C# 使用已启动的 List<> 取消任务

Question

我从列表开始任务并等待 Task.WhenAll

private async void btn_download_Click(object sender, EventArgs e)
    {
        .
        .
        .
        await DownloadMultipleFilesAsync(old_json);
        Console.WriteLine("Download completed.");
    }

这是我用列表开始任务的代码。

private async Task DownloadMultipleFilesAsync(List<media> doclist)
    {
        var token = cancelTokenSource.Token;
        await Task.WhenAll(doclist.Select(doc => DownloadFileAsync(doc)));
        btn_download.Enabled = true;
    }

还有我的下载方法

private async Task DownloadFileAsync(media media)
{
    .
    .
    .
    Console.WriteLine(media.no + media_ext + " started.");
    webClient.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;
    await webClient.DownloadFileTaskAsync(new Uri(media.url), @downloadToDirectory);
    Console.WriteLine(media.no + media_ext + " finished.");
    .
    .
    .
}

输出窗口如下：

1.jpg started.
2.jpg started.
3.jpg started.
4.jpg started.
5.jpg started.
6.jpg started.
7.jpg started.
8.jpg started.
9.jpg started.
10.jpg started.
11.jpg started.
12.jpg started.
13.jpg started.
14.jpg started.
15.jpg started.
16.jpg started.
17.jpg started.
18.jpg started.
19.jpg started.
1.jpg finished.
4.jpg finished.
2.jpg finished.
6.jpg finished.
8.jpg finished.
10.jpg finished.
3.jpg finished.
5.jpg finished.
12.jpg finished.
14.jpg finished.
7.jpg finished.
16.jpg finished.
18.jpg finished.
9.jpg finished.
11.jpg finished.
13.jpg finished.
15.jpg finished.
17.jpg finished.
19.jpg finished.
Download completed.

我想点击 btn_cancel 并取消启动任务并等待完成启动任务。

private void btn_cancel_Click(object sender, EventArgs e)
{
    cancelTokenSource.Cancel();
    cancelTokenSource = new CancellationTokenSource();
}

Answer 1

您需要做的是将取消令牌传递到调用链中，然后在可能的情况下使用它，您还需要注册一个取消回调以调用WebClient.CancelAsync() 以取消下载。

private async void btn_download_Click(object sender, EventArgs e)
{
    .
    .
    .
    var token = cancelTokenSource.Token;
    try
    {
        await DownloadMultipleFilesAsync(old_json, token);
        Console.WriteLine("Download completed.");
    }
    catch(OperationCanceledException ex)
    {
        //If something other than our token caused the cancel bubble up the exception.
        if(ex.CancellationToken != token)
            throw;
    }
}


private async Task DownloadMultipleFilesAsync(List<media> doclist, CancellationToken token)
{
    await Task.WhenAll(doclist.Select(doc => DownloadFileAsync(doc, token));
    btn_download.Enabled = true;
}


private async Task DownloadFileAsync(media media, CancellationToken token)
{
    .
    .
    .
    Console.WriteLine(media.no + media_ext + " started.");
    webClient.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;
    try
    {
        using(token.Register(() => webClient.CancelAsync()))
        {
            await webClient.DownloadFileTaskAsync(new Uri(media.url), @downloadToDirectory);
        }
    }            
    catch (WebException ex)
    {
        //Raise a OperationCanceledException if the request was canceled, otherwise bubble up the exception.
        if(ex.Status == WebExceptionStatus.RequestCanceled)
            throw new OperationCanceledException(token);
        else
            throw;
    }
    Console.WriteLine(media.no + media_ext + " finished.");
    .
    .
    .
}

Answer 2

WebClient 不接受 CancelationToken，这很奇怪。相反，它有一个方法CancelAsync，您可以调用它来取消挂起的调用。

似乎您必须保留您的 webClient 引用，并在单击取消按钮时调用该方法。