.NET 2.0+ C# FTP 操作完整指南：FtpWebRequest 上傳、下載、列表、刪除與進階設定

FtpWebRequest 核心類別

.NET Framework 2.0 引入 System.Net.FtpWebRequest，提供原生 FTP 客戶端支援，無需第三方庫。核心步驟：

1. 建立請求：`FtpWebRequest.Create(uri)`
2. 設定認證：`Credentials = new NetworkCredential(user, pass)`
3. 指定操作：`Method = WebRequestMethods.FTP.<操作>`
4. 設定傳輸模式：`UseBinary = true`（預設）、`UsePassive = true`（預設）
5. 取得請求/回應串流進行資料傳輸
6. 關閉串流與回應物件

常用 Method 列舉

上傳檔案完整範例

public void Upload(string localFilePath, string remoteFileName) {
    FileInfo fi = new FileInfo(localFilePath);
    string uri = $"ftp://{FtpServerIP}/{remoteFileName}";

    var req = (FtpWebRequest)WebRequest.Create(uri);
    req.Credentials = new NetworkCredential(FtpUser, FtpPass);
    req.Method = WebRequestMethods.FTP.UploadFile;
    req.UseBinary = true;
    req.KeepAlive = false;          // 傳完即關閉，避免連線佔用
    req.ContentLength = fi.Length;  // 讓伺服器預知大小

    const int BufferSize = 2048;
    byte[] buffer = new byte[BufferSize];

    using (FileStream fs = fi.OpenRead())
    using (Stream rs = req.GetRequestStream()) {
        int read;
        while ((read = fs.Read(buffer, 0, BufferSize)) > 0) {
            rs.Write(buffer, 0, read);
        }
    }
    // 可選：讀取回應確認
    using (var resp = (FtpWebResponse)req.GetResponse()) {
        Console.WriteLine($"上傳完成: {resp.StatusDescription}");
    }
}

下載檔案完整範例

public void Download(string remoteFileName, string localFolder) {
    string uri = $"ftp://{FtpServerIP}/{remoteFileName}";
    string localPath = Path.Combine(localFolder, remoteFileName);

    var req = (FtpWebRequest)WebRequest.Create(uri);
    req.Credentials = new NetworkCredential(FtpUser, FtpPass);
    req.Method = WebRequestMethods.FTP.DownloadFile;
    req.UseBinary = true;

    using (var resp = (FtpWebResponse)req.GetResponse())
    using (Stream rs = resp.GetResponseStream())
    using (FileStream fs = new FileStream(localPath, FileMode.Create)) {
        byte[] buffer = new byte[2048];
        int read;
        while ((read = rs.Read(buffer, 0, buffer.Length)) > 0) {
            fs.Write(buffer, 0, read);
        }
    }
}

取得檔案列表

public string[] ListFiles() {
    var req = (FtpWebRequest)WebRequest.Create($"ftp://{FtpServerIP}/");
    req.Credentials = new NetworkCredential(FtpUser, FtpPass);
    req.Method = WebRequestMethods.FTP.ListDirectory;
    req.UseBinary = true;

    using (var resp = (WebResponse)req.GetResponse())
    using (Stream rs = resp.GetResponseStream())
    using (StreamReader sr = new StreamReader(rs)) {
        string content = sr.ReadToEnd();
        return content.Split(new[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
    }
}

刪除與重命名

public void Delete(string fileName) {
    var req = (FtpWebRequest)WebRequest.Create($"ftp://{FtpServerIP}/{fileName}");
    req.Credentials = new NetworkCredential(FtpUser, FtpPass);
    req.Method = WebRequestMethods.FTP.DeleteFile;
    using (var resp = (FtpWebResponse)req.GetResponse()) { /* 檢查狀態 */ }
}

public void Rename(string oldName, string newName) {
    var req = (FtpWebRequest)WebRequest.Create($"ftp://{FtpServerIP}/{oldName}");
    req.Credentials = new NetworkCredential(FtpUser, FtpPass);
    req.Method = WebRequestMethods.FTP.Rename;
    req.RenameTo = newName; // 關鍵屬性
    using (var resp = (FtpWebResponse)req.GetResponse()) { /* 檢查狀態 */ }
}

關鍵屬性與進階設定

安全性與最佳實踐

• 啟用 SSL：`req.EnableSsl = true`，否則帳密明文傳輸。需伺服器支援 FTPS（Explicit/Implicit）。
• 例外處理：捕獲 `WebException`，檢查 `Response` 為 `FtpWebResponse` 取得狀態碼（550 不存在、530 未登入、553 權限不足等）。
• 大檔案斷點續傳：`FtpWebRequest` 原生不支援。需自行實作 `REST` 指令（`req.Method = "REST "`）或改用 FluentFTP 庫。
• 效能調優：批量操作時保持 `KeepAlive=true` 重用連線；緩衝區 8KB-64KB；平行上傳多檔案可用 `Parallel.ForEach` + 連線池。

現代替代庫推薦

• FluentFTP：功能最完整、支援非同步/續傳/SSL/列表解析/事件進度，NuGet 熱門首選。
• SSH.NET：若需 SFTP（SSH File Transfer Protocol），而非 FTPS。
• System.Net.FtpClient (舊版)：FluentFTP 前身，已停止維護。