How To Sort An Array List

Question

There are 2 listboxes to exchange items. Each list item has text and value properties eg: "Branch",1 ; "Leaves",23,...etc After adding an item from listbox2 to listbox1, I would like to sort listbox1.

lb1 and lb2 are the listboxes.

protected void btnRemove_Click()
{
  if(lb2.Items.count > 0)
   {
     if(lb2.selectedindex >=0)
      {
        ListItem li = lb2.selecteditem;
        lb1.items.add(li);

        //sort
         List<ListItem> al = new List<ListItem>();
         foreach(listitem l in lb1.items)
          {
           al.Add(l);
          }
        lb1.items.clear();
        al.sort();
        lb1.Datasource=al;
        lb1.databind();
     }
   }
}

At al.sort(); it gives an error - "Failed to compare 2 elements in the array". How should I correct this.

The code you've given wouldn't even compile (C# is case-sensitive) and we don't know what lb2 and lb1 are. Additionally, is there any reason you're using ArrayList rather than List<T>? — Jon Skeet
– Jon Skeet, Commented Dec 25, 2013 at 11:57
They are the names of the listboxes. No reason. I was just trying out omerkamal's suggestion - forums.asp.net/t/1094374.aspx — Ruby
– Ruby, Commented Dec 25, 2013 at 12:02
That's a suggestion from 6 1/2 years ago... back when some people were still unfortunately on .NET 1.1. There's very little reason not to use generic collections now... — Jon Skeet
– Jon Skeet, Commented Dec 25, 2013 at 12:03
That article is six years old! Be very careful about the age of articles on the Internet. — John Saunders
– John Saunders, Commented Dec 25, 2013 at 12:03
@jonskeet, I tried using List, but the same error at sort. could you plz check — Ruby
– Ruby, Commented Dec 25, 2013 at 12:18

Community · Accepted Answer · 2017-05-23 10:25:30Z

1

The problem with your existing code is that you are adding ListItem in ArrayList and to sort the ArrayList should contain objects which implements IComparer as stated in How can i sort Arraylist with a class?
You can add a custom IComparersomething like this,

public class MyListItemComparer : IComparer
{
    int IComparer.Compare(Object x, Object y)
    {
        //ListItem item1 = (ListItem)x;
        //ListItem item2 = (ListItem)y;
        return 1;//Your logic to compare and sort;
    }
}

and then you can call the sort method as

        ArrayList al = new ArrayList();
        ..... //Your other code
        IComparer myComparer = new MyListItemComparer();
        al.Sort(myComparer);

edited May 23, 2017 at 10:25

CommunityBot

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answered Dec 25, 2013 at 12:01

Deepak Bhatia

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Comments

Ankush Madankar · Accepted Answer · 2013-12-25 12:48:29Z

1

Try this to sort ur al array:

al.OrderBy(a => a);

edited Dec 25, 2013 at 12:48

answered Dec 25, 2013 at 12:03

Ankush Madankar

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1 Comment

Ruby Over a year ago

It did not error at this line now, but did at 'Databind(); - "Listitem does not contain property formname' But I have DataTextfield="formname" Datavaluefield="formid"...same for both listboxes

Zaheer Ahmed · Accepted Answer · 2013-12-25 12:07:45Z

0

Please make sure your array list contains the correct data and its not null. al.sort(); should work on arraylist of integers,string and other datatype if they inherit IComparable else you need to write your own comparer.

edited Dec 25, 2013 at 12:07

answered Dec 25, 2013 at 11:58

Zaheer Ahmed

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4 Comments

Ruby Over a year ago

which namespace. I have system.collections and generics but i dont get collections...

Ruby Over a year ago

it sure doesnt have nulls, each listitem has a string and int value

Zaheer Ahmed Over a year ago

have u tried parsing items to string al.Add(lb1.items[i].toString());

Ruby Over a year ago

yes. It did not error sort now, but did at 'Databind(); - "Listitem does not contain property formname' But I have DataTextfield="formname" Datavaluefield="formid"...same for both listboxes

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