SelectedIndexChanged is an event of ListBox in Asp.net. This example shows how to deal with Selected Index changed event. When post back occurs, make sure that you are not binding the listbox again. To avoid listbox binding, check IsPostBack property of the page and prevent binding. Here are common problems and solutions.

SelectedIndexChanged event Not working

Assuming you have set AutoPostBack property of ListBox to true and attached SelectedIndexChanged event. Even though your listBox selection is not functioning as per expectation. Solution: Makesure you are binding the listbox as shown in below code snippet.

protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostBack)
{
BindListBox();
}
}

You may have set the SelectedIndexChanged event to run code behind function. However, it's not working. Example:

protected void lstUsers_SelectedIndexChanged(object sender, EventArgs e)
{
}

Code will never work or selection will not cause post back.

SelectedIndexChanged and AutoPostBack property

The same code above will work and do post back. You need to set AutoPostBack property of ListBox to true.

<asp:ListBox ID="lstUsers"
             runat="server"
             OnSelectedIndexChanged="lstUsers_SelectedIndexChanged"
             AutoPostBack="True">
         </asp:ListBox>

 protected void lstUsers_SelectedIndexChanged(object sender, EventArgs e)
{
Label1.Text = lstUsers.SelectedItem.Text;
}