c# - 在 "await DownloadTaskAsync"上调用 WebClient.CancelAsync 时出现 WebException

Question

这很好用:

private WebClient _webClient;

private void ButtonStart_Click(object sender, RoutedEventArgs e) {
    using (_webClient = new WebClient()) {
        _webClient.DownloadFileTaskAsync("https://speed.hetzner.de/100MB.bin", @"D:\100MB.bin");
    }
}

private void ButtonStop_Click(object sender, RoutedEventArgs e) {
    _webClient.CancelAsync();
}

虽然这段代码(注意异步/等待模式)...:

private WebClient _webClient;

private async void ButtonStart_Click(object sender, RoutedEventArgs e) {
    using (_webClient = new WebClient()) {
        await _webClient.DownloadFileTaskAsync("https://speed.hetzner.de/100MB.bin", @"D:\100MB.bin");
    }
}

private void ButtonStop_Click(object sender, RoutedEventArgs e) {
    _webClient.CancelAsync();
}

...抛出以下异常:

System.Net.WebException

The request was aborted: The request was canceled.

   at System.Net.ConnectStream.EndRead(IAsyncResult asyncResult)
   at System.Net.WebClient.DownloadBitsReadCallbackState(DownloadBitsState state, IAsyncResult result)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at WpfApp1.MainWindow.<ButtonStart_Click>d__2.MoveNext() in WpfApp1\MainWindow.xaml.cs:line 19

如何在不引发异常的情况下取消以 await WebClient.DownloadFileTaskAsync() 开始的任务？

Answer 1

异常(exception)正是它应该的工作方式。

如果您不希望该异常从您的事件处理程序传播出去，则捕获该异常。