How do you set the UserState in the RunWorkerCompletedEventArgs object?

Question

HI all. I have an array of BackgroundWorker objects running instances of a Worker class. When I call the Worker class the object instance does it's thing and then runs out of code (the loop finishes). I'm able to listen to the RunWorkerCompleted() event but when it calls the delegate that I've set up I need to know which of my Worker objects just completed.

I see a UserState property in the RunWorkerCompletedEventArgs that comes to my delegate but I have no idea how to set this in my Worker object as it's finishing.

Any ideas?

snippet from my WorkManager.cs class

public Worker AddWorker()
{
    Worker w = new Worker();

    _workers.Add(w.WorkerID,w);

    BackgroundWorker bg = new BackgroundWorker();
    _bgworkers.Add(bg);

    bg.DoWork += w.Start;
    bg.WorkerReportsProgress = true;
    bg.WorkerSupportsCancellation = true;
    bg.ProgressChanged += ProcessWorkerMessage;
    bg.RunWorkerCompleted += WorkerFinished;


    w.WorkManager = this;
    w.BackgroundWorker = bg;

    bg.RunWorkerAsync(w);


    return w;

}


public void WorkerFinished(object sender, RunWorkerCompletedEventArgs e)
{
    if (_onManagerEvent != null)
        _onManagerEvent(new ManagerEvent { EventDate = DateTime.Now, Message = "Worker ??? successfully ended." });
}

So when my Worker object finishes the loop in its Start() method, what do I do to fill the userState property of the RunWorkerCompleteEventArgs object "e" that is passed to my WorkerFinished method()?

Thanks

Answer 1

Your Start method on the Worker class can set the Result property of the DoWorkEventArgs argument. Here's an example:

void Start(object sender, DoWorkEventArgs e)
{
   //Do your loop and other work.
   e.Result = this;
}

Then in the finish event handler, you can retrieve e.Result:

public void WorkerFinished(object sender, RunWorkerCompletedEventArgs e)
{
    //You should always check e.Cancelled and e.Error before checking e.Result!
    // ... even though I'm skipping that here

    Worker w = e.Result as Worker;
    if( w != null)
    {
        if (_onManagerEvent != null)
            _onManagerEvent(new ManagerEvent 
                    { 
                      EventDate = DateTime.Now, 
                      Message = String.Format("Worker {0} successfully ended."
                                              , w.ToString()) 
                    });
    }
}

Answer 2

That UserState thing is a known bug in BackgroundWorker:

http://www.pluralsight-training.net/community/blogs/mike/archive/2005/10/21/15783.aspx (archive.org link…original link is dead)

What I've done in the past when I've been in your situation is either use RunWorkerCompletedEventArgs.Result (as Philip suggests), or, if possible, have my worker derive from BackgroundWorker (then I can add as much extra state as I want, and get the whole worker as the sender argument to the events raised by BackgroundWorker, while still being able to use Result for its intended purpose).