解决 NET FTP Response error: (503) .

1. 在应用。NET 访问FTP的Response来返回时，出现了如下错误。

2. 参考如下MSDN 和一些网络资源，尝试后解决了此问题.

   > Ftp GetFileSize randomly throws FTP error 503 (Bad Sequence of Commands)

   >FtpWebRequest re-sending USER and PASS commands half way through download loop

3. 问题是在此处应用此个FTP Request 之前曾经使用些地址创建了一个登录Session， 所以在后面同一地址会话建立时会有这个异常报告。

The remote server returned an error: (503) Bad sequence of commands.

 解决办法就是将 request 对象的 KeepAlive 设为 False，其后基于同一IP创建的Session将不会抛出异常了。

FtpWebRequest reqFTP;
                    reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri(sUnixURL));//"ftp://" + "10.2.159.69" + "/" + fileInf.Name));
                    reqFTP.Credentials = new NetworkCredential("dwei", "Admin98");
                    reqFTP.Method = WebRequestMethods.Ftp.DownloadFile;
                    reqFTP.UseBinary = true;
                    reqFTP.KeepAlive = false;   // If keepalive is true will make the same IP request response lead to failed !
                    FileStream outputStream = new FileStream(FILE_NAME, FileMode.Create);
                    // Create the writer for data.
                    BinaryWriter w = new BinaryWriter(outputStream);
                    FtpWebResponse response = (FtpWebResponse)reqFTP.GetResponse();