如何将CancellationTokenSource附加到DownloadStringTaskAsync方法并取消异步调用? [英] How to attach CancellationTokenSource to DownloadStringTaskAsync method and cancel the async call?
问题描述
我正在创建一个示例示例,以使用异步和等待方法使用WebClient调用链接,现在我也想附加取消异步调用功能。但是我无法获取CancellationTokenSource令牌并将DownloadStringTaskAsync附加到此取消令牌。
I an creating a sample example to call link using WebClient using async and await method now I want to attach cancel async call functionality also. But I am not able to get CancellationTokenSource token and attach DownloadStringTaskAsync to this cancellation token. Following Is my code can anyone tell me how to accomplish this.
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时,不会取消取消DownloadStringTaskAsync调用。为什么取消按钮无法取消异步调用?
When my Cancel button calls cts.Cancel the DownloadStringTaskAsync call is not canceled. Why cancel button is not able to cancel the Async calls?
推荐答案
WebClient 的异步功能
早于.Net 4.5,因此它仅支持基于任务的异步模式部分地。这包括拥有自己的取消机制: CancelAsync()
方法,甚至可以使用新的 -TaskAsync
方法。若要取消 CancellationToken
时调用此方法,可以使用其 Register()
方法:
The async capabilities of WebClient
predate .Net 4.5, so it supports the Task-based Asynchronous Pattern only partially. That includes having its own cancellation mechanism: the CancelAsync()
method, which works even with the new -TaskAsync
methods. To call this method when a CancellationToken
is canceled, you can use its Register()
method:
cts.Token.Register(wc.CancelAsync);
作为替代,您可以使用新的 HttpClient
,正如Stephen所建议的那样,它完全支持TAP,包括 CancellationToken
s。
As an alternative, you could use the new HttpClient
, as Stephen suggested, which fully supports TAP, including CancellationToken
s.
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