asp.net后台输出提示 ，背景不变白

protected void MsgBox(string message, string url)
        {
            ClientScriptManager cs = Page.ClientScript;

            String csname1 = "PopupScript";
            Type cstype = this.GetType();
if (!cs.IsStartupScriptRegistered(cstype, csname1))
            {
                String cstext1 = "alert('" + Server.HtmlEncode(message) + "');window.location='" + url + "';";
                cs.RegisterStartupScript(cstype, csname1, cstext1, true);
            }
        }

然后调用此方法