overload and reduce your code to a line or two.

string[] formats = new string[] {"dd-MM-yyyy HH:mm:ss:fff",
                                 "dd-MM-yyyy H:mm:ss:fff",
                                 "dd-MM-yyyy HH:mm:ss:f",
                                 "dd-MM-yyyy HH:mm:ss", 
                                 ....};

finished = DateTime.ParseExact(a, formats, CultureInfo.InvariantCulture,
                DateTimeStyles.AssumeUniversal |
                DateTimeStyles.AdjustToUniversal);

If you don't know all the possible formats you could also read them from an external file to avoid recompiling your application if a new format pops up

Also, as said in the comments below, I prefer to use DateTime.TryParseExact to have more control on the outcome of the parsing and avoid a costly exception handling in case of a format not recognized.

Answer 2

What about using DateTime.Parse() instead of DateTime.ParseExact()? DateTime.Parse() is far more forgiving than ParseExact. This way you don't have to define all the possible formats?