我如何确定WebClient.DownloadFileTaskAsync是否已取消？

Question

How do i know if a call to WebClient.DownloadFileTaskAsync was cancelled?

My method call is like this:

WebClient webclient = new WebClient();
await webClient.DownloadFileTaskAsync(uri, filename);

The call is then cancelled elsewhere with:

webClient.CancelAsync();

When the await continues, how do i know if it was succesful or was cancelled?

Answer 1

When using await , then an OperationCanceledException object will be thrown if the object that you are awaiting on is canceled.

If the task is not canceled, execution will resume normally after the await point.

Answer 2

If I remember well, you can simply inspect the Task object that you awaited for.

WebClient webclient = new WebClient();
var task = webClient.DownloadFileTaskAsync(uri, filename);
await task;

later, you can inspect it:

task.Status
task.IsCancelled
task.IsCompleted  -> task.Result
task.IsFaulted    -> task.Exception

MSDN:Task

...or just check Jean Hominal's excellent point on that topic (upon task failure, the await simply throws instead of continuing).

Answer 3

As the DownloadFileTaskAsync method returns a Task you should be able to simply look at the IsCancelled property.

if (task.IsCancelled) {
     // Task was cancelled.
}

See this link:

http://msdn.microsoft.com/en-us/library/system.threading.tasks.task(v=vs.110).aspx

IsCancelled - Gets whether this Task instance has completed execution due to being canceled.