Question

I had posted a question on DateTime to String conversion, I got many satisfying answers for that .. so I thank StackOverflow very much ..
Here is one more problem of String manupulation, I am stuck with ..

I have to convert a string (from some external source) using C# code .. the string can have these expected format of DateTime ..

1. 02/31/2009 01:59:59           24 hours format
2. 02/31/2009 01:59:59 AM     12 hours format
3. 2/31/2009 1:59:59
4. 2/31/2009 1:59:59 AM
5. 02/01/2009 01:59:59 AM
6. 2/1/2009 1:59:59
7. and so on …….

I tried using DateTime(Convert.ToInt32(string_date.Substring(6,4)),Int,Int,Int,Int,Int,Int)
ie, By extracting the values of month, Day etc

But it doesn’t work .. because I can’t extract the values with substring perfectly .. as the length of string is Varying
I also have tried to extract the values referring the occurance of “/”, “space” and “:” but it becomes bottle neck to derive with (non-)Occurrence of AM/PM

Only the length of Day, Month and Hours can vary ..

Answer 1

You can use the DateTime.ParseExact overload that takes a list of formats:

private static string[] formats = new string[]
    {
        "MM/dd/yyyy HH:mm:ss tt",
        "MM/dd/yyyy HH:mm:ss",
        "M/dd/yyyy H:mm:ss tt",
        "M/dd/yyyy H:mm:ss"        
    };

private static DateTime ParseDate(string input)
{
    return DateTime.ParseExact(input, formats, CultureInfo.InvariantCulture, DateTimeStyles.None);
}

This will throw a FormatException if the passed string does not match any of the given formats. Notice that the formats expecting AM/PM should appear before identical formats without AM/PM ("MM/dd/yyyy HH:mm:ss tt" comes before "MM/dd/yyyy HH:mm:ss").

Update
As Henk points out in the comments, the same functionality is available when using TryParseExact which removes exception situation. Also, paired with nullable types this can be made a bit cleaner:

private static DateTime? ParseDate(string input)
{
    DateTime result;
    if (DateTime.TryParseExact(input, formats, CultureInfo.InvariantCulture, DateTimeStyles.None, out result))
    {
        return result;
    }
    return null;
}

Now it will simply return a null reference if it fails to parse the input.