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Introduction

I recently found a requirement in my ASP.NET application that needed to be able to sort a DropDownList control. Oddly enough I couldn't find a sort method within the framework, so I made my own. The code below shows what I did as a quick option, but you can of course use similar code to create a custom control that inherits from the DropDownList control.

Code

    Private Sub SortDropDown(ByVal dd As DropDownList)
        Dim ar As ListItem()
        Dim i As Long = 0
        For Each li As ListItem In dd.Items
            ReDim Preserve ar(i)
            ar(i) = li
            i += 1
        Next
        Dim ar1 As Array = ar

        ar1.Sort(ar1, New ListItemComparer)
        dd.Items.Clear()
        dd.Items.AddRange(ar1)
    End Sub
    Private Class ListItemComparer _
        Implements IComparer
        
        Public Function Compare(ByVal x As Object, _
              ByVal y As Object) As Integer _
              Implements System.Collections.IComparer.Compare
            Dim a As ListItem = x
            Dim b As ListItem = y
            Dim c As New CaseInsensitiveComparer
            Return c.Compare(a.Text, b.Text)
        End Function
    End Class

Summary

As I mentioned, this can easily be put into a custom control by simply creating a new class. Inherit from the DropDownList control and add the above code to the class. That's all. When you create your ASP.NET web form, you can drag and drop a normal DropDownList to your form, and edit the declaration in your code behind form, to use your custom class. Enjoy!

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

Domenic

I've been writing software applications since the early 90's from ASP and VB5 to C# and ASP.NET. I am currently working on law enforcement and criminal justice applications delivered over the web. I've always been astounded by the fact that the only 2 industries that refer to customers as "users" are narcotics and software development. Wink

Occupation:	Web Developer
Location:	United States

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Subject

Author

Date

Thanks!

dsilk

11:52 3 Jul '08

I put your code to use in a User Control, then registered the control in the web.config file so I can add it directly to any page like so:


<MyTag:SortableList ID="MyList" runat="server" ListType="DropDownList" />

The user control simply adds whatever type of list control you need and provides the means to sort the items by Text or by Value, ascending or descending. There's also a method to add an item to the top of the list (AddFirstItem) if you want to add a default item to the top of the list after you sort the items.

Just add a new User Control to your project and put the code below in the codebehind - no other controls are necessary unless you want to do more with it.


Partial Class Controls_SortableListControl
    Inherits System.Web.UI.UserControl

    Public Enum ListTypes
        DropDownList
        ListBox
        RadioButtonList
        CheckBoxList
        BulletedList
    End Enum

    Private _ListType As ListTypes
    Public Property ListType() As ListTypes
        Get
            If _ListType = Nothing Then
                _ListType = ListTypes.DropDownList
            End If
            Return _ListType
        End Get
        Set(ByVal value As ListTypes)
            _ListType = value
            ListSetup()
        End Set
    End Property

    Public Property ListControl() As Object
        Get
            If Me.FindControl("ListControl") Is Nothing Then
                ListSetup()
            End If
            Return Me.FindControl("ListControl")
        End Get
        Private Set(ByVal value As Object)
            If value IsNot Nothing Then
                If Me.FindControl("ListControl") Is Nothing Then
                    value.ID = "ListControl"
                    Me.Controls.Add(value)
                End If
            End If
        End Set
    End Property

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    End Sub

    Protected Sub ListSetup()
        Select Case ListType
            Case ListTypes.BulletedList
                ListControl = New BulletedList
            Case ListTypes.CheckBoxList
                ListControl = New CheckBoxList
            Case ListTypes.DropDownList
                ListControl = New DropDownList
            Case ListTypes.ListBox
                ListControl = New ListBox
            Case ListTypes.RadioButtonList
                ListControl = New RadioButtonList
            Case Else
                ListControl = Nothing
        End Select
    End Sub

    Public Sub AddFirstItem(ByVal li As ListItem)
        ListControl.Items.Insert(0, li)
    End Sub

    Public Sub SortList(ByVal SortByText As Boolean, ByVal SortAscending As Boolean)
        Dim ar(ListControl.Items.Count - 1) As ListItem
        ListControl.Items.CopyTo(ar, 0)

