如何解决FTP错误450

我想获得Ftp文件使用定时器(内部15秒)

但是我的程序不能正常运行，然后在

下面返回错误

"远程服务器返回错误:(450)文件不可用(例如，文件忙)"

这是我的代码。

private void timer1_Tick(object sender, EventArgs e)
        {//interval 15000
            GetFtpFile();
        }
private void GetFtpFile()
        {
            label1.Text = "File Name : ";
            label2.Text = "Process result : ";
            string date = DateTime.Now.ToString("yyMMdd");
            FtpUrl += date;
            try
            {
                FtpWebRequest ftpRequest = (FtpWebRequest)WebRequest.Create(FtpUrl);
                ftpRequest.Credentials = new NetworkCredential("root", "pass");
                ftpRequest.Method = WebRequestMethods.Ftp.ListDirectory;
                FtpWebResponse response = (FtpWebResponse)ftpRequest.GetResponse();
                using (StreamReader streamReader = new StreamReader(response.GetResponseStream()))
                {
                    string lastModified = response.LastModified.ToString();
                    List<string> FileName = new List<string>();
                    while (!streamReader.EndOfStream)
                    {
                        FileName.Add(streamReader.ReadLine().Split('/')[1]);
                    }
                    FileName.Sort();
                    if(FileName[FileName.Count-1].Equals(WriteFile) == false)
                    {
                        WriteFile = FileName[FileName.Count - 1];
                        label1.Text = "FileName : " + WriteFile;
                        using(WebClient wc = new WebClient())
                        {
                            wc.Credentials = new NetworkCredential("root", "pass");
                            byte[] FileData = wc.DownloadData(FtpUrl + @"'"+WriteFile);
                            using(FileStream fs = File.Create(di.FullName + @"'" + WriteFile))
                            {
                                fs.Write(FileData,0,FileData.Length);
                                fs.Close();
                            }
                            label2.Text = "Process result : Success";
                        }
                    }
                    streamReader.Close();
                    response.Close();
                }


            }
            catch(Exception ex)
            {
                _log.LogWrite(ex.Message);
            }