<customErrors mode="On" defaultRedirect="~/test2/404.htm">
      <error statusCode="403" redirect="NoAccess.htm" />
      <error statusCode="404" redirect="FileNotFound.htm" />
    </customErrors>
---------------------------在Application_Error中发生错误时转向另外一个页面------------------------
protected void Application_Error(object sender, EventArgs e)
        {
            string errorMsg = Context.Error.Message + Environment.NewLine + Context.Error.StackTrace + "【" + DateTime.Now.ToString() + "】" + Environment.NewLine + "====================================================" + Environment.NewLine;
            File.AppendAllText(Server.MapPath("~/log.txt"), errorMsg);
            Response.Redirect("~/test2/404.htm");
        }