ASP.NET ListBox Trouble

Question

Here is all my code:

Site.Master

<%@ Master Language="C#" AutoEventWireup="true"
CodeBehind="Site.master.cs" Inherits="WebApplication1.SiteMaster" %>

<!DOCTYPE html>
<html>
    <head runat="server">
        <title></title>
        <link href="~/Styles/Site.css" rel="stylesheet" type="text/css" />
        <asp:ContentPlaceHolder ID="HeadContent" runat="server">
        </asp:ContentPlaceHolder>
    </head>
    <body>
        <form runat="server">
            <div class="main">
                <asp:ContentPlaceHolder ID="MainContent" runat="server"/>
            </div>
        </form>
    </body>
</html>

Site.Master.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication1
{
    public partial class SiteMaster : System.Web.UI.MasterPage
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }
    }
}

Default.aspx

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master"
AutoEventWireup="true" CodeBehind="Default.aspx.cs"
Inherits="WebApplication1._Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <asp:ListBox ID="ListBox1" runat="server"></asp:ListBox>
</asp:Content>

Default.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication1
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            ListBox ListBox1 = new ListBox();
            ListBox1.FindControl("ListBox1");
            ListBox1.Items.Add(new ListItem("Hello"));
        }
    }
}

Question: Why is my ListBox not populating?

When running the code I get a page with a small box that is completely empty.

I tried the following already and I will tell you the result -

Remove

ListBox ListBox1 = new ListBox();
ListBox1.FindControl("ListBox1");

Result: The name 'ListBox1' does not exist in the current context

Add a reference to ListBox1 manually into the designer.

Result: Object reference not set to an instance of an object.

To fix that error adding the line ListBox ListBox1 = new ListBox(); works, but still does not display.

Any ideas? Thanks.

Christian Phillips · Accepted Answer · 2013-08-14 14:02:23Z

2

You should only need...

protected void Page_Load(object sender, EventArgs e)
        {
            ListBox1.Items.Add(new ListItem("Hello", "H"));
        }

See this link too

EDIT: Same code works for me.

enter image description here

edited Aug 14, 2013 at 14:02

answered Aug 14, 2013 at 13:42

Christian Phillips

18.8k8 gold badges55 silver badges85 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

Jeff Fritz Over a year ago

I would be willing to bet that your _Default.aspx.cs does not contain a definition for ListBox1. If this entry is not present, manually add it

Christian Phillips Over a year ago

@JohnAtrik, I have added the 2nd arg (this isn't actually required). I have seen this before, and either clean solution -> rebuild, or restart VS.

JohnAtrik Over a year ago

@Jeff Fritz I did do that as I said and got the error mentioned above.

JohnAtrik Over a year ago

@christiandev The addition of the "H" did nothing, still says that Object reference not set to an instance of an object.

Jeff Fritz Over a year ago

Ack, I listed the wrong filename... You should have a definition in Default.aspx.designer.cs That is where all controls in the markup are mapped to c-sharp.

|

Tory · Accepted Answer · 2013-08-14 14:09:06Z

To directly answer your question of why ListBox1 isn't getting populated:

protected void Page_Load(object sender, EventArgs e)
{
     ListBox ListBox1 = new ListBox();  //You are creating a new local ListBox named ListBox1.
     ListBox1.FindControl("ListBox1");  //You are trying to find the Control named ListBox1 that is held within the Control ListBox1 (and not doing anything with the result of the search)
     ListBox1.Items.Add(new ListItem("Hello"));  //You are adding a new ListItem to the local ListBox1.
}

As christiandev said, you should just be able to do this:

protected void Page_Load(object sender, EventArgs e)
{
     ListBox1.Items.Add(new ListItem("Hello"));
}

I have no idea why you can't seem to access the ListBox1 on the aspx page.

Collectives™ on Stack Overflow

ASP.NET ListBox Trouble

2 Answers 2

9 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

9 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related