ftp上载 包501异常

ftp下载 包501错误
ftp下载 包501错误,说参数有错或无效上代码

C# code

 public static bool DisplayFileFromServer(Uri serverUri,string pwd)         {            // The serverUri parameter should start with the ftp:// scheme.            if (serverUri.Scheme != Uri.UriSchemeFtp)            {                return false;            }                      WebClient request = new WebClient();                      //连接            request.Credentials = new NetworkCredential("forumDB", pwd);            try            {                byte[] newFileData = request.DownloadData(serverUri.ToString());                string fileString = System.Text.Encoding.UTF8.GetString(newFileData);                Console.WriteLine(fileString);            }            catch (WebException e)            {                Console.WriteLine(e.ToString());            }            return true;        } static void Main(string[] args)        {           // Download(@"c:/","a.txt");            DisplayFileFromServer(new Uri("ftp://117.79.93.200//13_2_db120617.rar"),"ddddd");}