C#ftp download file has a remote server return error: (500) syntax error, command not recognized

If you download multiple files, sometimes inexplicably 500 server errors, it is possible that the KeepAlive property is not set to cause.

An unhandled exception occurred in the application: 2015/1/6 11:40:56
Exception type: WebException
Exception message: The remote server returned an error: (500) syntax error, command not recognized.

Reference: http://www.cnblogs.com/webabcd/archive/2007/01/21/626242.html

KeepAlive-Specifies whether the connection should be closed or closed after the request is complete, by default true

/// <summary>
        /// FTP download file (with progress bar)
        /// </summary>
        /// <param name="filename"></param>
        Public void DownloadFile(string filename)
        {
            Float percent = 0;
            String filePathName = string.Empty;
            String url = string.Empty;
            filePathName = Path.Combine(Application.StartupPath, filename);
            String dirPath = GetDirPath(filePathName);
            If (!Directory.Exists(dirPath))
                Directory.CreateDirectory(dirPath);
            //=>Replace the path in the file directory as the network path
            Filename = filename.Replace("\\", "/");
            Url = "ftp://" + clientUpdateInfo.UpdateFTPIP + "/" + clientUpdateInfo.UpdatePath + "/" + filename;
            Var reqFtp = (FtpWebRequest)FtpWebRequest.Create(new Uri(url));
            reqFtp.Method = WebRequestMethods.Ftp.DownloadFile;
            reqFtp.UseBinary = true;
            reqFtp.KeepAlive = false;//Be sure to set this property, otherwise an exception will occur when downloading multiple files at once.
            reqFtp.Credentials = new NetworkCredential(clientUpdateInfo.FtpUserName, clientUpdateInfo.FtpUserPwd);
            Var response = (FtpWebResponse)reqFtp.GetResponse();
            Long totalBytes = response.ContentLength;
            If (prog != null)
            {
                this.BeginInvoke(new MethodInvoker(delegate()
                {
                    prog.Maximum = (int)totalBytes;
                }));
            }
            Stream st = response.GetResponseStream();
            Var so = new FileStream(filePathName, FileMode.Create);
            Long totalDownloadedByte = 0;
            Byte[] by = new byte[1024];
            Int osize = st.Read(by, 0, (int)by.Length);
            While (osize > 0)
            {
                totalDownloadedByte = osize + totalDownloadedByte;
                so.Write(by, 0, osize);
                If (prog != null)
                {
                    this.BeginInvoke(new MethodInvoker(delegate()
                    {
                        prog.Value = (int)totalDownloadedByte;
                    }));

                }
                Osize = st.Read(by, 0, (int)by.Length);
                Percent = (float)totalDownloadedByte * 1.0f / (float)totalBytes * 100;
                Application.DoEvents();
                this.BeginInvoke(new MethodInvoker(delegate()
                {
                    lbDownInfo.Text = "Downloading" + filename + ", download progress is: " + Math.Round(percent, 2) + "%";
                    lbDownInfo.Refresh();
                }));
                Application.DoEvents();
            }
            so.Close();
            st.Close();
            response.Close();
        }

        Private void FtpDownload(string filename)
        {
            String filePathName = string.Empty;
            String url = string.Empty;
            filePathName = Path.Combine(Application.StartupPath, filename);
            String dirPath = GetDirPath(filePathName);
            If (!Directory.Exists(dirPath))
                Directory.CreateDirectory(dirPath);
            //=>Replace the path in the file directory as the network path
            Filename = filename.Replace("\\", "/");
            Url = "ftp://" + clientUpdateInfo.UpdateFTPIP + "/" + clientUpdateInfo.UpdatePath + "/" + filename;
            FtpWebRequest reqFTP;
            this.BeginInvoke(new MethodInvoker(delegate()
            {
                this.lbDownInfo.Text = "Start downloading...";
            }));
            FileStream outputStream = new FileStream(filePathName, FileMode.Create);
            reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri(url));
            reqFTP.Credentials = new NetworkCredential(clientUpdateInfo.FtpUserName, clientUpdateInfo.FtpUserPwd);
            reqFTP.Method = WebRequestMethods.Ftp.DownloadFile;
            reqFTP.UseBinary = true;
            reqFTP.KeepAlive = false;

            FtpWebResponse response = (FtpWebResponse)reqFTP.GetResponse();
            Stream ftpStream = response.GetResponseStream();
            Int bufferSize = 1024;
            Int readCount;
            Byte[] buffer = new byte[bufferSize];
            readCount = ftpStream.Read(buffer, 0, bufferSize);
            / / The size of the file on FTP
            Int allbye = GetFtpFileSize(filename);// (int)response.ContentLength;
            Int startbye = 0;
            this.BeginInvoke(new MethodInvoker(delegate()
              {
                  this.prog.Maximum = allbye;
                  this.prog.Minimum = 0;
                  this.prog.Visible = true;
                  this.lbDownInfo.Visible = true;
              }));
            While (readCount > 0)
            {
                outputStream.Write(buffer, 0, readCount);
                readCount = ftpStream.Read(buffer, 0, bufferSize);
                Startbye += readCount;
                this.BeginInvoke(new MethodInvoker(delegate()
                  {
                      this.lbDownInfo.Text = "Downloaded:" + (int)(startbye / 1024) + "KB/" + "Total length:"
                                            + (int)(allbye / 1024) + "KB" + " " + " filename:" + filename;
prog.Value = startbye;
                       this.lbDownInfo.Refresh();
                   }));
                 Application.DoEvents();
                 Thread.Sleep(5);
             }
             this.BeginInvoke(new MethodInvoker(delegate()
             {
                 this.prog.Visible = false;
                 this.lbDownInfo.Text = "Download successful!";
             }));
             ftpStream.Close();
             outputStream.Close();
             response.Close();
         }

C#ftp download file has a remote server return error: (500) syntax error, command not recognized