返回自定义错误页面

public ActionResult Details( string product)
{
   ....
    if (productNotFound) {
        // http://www.eidias.com/blog/2014/7/2/mvc-custom-error-pages
        Response.Clear();
        Response.TrySkipIisCustomErrors = true;
        Response.Write(product + " product not exists");
        Response.StatusCode = (int)HttpStatusCode.NotFound;
        Response.End();
        return null;
    }

}