c# - 下载前检查文件在ftp服务器上是否存在

Question

从ftp服务器下载一个文件之前，我想检查它是否存在，如果不存在则must 抛出异常。当文件不存在时，代码示例有效。但是，当文件存在时，执行该行后； "ftpRequest.Method = WebRequestMethods.Ftp.DownloadFile;"它跳到第二个 catch block 并打印“错误:提交请求后无法执行此操作。”我看不到的重点是什么..感谢您的回答。

public void fileDownload(string fileName)
        {
            stream = new FileStream(filePath + fileName, FileMode.Create);
            ftpRequest = (FtpWebRequest)FtpWebRequest.Create(new Uri(ftpPath + fileName));
            ftpRequest.Credentials = new NetworkCredential(userName, password);
            ftpRequest.Method = WebRequestMethods.Ftp.GetFileSize;

            try
            {
                response = (FtpWebResponse)ftpRequest.GetResponse();
                ftpRequest.Method = WebRequestMethods.Ftp.DownloadFile;
                ftpRequest.UseBinary = true;
                response = (FtpWebResponse)ftpRequest.GetResponse();
                ftpStream = response.GetResponseStream();
                cl = response.ContentLength;
                bufferSize = 2048;
                buffer = new byte[bufferSize];
                readCount = ftpStream.Read(buffer, 0, bufferSize);

                while (readCount > 0)
                {
                    stream.Write(buffer, 0, readCount);
                    readCount = ftpStream.Read(buffer, 0, bufferSize);
                }

                ftpStream.Close();
                stream.Close();
                response.Close();
                Console.WriteLine("File : " + fileName + " is downloaded from ftp server"); 
            }
            catch (WebException ex)
            {
                 FtpWebResponse res = (FtpWebResponse)ex.Response;
                 if (res.StatusCode == FtpStatusCode.ActionNotTakenFileUnavailable)
                 {
                     stream.Close();
                     File.Delete(filePath + fileName);
                     Console.WriteLine("File : " + fileName + " does not exists on ftp server");
                     System.Diagnostics.Debug.WriteLine("Error: " + fileName + " is not available on fpt server");
                 }
            }   
            catch (Exception ex)
            {
                System.Diagnostics.Debug.WriteLine("Error: " + ex.Message);
            }
        }

Answer 1

我的理解是您必须为您发出的每个请求创建一个新的 FtpWebRequest。因此，在再次设置 Method 之前，您必须创建一个新方法并再次设置凭据。如此之多以至于您不得不重复以下两行:

ftpRequest = (FtpWebRequest)FtpWebRequest.Create(new Uri(ftpPath + fileName));
ftpRequest.Credentials = new NetworkCredential(userName, password);