使用HttpClient下载文件时，如何添加下载完成事件？

核心思路 #

通过自定义事件和事件参数类，在单个文件下载完成（无论成功/失败）时触发完成事件，同时手动读取流来实现进度跟踪。

步骤1：定义事件参数类 #

用来传递下载过程中的关键信息：

public class FileDownloadCompletedEventArgs : EventArgs
{
    public string FileUrl { get; }
    public string LocalFilePath { get; }
    public bool IsSuccess { get; }
    public Exception? Exception { get; }

    public FileDownloadCompletedEventArgs(string fileUrl, string localFilePath, bool isSuccess, Exception? exception = null)
    {
        FileUrl = fileUrl;
        LocalFilePath = localFilePath;
        IsSuccess = isSuccess;
        Exception = exception;
    }
}

public class FileDownloadProgressEventArgs : EventArgs
{
    public string FileUrl { get; }
    public string LocalFilePath { get; }
    public long BytesDownloaded { get; }
    public long TotalBytes { get; }
    public double ProgressPercentage => TotalBytes > 0 ? (double)BytesDownloaded / TotalBytes * 100 : 0;

    public FileDownloadProgressEventArgs(string fileUrl, string localFilePath, long bytesDownloaded, long totalBytes)
    {
        FileUrl = fileUrl;
        LocalFilePath = localFilePath;
        BytesDownloaded = bytesDownloaded;
        TotalBytes = totalBytes;
    }
}

步骤2：在窗体类中声明事件 #

public event EventHandler<FileDownloadCompletedEventArgs>? FileDownloadCompleted;
public event EventHandler<FileDownloadProgressEventArgs>? FileDownloadProgressChanged;

步骤3：修改下载方法，触发事件 #

替换原有的Download方法，添加进度跟踪和完成事件触发逻辑：

private async Task Download()
{
    FilesDownoads downloads = new FilesDownloads(@"d:\testfiles");
    downloads.PrepareLinks();
    using var client = new HttpClient();

    for (int i = 0; i < files.Count; i++)
    {
        string fileUrl = files[i];
        string localPath = @"e:\files1\file" + i.ToString() + ".txt";
        bool isSuccess = false;
        Exception? ex = null;

        try
        {
            // 获取响应头先拿到文件总大小
            using var response = await client.GetAsync(fileUrl, HttpCompletionOption.ResponseHeadersRead);
            response.EnsureSuccessStatusCode();
            long totalBytes = response.Content.Headers.ContentLength ?? -1;

            using var s = await response.Content.ReadAsStreamAsync();
            using var fs = new FileStream(localPath, FileMode.OpenOrCreate);
            
            byte[] buffer = new byte[8192];
            long bytesDownloaded = 0;
            int bytesRead;

            // 手动读写流，实时计算进度
            while ((bytesRead = await s.ReadAsync(buffer)) > 0)
            {
                await fs.WriteAsync(buffer.AsMemory(0, bytesRead));
                bytesDownloaded += bytesRead;

                // 触发进度更新事件
                FileDownloadProgressChanged?.Invoke(this, new FileDownloadProgressEventArgs(fileUrl, localPath, bytesDownloaded, totalBytes));
            }

            isSuccess = true;
        }
        catch (Exception exception)
        {
            ex = exception;
        }
        finally
        {
            // 无论成功失败，都触发完成事件
            FileDownloadCompleted?.Invoke(this, new FileDownloadCompletedEventArgs(fileUrl, localPath, isSuccess, ex));
        }
    }
}

步骤4：订阅并处理事件 #

在窗体初始化时订阅事件，编写完成后的处理逻辑（注意UI操作需切换到主线程）：

public YourFormName()
{
    InitializeComponent();
    FileDownloadCompleted += OnFileDownloadCompleted;
    FileDownloadProgressChanged += OnFileDownloadProgressChanged;
}

private void OnFileDownloadCompleted(object? sender, FileDownloadCompletedEventArgs e)
{
    // UI操作必须回到主线程
    if (InvokeRequired)
    {
        Invoke(new Action(() => OnFileDownloadCompleted(sender, e)));
        return;
    }

    if (e.IsSuccess)
    {
        // 这里写文件下载成功后的操作，比如校验、解析、记录日志
        MessageBox.Show($"文件 {Path.GetFileName(e.LocalFilePath)} 下载完成");
    }
    else
    {
        MessageBox.Show($"下载失败：{e.FileUrl}\n错误信息：{e.Exception?.Message}");
    }
}

private void OnFileDownloadProgressChanged(object? sender, FileDownloadProgressEventArgs e)
{
    if (InvokeRequired)
    {
        Invoke(new Action(() => OnFileDownloadProgressChanged(sender, e)));
        return;
    }

    // 更新进度显示，比如进度条或标签
    labelProgress.Text = $"当前文件：{Path.GetFileName(e.LocalFilePath)}\n进度：{e.ProgressPercentage:F1}%";
}

关键说明 #

• 完成事件：在finally块中触发，确保每个文件下载结束后都会通知，不管成功还是异常。
• 进度跟踪：通过手动读写流的方式统计已下载字节数，实时触发进度事件；如果服务器返回的响应头没有Content-Length，进度百分比会显示0。
• 线程安全：因为下载操作在后台线程执行，UI操作必须通过Invoke切换到主线程，避免跨线程操作异常。

内容的提问来源于stack exchange，提问作者Daniel Lip