如何将CancellationTokenSource附加到DownloadStringTaskAsync方法并取消异步调用？

Question

我创建了一个示例示例,使用async和await方法使用WebClient调用链接现在我想附加取消异步调用功能.但我无法获得CancellationTokenSource令牌并将DownloadStringTaskAsync附加到此取消令牌.以下是我的代码可以告诉我如何实现这一目标.

private async void DoWork()
        {
            this.Cursor = Cursors.WaitCursor;
            Write("DoWork started.");
            cts = new CancellationTokenSource();
            WebClient wc = new WebClient();
            string result = await wc.DownloadStringTaskAsync(new Uri("http://gyorgybalassy.wordpress.com"));

            if (result.Length < 100000)
            {
                Write("The result is too small, download started from second URL.");
                result = await wc.DownloadStringTaskAsync(new Uri("https://www.facebook.com/balassy"));
            }
            Write("Download completed. Downloaded bytes: " + result.Length.ToString());
            Write("DoWork ended.");
            this.Cursor = Cursors.Default;
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            Write("Cancellation started.");
            this.cts.Cancel();
            Write("Cancellation ended.");
        }

当我的取消按钮调用cts.Cancel时,不会取消DownloadStringTaskAsync调用.为什么取消按钮无法取消异步通话？

Answer 1

WebClient早于.Net 4.5 的异步功能,因此它仅部分支持基于任务的异步模式.这包括拥有自己的取消机制:该CancelAsync()方法,即使使用新-TaskAsync方法也能工作.要在CancellationToken取消a时调用此方法,可以使用其Register()方法:

cts.Token.Register(wc.CancelAsync);

作为替代方案,你可以使用新的HttpClient,正如斯蒂芬建议的那样,它完全支持TAP,包括CancellationTokens.

Answer 2

基于 svick 答案的扩展方法：

public static async Task<string> DownloadStringTaskAsync(this WebClient webClient, string url, CancellationToken cancellationToken) {
    using (cancellationToken.Register(webClient.CancelAsync)) {
        return await webClient.DownloadStringTaskAsync(url);
    }
}

public static async Task<string> DownloadStringTaskAsync(this WebClient webClient, Uri uri, CancellationToken cancellationToken) {
    using (cancellationToken.Register(webClient.CancelAsync)) {
        return await webClient.DownloadStringTaskAsync(uri);
    }
}