业务场景：

应用程序服务器要与文件服务器分离，因此需要实现跨服务器操作附件，最终决定使用FTP方式

实现方式：

创建FileService类

 public class FileService
    {
        //FTP路径
        public static string url = "ftp://x.x.x.x/";
        //FTP登录名
        public static string loginName = "ftpuser";
        //FTP密码
        public static string passWord = "123456";
        /// <summary>
        /// FTP上传
        /// </summary>
        /// <param name="file">http获取到的文件</param>
        /// <param name="filename">文件名</param>
        /// <returns></returns>
        public static bool FtpUpload(HttpPostedFileBase file, string filename)
        {
            try
            {
                MemoryStream fs = new MemoryStream();
                file.InputStream.CopyTo(fs);
                long length = fs.Length;
                FtpWebRequest ftpReq = (FtpWebRequest)FtpWebRequest.Create(new Uri(url + filename));
                ftpReq.Credentials = new NetworkCredential(loginName, passWord);   //默认为true，连接不会关闭，设置为false，执行之后关闭连接
                ftpReq.KeepAlive = false;
                ftpReq.Method = WebRequestMethods.Ftp.UploadFile;
                ftpReq.UseBinary = true;
                ftpReq.UsePassive = false;
                Stream fsftp = ftpReq.GetRequestStream();

                //设置缓冲大小
                int BufferLength = 5120;
                byte[] b = new byte[BufferLength];
                fs.Seek(0, SeekOrigin.Begin);
                int i;
                while ((i = fs.Read(b, 0, BufferLength)) > 0)
                {
                    fsftp.Write(b, 0, i);
                }

                return true;

            }
            catch (Exception ex)
            {
                return false;
            }

        }





        /// <summary>
        /// 获取附件文件流
        /// </summary>
        /// <param name="fileName"></param>
        /// <returns></returns>
        public static byte[] FtpDownload(string fileName)
        {
            try
            {
                MemoryStream outputStream = new MemoryStream();
                FtpWebRequest ftp = (FtpWebRequest)FtpWebRequest.Create(new Uri(url + fileName));
                ftp.Method = WebRequestMethods.Ftp.DownloadFile;
                ftp.UseBinary = true;
                ftp.UsePassive = false;
                ftp.Credentials = new NetworkCredential(loginName, passWord);
                FtpWebResponse response = (FtpWebResponse)ftp.GetResponse();
                Stream ftpStream = response.GetResponseStream();
                int bufferSize = 2048;
                int readCount;
                byte[] buffer = new byte[bufferSize];
                readCount = ftpStream.Read(buffer, 0, bufferSize);
                while (readCount > 0)
                {
                    outputStream.Write(buffer, 0, readCount);
                    readCount = ftpStream.Read(buffer, 0, bufferSize);
                }
                byte[] file = outputStream.GetBuffer();
                ftpStream.Close();
                outputStream.Close();
                response.Close();
                return file;
            }
            catch (Exception ex)
            {
                return null;
            }
        }
    }

Controller代码

 1  /// <summary>
 2         /// 上传
 3         /// </summary>
 4         /// <returns></returns>
 5         public ActionResult Upload()
 6         {
 7 
 8             if (Request.Files.Count == 0)
 9             {
10                 //Request.Files.Count 文件数为0上传不成功 
11                 return Content("上传文件失败，请重新选择！");
12             }
13             var file = Request.Files[0];
14             if (file.ContentLength == 0)
15             {
16                 //文件大小大（以字节为单位）为0时，做一些操作
17                 return Content("上传文件失败，请重新选择！");
18             }
19             string filename = DateTime.Now.ToString("ffffff") + file.FileName;//取得文件名字 
20             return FileService.FtpUpload(file, filename)! ? Content("操作成功！") : Content("上传文件失败，请重新选择！");
21         }
22 
23 
24 
25         /// <summary>
26         /// 下载
27         /// </summary>
28         /// <param name="filename"></param>
29         /// <returns></returns>
30         public ActionResult Download(string filename)
31         { 
32             var file = FileService.FtpDownload(filename);
33             Stream stream = new MemoryStream(file);
34             stream.Read(file, 0, file.Length);
35             stream.Close();
36             Response.Charset = "UTF-8";
37             Response.ContentType = "application/octet-stream";
38             Response.ContentEncoding = Encoding.GetEncoding("GB2312");
39             Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(filename));
40             Response.BinaryWrite(file);
41             Response.Flush();
42             Response.End();
43             return new EmptyResult();
44         }

调试期间一直报错：227 Entering Passive Mode

网上查了半天也没得到解决方案，最后在一个2008年的老帖子里发现，是FTP访问模式问题，FtpWebRequest默认是被动模式，需要手动改成主动模式，所以多加一行ftpReq.UsePassive = false;就可以了

原文链接：The remote server returned an error: 227 Entering Passive Mode (67,228,53,42,12,130) (microsoft.com)