C＃取消执行包含多个任务的任务

Question

I'd like to cancel a task that has multiple tasks in it, for example:

 private async Task<bool> myTask0() {
       var val2 = await myTask2();
       var val3 = await myTask3();            
       return true;
    }

where:

private async Task<bool> myTask1() {
       //run some tasks in paralell
        var myParallel= arrayValues.Select(fileBEanListItem => manageSrcFilesDownload().ToList();
        return true;
    }

   private async Task<bool> myTask2() {
        return await myTask3();
    }

So I'd like to gave the option to cancel myTask0 with all tasks in it - just abort it! Is there a solution:

I am trying:

 var ts = new CancellationTokenSource();
 CancellationToken ct = ts.Token;

 await Task.Factory.StartNew(async () => {
                await myTask0();
        },ct);

and when I need to close it, I use ts.Cancel(true); but no effect, tasks are running anyway.

Answer 1

Cancellation is cooperative . So, you should pass the CancellationToken down through your methods:

private async Task<bool> myTask0(CancellationToken token) {
   var val2 = await myTask2(token);
   var val3 = await myTask3(token);            
   return true;
}

private async Task<bool> myTask1(CancellationToken token) {
   //run some tasks in paralell
   var myParallel= arrayValues.Select(fileBEanListItem => manageSrcFilesDownload(token).ToList();
   return true;
}

private async Task<bool> myTask2(CancellationToken token) {
    return await myTask3(token);
}

And so on, until you either pass the token to APIs that can take it (eg, file download), or until you have your own code that uses CancellationToken.ThrowIfCancellationRequested or CancellationToken.Register to respond to cancellation.

On a side note, StartNew is an anti-pattern . As I explain on my blog, you should use Task.Run instead. In particular, the CancellationToken parameter (for both StartNew and Run ) only cancels the scheduling of the delegate ; they won't abort your code. For proper cancellation support, you have to write code that responds to a CancellationToken .

Answer 2

In addition to Stephen Cleary

MSDN states:

In the Task classes, cancellation involves cooperation between the user delegate, which represents a cancelable operation and the code that requested the cancellation. A successful cancellation involves the requesting code calling the CancellationTokenSource.Cancel method, and the user delegate terminating the operation in a timely manner.