        If SortByText Then
            Array.Sort(ar, New ListItemCompareText)
        Else
            Array.Sort(ar, New ListItemCompareValue)
        End If

        If Not SortAscending Then
            Array.Reverse(ar)
        End If

        ListControl.Items.Clear()
        ListControl.Items.AddRange(ar)
    End Sub

    Private Class ListItemCompareText
        Implements IComparer

        Public Function Compare(ByVal x As Object, _
              ByVal y As Object) As Integer _
              Implements System.Collections.IComparer.Compare
            Dim a As ListItem = x
            Dim b As ListItem = y
            Dim c As New CaseInsensitiveComparer
            Return c.Compare(a.Text, b.Text)
        End Function
    End Class
    Private Class ListItemCompareValue
        Implements IComparer

        Public Function Compare(ByVal x As Object, _
              ByVal y As Object) As Integer _
              Implements System.Collections.IComparer.Compare
            Dim a As ListItem = x
            Dim b As ListItem = y
            Dim c As New CaseInsensitiveComparer
            Return c.Compare(a.Value, b.Value)
        End Function
    End Class
End Class

Error

halliej2x

22:45 1 Aug '06

I'm having problem with this code... Private Class ListItemComparer _ Implements IComparer Public Function Compare(ByVal x As Object, _ ByVal y As Object) As Integer _ Implements System.Collections.IComparer.Compare Is the "IComparer" a reference?
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Re: Error

Domenic

3:27 2 Aug '06

put imports System.Collections at the top of your file before the class declaration. IComarer is part of this namespace. You could alternatively use the fully qualified name of: Private Class ListItemCompater Implements System.Collections.IComparer
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Re: Error

halliej2x

20:16 2 Aug '06

Still not working to me....kindly please send me a sample project for this i possible. Thank you in advance.
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Re: Error

Domenic

10:41 3 Aug '06

I'm sorry, but I don't have time to do that. As you can see from the posts below, this code should work as is in a VB ASP.NET application. There's truely not much to it.
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Re: Error

Gregorio Cybill

18:58 11 Aug '08

the same problem is happening to me. I couldn't find IComparer in Collections Namespace. Only Comparer is there. I use VS 2003 and .NET framework 1.1. Is there something wrong or not applicable yet?
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Re: Error

Mohammed Aziz Elnahrawi

21:31 5 Oct '06

The error is here : Private Class ListItemComparer _ Implements IComparer You must remove the underscore "_" from the end of line Muhammad Muhammad Elnahrawi www.mycodebank.com
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Shortcut using CopyTo method on prealloced array

hafthor

6:22 27 Feb '06

Thanks for the idea Domenic. I made a slight tweak. Private Sub SortDropDown(ByVal dd As DropDownList) Dim ar(dd.Items.Count - 1) As ListItem dd.Items.CopyTo(ar, 0) dd.Items.Clear() ar.Sort(ar, New ListItemComparer) dd.Items.AddRange(ar) End Sub
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Good code, but let's make it even better

HamidTheProgrammer

11:34 9 Aug '05

Nice code. It worked immediately. However, when you have an item like "Select one" and want to have it displayed as the default selected item, it will fall down the list of items in the dropdown list. So to make this work, I added on line to your code to add "Select one" item to the top of the list after the sort. Here is your code with the one extra line of code I added:

Private Sub SortDropDown(ByVal dd As DropDownList)
Dim ar As ListItem()
Dim i As Long = 0
For Each li As ListItem In dd.Items
ReDim Preserve ar(i)
ar(i) = li
i += 1
Next
Dim ar1 As Array = ar

ar1.Sort(ar1, New ListItemComparer)
dd.Items.Clear()
'Add an item that will appear as the default selected item in the dropdown list
dd.Items.Add(New ListItem("Select one", "Select one"))
dd.Items.AddRange(ar1)
End Sub

Private Class ListItemComparer
Implements IComparer

Public Function Compare(ByVal x As Object, _
ByVal y As Object) As Integer _
Implements System.Collections.IComparer.Compare
Dim a As ListItem = x
Dim b As ListItem = y
Dim c As New CaseInsensitiveComparer
Return c.Compare(a.Text, b.Text)
End Function
End Class

5.00/5 (1 vote)

Re: Good code, but let's make it even better

Domenic

3:08 10 Aug '05

Makes sense to me. Thanks for your comments and suggestion!
Sign In·View Thread·PermaLink	1.00/5 (1 vote)

Would just like to say...

