如何获取FTP文件根目录文件夹

Question

这个方法只能获取列表，不能获取文件夹。问下这个怎么解决。
 public List<FileSystemItem> GetAllList(string url, string userid, string password)
        {
            List<FileSystemItem> list = new List<FileSystemItem>();
            FtpWebRequest req = (FtpWebRequest)WebRequest.Create(new Uri(url));
            req.Credentials = new NetworkCredential(userid, password);
            req.Method = WebRequestMethods.Ftp.ListDirectory;
            req.UseBinary = true;
            req.UsePassive = true;
            try
            {
                using (FtpWebResponse res = (FtpWebResponse)req.GetResponse())
                {
                    using (StreamReader sr = new StreamReader(res.GetResponseStream()))
                    {
                        string s;
                        while ((s = sr.ReadLine()) != null)
                        {                           
                            FileSystemItem item = new FileSystemItem();
                            DirectoryInfo di = new DirectoryInfo(s);
                            item.Name = di.Name;
                            item.FullName = di.FullName;
                            item.CreationDate = di.CreationTime;
                            item.IsFolder = true;
                            list.Add(item);
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                throw (ex);
            }
            return list;
        }