Removing element at index 0, throws IndexOutOfRangeException but array not empty

Question

To process data in a large array that is being filled fast, I made few threads to process it in parallel. Deleting the first element and logging the result has to be synchronised so I used AutoResetEvent. The issue is that at some point, when deleting the element, it throws an error that the 0 index is out of range even though it has elements in it. I have no idea how this is happening.

private void InitializeProcessingThreads()
{
    for (int i = 0; i < numberOfThreads; i++)
    {
        ares[i] = new AutoResetEvent(false);
        int index = i;
        threads[i] = new Thread(() =>
        {
            try
            {
                while (true)
                {
                    if (BigArray.Count < index + 1)
                    {
                        Debug.WriteLine("BigArray is smaller than " + (index + 1));
                        continue;
                    }
                    byte[] data = BigArray[index];
                    Processing(data);
                    // do other processing here
                    if (index > 0)
                    {
                        ares[index - 1].WaitOne();
                    }
                    else
                    {
                        ares[numberOfThreads - 1].WaitOne();
                    }
                    Debug.WriteLine(Thread.CurrentThread.Name + " Completed processing");
                    if (BigArray.Count != 0)
                        BigArray.RemoveAt(0);   // Or call another thread to remove all the packets and do some general work
                    ares[index].Set();
                    if (index > 0)
                    {
                        ares[index - 1].Reset();
                    }
                    else
                    {
                        ares[numberOfThreads - 1].Reset();
                    }
                }
            }
            catch (Exception e)
            {
                Debug.WriteLine(Thread.CurrentThread.Name + ", " + BigArray.Count + ", " + e);
                throw;
            }

        });
        threads[i].Name = "Thread" + i;
    }
    ares[numberOfThreads - 1].Set();
}

This circularly runs for ever.

I have no idea how this is happening. I tried to remove element only if size of array is not zero but it still runs and gives an error.

Answer 1

It looks like you need to use another solution for your task. Instead of deleting elements, use the array as a circular buffer with pointers to the first element and to the last element.

This will not only relieve you of the problem described but will also increase productivity.

Use a single object to maintain the buffer for multithreading purposes. Create a dispatcher method to queue requests and allocate the appropriate number of elements per request.