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Pausable & resumable async task queue

I got this working and thought I should outline my solution here.

It turns out my use of cancellation tokens was a little haphazard, which was preventing this class from functioning as intended. For example, the following issues were relevant:

  1. If Cancel was called after the last cancellation check had passed in the command, a new command would be loaded (along with its own new cancellation token), and as such the cancel call would have been lost / ignored. This was solved with if (_curCommand.Cts.Token.IsCancellationRequested) return; right after _queue.Dequeue();.

  2. After a cancel had been called, if the command were to be resumed later, then it would need a new cancellation token (otherwise the existing one with cancel = true would still be active). The line _curCommand.InvalidateCancellationToken(); does this by setting the token to null, and then my command refreshes the token when next called.

The full code I used:

public class WqController
    private readonly Queue<ICommand> _queue
= new Queue<ICommand>();
    private Task _queueProcessor;
    private ICommand _curCommand;

    public void Enqueue(ICommand command)

        if (_queueProcessor == null)
_queueProcessor = ProcessQueue();

    private async Task ProcessQueue()
            while (_queue.Count != 0)
                _curCommand = _queue.Peek();

                    await Task.Run(() =>
                catch (OperationCanceledException)
                catch (Exception)
                    Console.WriteLine("FAILED TO
            _queueProcessor = null;
            _curCommand = null;

    public async Task Cancel()
        await _queueProcessor;

    public void Resume()
        _queueProcessor = ProcessQueue();

This all seems to work very smoothly now, and is a big improvement on the Background worker queue implementation I had been using prior.

Categories : C#

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