<system.web>    <customErrors mode="On" defaultRedirect="error.asp" redirectMode="ResponseRewrite">      <error statusCode="404" redirect="404.aspx" />    </customErrors>    </system.web>  <system.webServer>    <httpErrors errorMode="Custom">      <!--有3中 Custom, DetailedLocalOnly, Detailed-->      <remove statusCode="404" />      <error statusCode="404" path="/404.aspx" responseMode="ExecuteURL"  />    </httpErrors>  </system.webServer>

    protected void Page_Load(object sender, EventArgs e)    {        Response.StatusCode = 404;    }

    try    {        throw new HttpException(404, "PageNotFound"); //可以 throw to 404page    }    catch (Exception ex)     {        throw ex;    }