Anonymous

1:51 4 Aug '05

What balls programming. Whoever wrote this should really work for the framework development team... their programming is little better.
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Re: Would just like to say...

Domenic

3:18 4 Aug '05

Thanks!
Sign In·View Thread·PermaLink	5.00/5 (1 vote)

Syntax error

chatelainj

9:41 1 Sep '04

Just wanted to let you know there's an error when i compile this code. This code work perfectly when the "_" sign is removed in the Class declaration. Public Class ListItemComparer Implements IComparer _ Thanks for the tip. Jean-Francois Chatelain
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Re: Syntax error

Anonymous

9:21 3 May '05

Public Class ListItemComparer Implements IComparer _ remove the _ Then it will work.
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c# version wrapped in a SortableDropDownList

Steinar Herland

23:57 2 Apr '04

Thanks for the sample. I needed this in c#, so I wrapped it in a SortableDropDownList


public class SortableDropDownList : DropDownList {
   class ListItemComparer : IComparer{
      public int Compare(object a, object b){
         return new CaseInsensitiveComparer().Compare(((ListItem)a).Text, ((ListItem)b).Text);
      }
   }
   public void SortItems(){
      ListItem[] items = new ListItem[Items.Count];
      int i = 0;
      foreach(ListItem item in Items)
         items[i++] = item;
      Array.Sort(items, new ListItemComparer());
      Items.Clear();
      Items.AddRange(items);
   }
}

Re: c# version wrapped in a SortableDropDownList

Paul Rogan

1:32 10 Jan '06

muchos gracias!
Sign In·View Thread·PermaLink	1.00/5 (1 vote)

Re: c# version wrapped in a SortableDropDownList

banditking

5:41 20 Apr '06

Your code returns a null array of listitems, here it the correct version

public void SortDropDown(ref DropDownList ddl)
{
ListItem[] liArray = new ListItem[ddl.Items.Count];
ddl.Items.CopyTo(liArray, 0);
Array.Sort(liArray, new ListItemComparer());
ddl.Items.Clear();
ddl.Items.AddRange(liArray );
}

public class ListItemComparer : IComparer
{
public int Compare(object a, object b)
{
return new CaseInsensitiveComparer().Compare(((ListItem)a).Text, ((ListItem)b).Text);
}
}

4.50/5 (4 votes)

Re: c# version wrapped in a SortableDropDownList

Ryan McFarren

7:25 14 Jul '06

Works great. Minor nit - "ref" is not necessary.
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Re: c# version wrapped in a SortableDropDownList

ashushere

12:56 8 Jun '07

Thanks a lot. That was helpful
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Re: c# version wrapped in a SortableDropDownList

ruileier

22:15 18 Sep '07

I try to improve on the SortMethod.

public void SortListItem(DropDownList dropDownList, Enum sortType)
{
ListItem[] listItem = new ListItem[dropDownList.Items.Count];

dropDownList.Items.CopyTo(listItem, 0);

if (sortType.ToString().Equals("Text"))
{
Array.Sort(listItem, new ListItemTextComparer());
}
else
{
Array.Sort(listItem, new ListItemValueComparer());
}

dropDownList.Items.Clear();
dropDownList.Items.AddRange(listItem);
}

internal class ListItemTextComparer : IComparer
{
public int Compare(object x, object y)
{
return new CaseInsensitiveComparer().Compare((x as ListItem).Text, (y as ListItem).Text);
}
}

internal class ListItemValueComparer : IComparer
{
public int Compare(object x, object y)
{
return new CaseInsensitiveComparer().Compare((x as ListItem).Value, (y as ListItem).Value);
}
}

my Blog : ruilei.cnblogs.com

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2.00/5 (1 vote)

hello!??

noahart

23:05 24 Feb '04

ListItem's Forecolor is "Black" I want turn the forecolor of one of ListItems in the DropDownList to "Red",and others are "Black", How can I do this?
Sign In·View Thread·PermaLink	3.00/5 (1 vote)

Re: hello!??

Anonymous

1:45 4 Aug '05

You cant. Its bugged.
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