Sorting objects in ArrayList by DateTime variable inside the objects

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?

Do you have to stick to the old and ugly ArrayList? If you used generic collections and LINQ, this whole task could be done in 1 line of code. — Jacek Gorgoń, Mar 01 '12 at 15:34
I'd recommend not to use LINQ for something like this. You may get only a single line of code but it'll be significantly slower than just using `ArrayList.Sort()` or `List.Sort()`. — xxbbcc, Mar 01 '12 at 16:35
I'm right now trying to do List.Sort() using Miky's example with Lambda expression, but it does not work =_= and I don't know why. It just gives this: Cannot Convert lambda expression to type: 'System.Collections.Generic.IComparer because it's not a delegate type. — eltaro, Mar 01 '12 at 16:47
xxbbcc: http://stackoverflow.com/questions/1832684/c-sharp-sort-and-orderby-comparison says otherwise. — Jacek Gorgoń, Mar 02 '12 at 08:31

score 0 · Answer 1 · answered Mar 01 '12 at 15:24

Use this overload of ArrayList.Sort() - you'll get a callback for every comparison with your objects and you can then easily compare two dates inside your objects.

public class MyDateComparer : IComparer
{
    int IComparer.Compare ( Object x, Object y )
    {
        // Cast x and y to your object type and then compare the dates. Return
        // -1, 0 or 1 depending on your comparison
        return ( ... );
    }
}

...

myList.Sort ( 0, myList.Length, new MyDateComparer () );

Shai · Answer 2 · 2012-03-01T15:31:33.400

0

That's a wrong approach imo, a better way to do this is to use a Custom comparer.

   public class SortByDateTime: IComparer  {
      // Calls CaseInsensitiveComparer.Compare with the parameters reversed.
      int IComparer.Compare( Object x, Object y )  {
          // Compare objects according to the date
          }
      }

   ...
   // Sort the arraylist using the custom comparer
   IComparer myComparer = new SortByDateTime();
   ArrayList mList = new ArrayList();
   mList.Sort(myComparer);

edited Mar 01 '12 at 15:31

answered Mar 01 '12 at 15:24

Shai

25,159
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I don't think ArrayList is a generic type. You may want to correct your example. – xxbbcc Mar 01 '12 at 15:29
thank you for your fast replies. I am right now trying to understand this piece of code and adapt it to my program. – eltaro Mar 01 '12 at 15:34

Mike Dinescu · Accepted Answer · 2012-03-01T15:37:42.807

0

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)));

edited Mar 01 '12 at 15:37

answered Mar 01 '12 at 15:30

Mike Dinescu

54,171
16
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whats a lambda expression? I tried this code, it gives out an error, Cannot Convert lambda expression to type: 'System.Collections.Generic.IComparer because it's not a delegate type. – eltaro Mar 01 '12 at 15:44
This comment box is not enough space to explain what a lambda expression is, so I'm just going to try to give you an idea as it applies in your case: Think of it as an anonymous function, declared inline, that is interpreted by the compiler as a delegate. In my example the lambda expression is (d1,d2) => ... Which is like declaring a function that takes two parameters d1 and d2 of type Event in that case, and then returns (=>) the result of a computation using the two parameters. For details, see he MSDN page on lambda expressions here: http://msdn.microsoft.com/en-us/library/3da4abas.aspx – Mike Dinescu Mar 01 '12 at 15:49
I've converted EventHolder to List, and added your code. It gives me an error still. Adding using System.Linq; did not help either. – eltaro Mar 01 '12 at 16:32
Thanks, I've fixed it. Visual Studio was unhappy for no reason. I've transfered the code to another project and it worked. – eltaro Mar 01 '12 at 17:04

score 0 · Answer 4 · answered Mar 01 '12 at 15:44

If you don't really need ArrayList and have .NET 3.5, try the following instead:

List<Event> eventHolder = ... // Just to make certain it's a List<Event>
eventHolder = eventHolder
    .OrderBy(e => DateTime.ParseExact(e.Date_And_Time,"dd/MM/yyyy HH:mm",region)
    .ToList();

Sorting objects in ArrayList by DateTime variable inside the objects

4 Answers4