Producer Consumer With AutoResetEvent

Question

I'm trying to use the producer consumer pattern to process and save some data. I'm using AutoResetEvent for signalling between the two therads here is the code I have

Here is the producer function

 public Results[] Evaluate()
    {
        processingComplete = false;
        resultQueue.Clear();
        for (int i = 0; i < data.Length; ++i)
            {
                if (saveThread.ThreadState == ThreadState.Unstarted)
                    saveThread.Start();
               //-....
               //Process data 
               // 
                lock (lockobject)
                {
                    resultQueue.Enqueue(result);
                }

                signal.Set();
            }
            processingComplete = true;
        }

And here is the consumer function

   private void SaveResults()
    {
        Model dataAccess = new Model();

        while (!processingComplete || resultQueue.Count > 0)
        {
            if (resultQueue.Count == 0)
                signal.WaitOne();
            ModelResults result;
            lock (lockobject)

            {
                result = resultQueue.Dequeue();
            }
            dataAccess.Save(result);
        }
        SaveCompleteSignal.Set();
    }

So my issue is sometimes resultQueue.Dequeue() throws InvalidOperation exception because the Queue is empty. I'm not sure what I'm doing wrong shouldn't the signal.WaitOne() above that block the the queue is empty?

Answer 1

You check the Queue's Count outside of a synchronized context. Since the Queue is not threadsafe, this could be a problem (possibly while Enqueue is in process Count return 1 but no item can be dequeued), and it would go seriously wrong if you were to use more than one consumer anyways.

You may want to read the threading articles written by Joseph Albahari, he has also a good sample for your problem as well as a "better" solution without OS synchronization objects.