How to Recursively Download All Files and Subdirectories via FTP in C#?

C# FTP recursive download of files and subdirectories

When using FTP to automatically synchronize files, you often encounter problems downloading files and subdirectories. This is usually accompanied by a 550 error since the folder is treated as a file to be downloaded.

Identify files and directories

FTP itself does not clearly distinguish between files and directories. Therefore, other methods are needed to identify their types.

• Attempting to download file: If an attempt to download "file" fails, it may be a directory.
• File extension: This can be used to differentiate between files and directories when the file extensions for files in a specific directory are consistent.
• Server-specific directory listing: Parses the server's response to a directory listing command (e.g., LIST). Some servers use specific characters or symbols to indicate directories.

Recursive file download

To download all files (including those in subdirectories), follow these steps:

1. List remote directories and retrieve directory listings.
2. Loop through the list and differentiate between files and directories.
3. For files, download them as usual.
4. For directories, repeat steps 1-3 recursively to retrieve files in subdirectories.

Code implementation

The following code example demonstrates recursive file downloading using the FtpWebRequest class, assuming *nix-style directory listings are used:

using System;
using System.IO;
using System.Net;

public class FtpDownload
{
    public static void DownloadFtpDirectory(string url, NetworkCredential credentials, string localPath)
    {
        FtpWebRequest listRequest = (FtpWebRequest)WebRequest.Create(url);
        listRequest.Method = WebRequestMethods.Ftp.ListDirectoryDetails;
        listRequest.Credentials = credentials;

        var lines = new System.Collections.Generic.List<string>();

        using (var listResponse = (FtpWebResponse)listRequest.GetResponse())
        using (Stream listStream = listResponse.GetResponseStream())
        using (var listReader = new StreamReader(listStream))
        {
            while (!listReader.EndOfStream)
            {
                lines.Add(listReader.ReadLine());
            }
        }

        foreach (string line in lines)
        {
            string[] tokens = line.Split(new[] { ' ' }, 9, StringSplitOptions.RemoveEmptyEntries);
            string name = tokens[8];
            string permissions = tokens[0];

            string localFilePath = Path.Combine(localPath, name);
            string fileUrl = url + name;

            if (permissions[0] == 'd')
            {
                if (!Directory.Exists(localFilePath))
                {
                    Directory.CreateDirectory(localFilePath);
                }

                DownloadFtpDirectory(fileUrl + "/", credentials, localFilePath);
            }
            else
            {
                FtpWebRequest downloadRequest = (FtpWebRequest)WebRequest.Create(fileUrl);
                downloadRequest.Method = WebRequestMethods.Ftp.DownloadFile;
                downloadRequest.Credentials = credentials;

                using (FtpWebResponse downloadResponse = (FtpWebResponse)downloadRequest.GetResponse())
                using (Stream sourceStream = downloadResponse.GetResponseStream())
                using (Stream targetStream = File.Create(localFilePath))
                {
                    byte[] buffer = new byte[10240];
                    int read;
                    while ((read = sourceStream.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        targetStream.Write(buffer, 0, read);
                    }
                }
            }
        }
    }
}

Alternative library

For enhanced functionality and support for MLSD commands, consider using a third-party library such as WinSCP.NET. This library simplifies recursive downloading and supports various server-specific directory listing formats.

using System;
using System.IO;
using WinSCP;

public class FtpDownloadWithLibrary
{
    public static void DownloadFtpDirectory(string url, NetworkCredential credentials, string localPath)
    {
        // 设置会话选项
        SessionOptions sessionOptions = new SessionOptions
        {
            Protocol = Protocol.Ftp,
            HostName = host, // 请替换为你的主机名
            UserName = user, // 请替换为你的用户名
            Password = password, // 请替换为你的密码
        };

        using (Session session = new Session())
        {
            // 连接
            session.Open(sessionOptions);

            // 下载文件
            session.GetFiles(url, localPath).Check();
        }
    }
}

Please note that in the FtpDownloadWithLibrary example you need to replace host, user, password with your actual FTP server information. And you need to install the WinSCP.NET library.

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