listbox1.SelectedIndex (arguement out of range exception) c#/.net - listbox

Question

I am trying to browse web pag1es using a listbox, I have added three links to it. All three links are loading well but when third link finishes loading I got this exception.
Exception is:
InvelidArguement = value of '3' is not valid for 'SelectedIndex'. Parameter name:
SelectedIndex
Warning is:
`The result of the exception is always 'true' since a value of type 'int' is
never equal to 'null' of type 'int?'
this is my program image:
this is my program code:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
listBox1.Items.Add("www.google.com");
listBox1.Items.Add("www.facebook.com");
listBox1.Items.Add("www.yahoo.com");
listBox1.SelectedIndex = 0;
listBox1.DataSource = listBox1.Items;
}
private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
}
private void button1_Click(object sender, EventArgs e)
{
do
{
webBrowser1.Navigate(listBox1.SelectedItem.ToString());
while (webBrowser1.ReadyState != WebBrowserReadyState.Complete)
{
Application.DoEvents();
if(webBrowser1.ReadyState == WebBrowserReadyState.Complete)
{
listBox1.SelectedIndex = listBox1.SelectedIndex+1;
}
}
} while (listBox1.SelectedIndex != null);
}
private void Form1_Load(object sender, EventArgs e)
{
}
}
}

Answer 1

This statement here is causing an issue. When the selected index (in your example 0 - 2) reaches 3, it will throw an exception because there isn't an index of 3 available.
listBox1.SelectedIndex = listBox1.SelectedIndex+1;
Your loop will also never end because the SelectedIndex returns an integer which can never return null. You'll want to modify your code to check for length instead using an new index integer. Keep in mind the count will always return one value higher than the index (count starts at 1, index starts at 0).