Question

I have a class WorkQueue, just take note about the DoNext() for now, the rest are mainly helpers. Basically, WorkQueue is just a Queue of WorkItems. DoNext() is responsible for “Starting a Pending WorkItem with a Free Background Worker”. Also note that it will set the WorkItem.Worker property.

public class WorkQueue<Tin, Tout> :
    INotifyCollectionChanged, IEnumerable<WorkItem<Tin, Tout>>
{
    public bool DoNext()
    {
        // check if any work left in queue
        WorkItem<Tin, Tout> item = GetWork();
        if (item != null)
        {
            // check if any free workers
            BackgroundWorker worker = GetWorker();
            Debug.WriteLine(
                "[WorkQueue.DoNext] Setting Worker to WorkItem: " + worker);
            item.Worker = worker;
            if (worker != null)
            {
                worker.RunWorkerAsync(item);
                return true;
            }
        }
        return false;
    }
    public void AddWork(WorkItem<Tin, Tout> item)
    {
        _queue.Add(item);
        RaiseCollectionChanged(
            new NotifyCollectionChangedEventArgs(
                NotifyCollectionChangedAction.Add, item));
    }

    public WorkItem<Tin, Tout> GetWork()
    {
        return (from i in _queue
                where i.Status == WorkStatus.Pending
                select i).FirstOrDefault();;
    }

    public BackgroundWorker GetWorker()
    {
        return (from worker in _workers
                where worker.IsBusy == false
                select worker).FirstOrDefault();
    }
}

The problem I am having is when I do something like below,

foreach (string filename in fileNames) {
    UploadQueue.AddWork(new WorkItem<string, UploadedImage>(filename));
    UploadQueue.DoNext();
}

Where UploadQueue is a WorkQueue<string, UploadedImage>. On the 1st (first only) DoNext(), the WorkItem.Worker is null. I know that because my Cancel button bound to WorkItem.CancelCommand is disabled. When debugging, I discovered the reason was because worker is null.

_cancelCommand = new RelayCommand(... () =>
{
    // Returns true if WorkItem is being processed with a worker that supports
    // cancellation or if the WorkItem is still Pending
    // False if otherwise, eg. already completed, cancelled etc
    if (Status == WorkStatus.Processing)
    {
        if (_worker != null && _worker.WorkerSupportsCancellation)
            return true;
    } else if (Status == WorkStatus.Pending) {
        return true;
    }
    return false;
});

The solution is to move the DoNext() out of the loop,

foreach (string filename in fileNames)
    UploadQueue.AddWork(new WorkItem<string, UploadedImage>(filename));
UploadQueue.DoNext();

but whats the problem with it inside, why is worker set to null? If its null, from the if clause, the BackgroundWorker should not start?

if (worker != null)
    worker.RunWorkerAsync(item);

Video Demonstrating the Problem

Answer 1

public BackgroundWorker GetWorker()
{
    return (from worker in _workers
            where worker.IsBusy == false
            select worker).FirstOrDefault();
}

if all of the workers are busy, this function will return null;

foreach (string filename in fileNames)
    UploadQueue.AddWork(new WorkItem<string, UploadedImage>(filename));
    UploadQueue.DoNext();

UploadQueue.DoNext() executed multiple times

foreach (string filename in fileNames)
    UploadQueue.AddWork(new WorkItem<string, UploadedImage>(filename));
UploadQueue.DoNext();

UploadQueue.DoNext() executed once.

Then it’s quite clear, if you execute UploadQueue.DoNext() multiple times in a short time period, there will be no worker which is not busy, so you get a null worker