Question

Use Listbox.SelectedItem in an asp.net web page

Asked by: sebastiz

I have a web page. on  On it is a dynamically filled listbox (called ListFavoriteURLs). I would like a user to be able to select an item and then press a button (called Remove)to remove that item from the users profile. The problem is that the page does not seem to recognise that an item has been selected, and therefore I get a null exception reference returned. The code is below. There are two event handlers- the Page_load event handler (not shown) and the remove_Click handler which deals with deleting the selected Item from the list. The listbox properties are set to autoPostBack=False and ViewState=True
Any ideas?
Protected Sub Remove_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Remove.Click

        Dim i As Integer
        If listFavoriteURLs.SelectedItem Is Nothing Then MsgBox("No items selected!")
        If listFavoriteURLs.SelectedItem Is Nothing Then MsgBox("No value selected!")

        For i = 0 To Profile.FavoriteURLs.Count - 1
            If String.Compare(Profile.FavoriteURLs.Item(i).Trim(), listFavoriteURLs.SelectedItem.ToString.Trim, True) = 0 Then
                Profile.FavoriteURLs.RemoveAt(i)
            End If
        Next
       
    End Sub

PS when I run this both the message boxes come up!

Seb

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Asked On
2007-12-24 at 08:05:06ID23041581
Tags

listbox

,

item

,

selected

Topics

Programming for ASP.NET

,

Microsoft Visual Basic.Net

,

Visual Studio

Participating Experts
2
Points
500
Comments
8

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Answers

 

by: jpaulinoPosted on 2007-12-24 at 08:07:25ID: 20525063

Do you fill the listbox in the page load event this way  ?

If Not Page.IsPostBack Then
  ' Fill your listbox
End If

 

by: sebastizPosted on 2007-12-24 at 08:35:37ID: 20525150

No the page load simply has a call to the method DisplayFavoriteURLs. This method's code is below. Do i need to use the If Not Page.IsPostBack and if so, do I use it within the DiplayFavoriteURLs sub or in the Page_load handler code?:

Sub DisplayFavoriteURLs()
        If Profile.FavoriteURLs.ToString.Length > 0 Then
            Dim ar As New ArrayList
            For Each s As String In Profile.FavoriteURLs
                ar.Add(Int32.Parse(s))
            Next
            ar.Sort()
            listFavoriteURLs.DataSource = ar
            listFavoriteURLs.DataBind()
        End If
    End Sub
Seb

 

by: jpaulinoPosted on 2007-12-24 at 09:01:17ID: 20525225

Ok,

In the page load event you have only this:

Call DisplayFavoriteURLs
or simple
DisplayFavoriteURLs

This will that when that page reloads (after the Remove button click) it fills the list agains and you have no selection. Try this way:

If Not Page.IsPostBack Then
   Call DisplayFavoriteURLs
End If

 

by: sebastizPosted on 2007-12-24 at 13:05:29ID: 20525853

That is great. Before I give you the points, can you tell me why I have to use If Not Page.IsPostback?

Seb

 

by: jpaulinoPosted on 2007-12-24 at 14:42:59ID: 20526002

Shure!

If Not Page.IsPostback avoids to run the code everytime the page reloads. Page.IsPostback indicating whether the user control is being loaded in response to a client postback, or if it is being loaded and accessed for the first time.

This way if you don't put your code inside that scrope (If Not Page.IsPostback Then ...End If) the code runs everytime any control refreshes the page. This is usefull to grids that must be updated everytime but not for dropdowns.

In your code your add itens to the dropdown on every refresh so never had a selected item.

jpaulino

 

by: jpaulinoPosted on 2007-12-27 at 02:53:47ID: 20533296

Was I clear enough ?

 

by: Computer101Posted on 2008-03-23 at 06:20:30ID: 21189474

Forced accept.

Computer101
EE Admin

20120131-EE-VQP-002

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