webBrowser 禁止屏蔽alert confirm open showModalDialog

private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
 {
        IHTMLWindow2 ihtmlWin = webBrowser1.Document.Window.DomWindow as IHTMLWindow2;
        //屏蔽alert
        string s = "var aaa=function(){return true;}
window.alert = aaa;
window.confirm = aaa;
window.open = aaa;
window.showModalDialog = aaa;";
        ihtmlWin.execScript(s, "javascript");
}