index.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
    // handle the AJAX postback
    string eventTarget =
        Convert.ToString(Request.Params.Get("__EVENTTARGET"));
    string eventArgument =
        Convert.ToString(Request.Params.Get("__EVENTARGUMENT"));

    // if the event argument is set, toggle the checkbox
    if (eventArgument != null)
    {
        ToggleCheckBox(eventArgument);
    }
}