Listbox,该如何解决

Listbox
      AutoPostBack=true
      protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
      {
       }
     

   Listbox添加该事件后,Item永远选不中,SelectedIndex=-1,我想获得选中某个Item时的SelectedIndex,求各位大侠给个解决方案

------解决方案--------------------
protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
Listbox,该如何解决
        }
    }