如何使用IHttpClientFactory替代WebClient向FTP Server上传文件？

问题背景 #

原有代码使用已过时的WebClient实现FTP文件上传，收到官方过时警告：

SYSLIB0014 'WebClient.WebClient()' is obsolete: 'WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead.'

原有实现代码：

public async Task<BaseResponse<Void>> Handle(FTPUploadRequest request, CancellationToken cancellationToken)
{
    using WebClient client = new()
    {
        Credentials = new NetworkCredential(configuration.FTP.User, configuration.FTP.Password.Decrypt())
    };

   static string BuildUri(FTPUploadRequest request, IOptionsSnapshot<Configuration> configuration) 
       => $"ftp://{configuration.FTP.Server}/{request.DirectoryPath}/{request.FileName}";

   client.UploadFile(
       BuildUri(request, configuration),
       WebRequestMethods.Ftp.UploadFile,
       request.FilePath + "/" + request.FileName);

   return await BaseResponse<Void>.Create(true);
}

需求是通过IHttpClientFactory替代现有实现，但不清楚FTP上传的正确方式，疑惑是否需要使用multipart请求。

关键说明 #

HttpClient不支持FTP协议，它仅为HTTP/HTTPS通信设计，因此无法直接用IHttpClientFactory实现FTP上传，也不需要使用multipart-message（这是HTTP文件上传的规范，和FTP协议无关）。

以下是两种可行的替代方案：

方案一：基于FtpWebRequest重构（原生兼容，支持依赖注入） #

虽然FtpWebRequest也标记为过时，但目前仍能正常运行，可通过封装接口的方式提升可测试性，同时适配依赖注入。

1. 定义FTP客户端接口：

public interface IFtpClient
{
    Task UploadFileAsync(string server, string username, string password, string remotePath, string localFilePath, CancellationToken cancellationToken);
}

2. 实现接口（基于FtpWebRequest）：

public class FtpWebRequestClient : IFtpClient
{
    public async Task UploadFileAsync(string server, string username, string password, string remotePath, string localFilePath, CancellationToken cancellationToken)
    {
        var ftpUri = new Uri($"ftp://{server}/{remotePath}");
        using var request = (FtpWebRequest)WebRequest.Create(ftpUri);
        request.Method = WebRequestMethods.Ftp.UploadFile;
        request.Credentials = new NetworkCredential(username, password);

        using var fileStream = new FileStream(localFilePath, FileMode.Open, FileAccess.Read);
        using var requestStream = await request.GetRequestStreamAsync(cancellationToken);
        await fileStream.CopyToAsync(requestStream, cancellationToken);

        using var response = (FtpWebResponse)await request.GetResponseAsync(cancellationToken);
        // 可根据需求检查响应状态，比如response.StatusCode
    }
}

3. 注册依赖到容器：

services.AddScoped<IFtpClient, FtpWebRequestClient>();

4. 重构原有Handle方法：

private readonly IFtpClient _ftpClient;
private readonly IOptionsSnapshot<Configuration> _configuration;

// 构造函数注入依赖
public YourHandler(IFtpClient ftpClient, IOptionsSnapshot<Configuration> configuration)
{
    _ftpClient = ftpClient;
    _configuration = configuration;
}

public async Task<BaseResponse<Void>> Handle(FTPUploadRequest request, CancellationToken cancellationToken)
{
    var ftpConfig = _configuration.FTP;
    var remotePath = $"{request.DirectoryPath}/{request.FileName}";
    var localFilePath = Path.Combine(request.FilePath, request.FileName);

    await _ftpClient.UploadFileAsync(
        ftpConfig.Server,
        ftpConfig.User,
        ftpConfig.Password.Decrypt(),
        remotePath,
        localFilePath,
        cancellationToken);

    return await BaseResponse<Void>.Create(true);
}

方案二：使用第三方FTP库（推荐，无过时问题，易测试） #

推荐使用FluentFTP成熟第三方库，它支持.NET Core/.NET 5+，原生支持异步操作和依赖注入，且API设计更友好，便于单元测试。

1. 安装NuGet包：

Install-Package FluentFTP

2. 注册FluentFTP到依赖注入容器：

services.AddFluentFTP(c => {
    c.ConfigureClient = (client, serviceProvider) => {
        var config = serviceProvider.GetRequiredService<IOptionsSnapshot<Configuration>>().FTP;
        client.Host = config.Server;
        client.Credentials = new NetworkCredential(config.User, config.Password.Decrypt());
        // 可按需配置端口、加密方式等FTP参数
    };
});

3. 重构Handle方法：

private readonly IFtpClient _ftpClient;
private readonly IOptionsSnapshot<Configuration> _configuration;

public YourHandler(IFtpClient ftpClient, IOptionsSnapshot<Configuration> configuration)
{
    _ftpClient = ftpClient;
    _configuration = configuration;
}

public async Task<BaseResponse<Void>> Handle(FTPUploadRequest request, CancellationToken cancellationToken)
{
    var localFilePath = Path.Combine(request.FilePath, request.FileName);
    var remotePath = $"/{request.DirectoryPath}/{request.FileName}"; // FluentFTP建议使用绝对路径

    await _ftpClient.ConnectAsync(cancellationToken);
    try
    {
        var uploadStatus = await _ftpClient.UploadFileAsync(localFilePath, remotePath, FtpRemoteExists.Overwrite, true, cancellationToken);
        // 可根据uploadStatus判断上传结果
    }
    finally
    {
        await _ftpClient.DisconnectAsync(cancellationToken);
    }

    return await BaseResponse<Void>.Create(true);
}

该方案不仅规避了过时API问题，还提供了更丰富的FTP操作能力（如文件夹创建、文件列表查询等），单元测试时可通过Moq等框架模拟IFtpClient行为，提升测试效率。

内容的提问来源于stack exchange，提问作者decius