Question

I have a problem.

I have an ArrayList of objects. Each object contains a DateTime data field.

I need to sort this ArrayList by this DateTime data field.

This is my current code and it does not work:

        for (int i = 0;i<EventHolder.Count;i++)
        {
            Event obj = (Event) EventHolder[i];
            try
            {
                obj2=(Event)EventHolder[i+1];
            }
            catch
            {
                break;
            }
            DateTime date1 = DateTime.ParseExact(obj.Date_And_Time,"dd/MM/yyyy HH:mm",region);
            DateTime date2 = DateTime.ParseExact(obj2.Date_And_Time, "dd/MM/yyyy HH:mm", region);
            if (DateTime.Compare(date1,date2)>0)
            {
                Event tempobj=obj2;
                EventHolder[i+1]=obj;
                EventHolder[i]=tempobj;
            }
        }
        foreach (Event i in EventHolder)
        {
            Console.WriteLine(i.Date_And_Time);
        }
        Console.ReadLine();

I use these 3 dates to test if it works:

23/11/2011 12:15

28/06/2010 04:05

02/09/1992 03:54

The output is always:

28/06/2010 04:05

02/09/1992 03:54

23/11/2011 12:15

I tried using just > and < operators for comparing and got same results. I also tried using different commands to convert the strings to DateTime objects, like Convert.ToDateTime, DateTime.Parse. Tried entering without HH:mm (adjusted the code accordingly) and still had the same output. I tried not using IFormatProvider (set it to null) and got nowhere again.

By chance, I found this thread:

How to sort ArrayList of DateTime objects in descending order?

I extracted all the dates from the objects and put them into this DateTime list. Used List.Sort() and it produced correct output. But then I got stuck at how to use this sorted list to sort my EventHolder ArrayList. I’ve been banging on this wall for a while, and produced no result.

I am very new to programming, so I hardly know any good way of doing what I’m doing. And it doesn’t even work.
Can anybody explain to me where I’m going wrong?

Answer 1

What you’re trying to do there is implement a sorting algorithm but unfortunately yours is not a correct one. It almost looks like Bubble Sort which is a pretty slow algorithm to begin with but it’s not quite correct.

A much better way to accomplish what you’re trying to do is to use the Sort method or ArrayList or of the generic List<> classes. Those sort methods allow you to provide an IComparer implementation (or a comparison delegate in the case of the generic List) to specify the sort criteria and internally they use a correct and efficient sort algorithm.

If you use the generic List<> you could just pass in a lambda expression and have it dictate the sorting criteria based on the date field, like so:

// Assuming EventHolder is a List<Event>
EventHolder.Sort((d1, d2) => DateTime.ParseExact(d1.Date_And_Time,"dd/MM/yyyy HH:mm",region).CompareTo(DateTime.ParseExact(d2.Date_And_Time,"dd/MM/yyyy HH:mm",region)));