从无源服务器 C# 下载原始 FTP 文件

我必须从ftp服务器获取一个文件，但是我遇到了一些特定的问题。 代码的一般思路是我必须登录三次，然后从服务器获得文件列表。 这很好，但是当我去我需要的两个文件时，我得到一个套接字异常："主机中的软件中止了已建立的连接"

需要明确的是，第一个文件不会生成此异常，但它也不会下载任何内容。 第二个完全失败了。

代码如下：

static void Main(string[] args)
    {
        IPHostEntry dns = Dns.GetHostEntry("SERVER");
        IPEndPoint serverIP = new IPEndPoint(dns.AddressList[0], 21);
        Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

        sock.Connect(serverIP);
        FTPRequest("USER firstusername'n", sock);
        FTPRequest("PASS firstpass'n", sock);
        FTPRequest("USER secondusername'n", sock);
        FTPRequest("PASS secondpass'n", sock);
        FTPRequest("USER thirdusername'n", sock);
        FTPRequest("PASS thirdpass'n", sock);
        FTPRequest("TYPE A'n", sock);
        string s = FTPRequest("PASV'n", sock);
        IPEndPoint listenIP = IPEnd(s);
        Socket sock2 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        sock2.Connect(listenIP);
        FTPRequest("LIST'n", sock);

        ArrayList al = new ArrayList();

        try
        {
            byte[] buffer = new byte[25000];
            int bytes = 0;
            bytes = sock2.Receive(buffer);
            Thread.Sleep(1000);
            string stringList = Encoding.ASCII.GetString(buffer, 0, bytes);

            foreach (string st in stringList.Split(' ', ''n'))
            {
                if (st.Contains(".CSV"))
                    al.Add(st.Trim());
            }
        }
        catch (Exception)
        {
            return;
        }
        sock2.Disconnect(false);
        DateTime dt = new DateTime();
        foreach (string st in al)
        {
            DateTime temp = DateTime.ParseExact(st.Split('_', '.')[4], "yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture);
            if (temp > dt)
                dt = temp;
        }
        string file1 = "";
        string file2 = "";
        foreach (string st in al)
        {
            if (st.Contains(dt.ToString("yyyyMMdd")))
            {
                if (st.Contains("CAN"))
                    file1 = st;
                else
                    file2 = st;
            }
        }
        FTPRequest("TYPE I'n", sock);
        foreach (string file in new string[] { file1, file2 })
        {
            if (file != "")
            {
                Socket sock3 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
                FTPRequest("TYPE I'n", sock);
                sock3.Connect(IPEnd(FTPRequest("PASV'n", sock)));
                FTPRequest("RETR" + file + ''n', sock);
                try
                {
                    byte[] buffer = new byte[25000];
                    int bytes = 0;
                    bytes = sock3.Receive(buffer);

                    FileStream writeStream = new FileStream(@"MYPATH" + file1, FileMode.Create);
                    while (bytes > 0)
                    {
                        writeStream.Write(buffer, 0, bytes);
                        bytes = sock3.Receive(buffer, 0);
                    }

                }
                catch (Exception)
                {
                    return;
                }
                finally
                {
                    sock3.Disconnect(false);
                }
            }
        }
        sock.Disconnect(false);
        Console.Read();
    }

    static string FTPRequest(string command, Socket sock)
    {
        string resp = string.Empty;
        Byte[] bytesReceived = new Byte[1024];
        int bytes = 0;
        Byte[] bytesSent = Encoding.ASCII.GetBytes(command);
        sock.Send(bytesSent, bytesSent.Length, 0);
        Thread.Sleep(1000);
        try
        {
            bytes = sock.Receive(bytesReceived, bytesReceived.Length, 0);
        }
        catch (Exception)
        {
        }
        return Encoding.ASCII.GetString(bytesReceived, 0, bytes);
    }
    static IPEndPoint IPEnd(string pasvResponse)
    {
        string s = pasvResponse.Split(' ')[4];
        s = s.Trim();
        s = s.Trim('(', ')');
        string[] sp = s.Split(',');
        IPHostEntry dns = new IPHostEntry();
        try
        {
            dns = Dns.Resolve(string.Format("{0}.{1}.{2}.{3}", sp[0], sp[1], sp[2], sp[3]));
        }
        catch (Exception)
        {
        }
        int port = int.Parse(sp[4]) * 256 + int.Parse(sp[5]);
        return new IPEndPoint(dns.AddressList[0], port);
    }

我对将 c# 用于 FTP 有点陌生，因此非常感谢任何帮助。