asp.net onbeforeunload

String csname1 = "OnQuitScript";
Type cstype = this.GetType();

ClientScriptManager cs = Page.ClientScript;

if (!cs.IsStartupScriptRegistered(cstype, csname1)) {
        String cstext1 = "window.onbeforeunload = confirmQuit;";
        cs.RegisterStartupScript(cstype, csname1, cstext1, true);
}