unexplainable Null exception error

Question

In one of my applications I've created a sort of queue-list so that my application can send out midi-signals in the correct order.

So whenever I want to play a midi-note, I will add that midinote to a list.

I also have a backgroundworker running which will continuously check if there is note waiting in the "queue" and if there is, he will send it to the outputdevice.

Now I wanted to stresstest this, and 19 out of the 20 it works perfectly. Once every now and then, I get a nullexception error. The truth is that I don't know why it's giving me this error.

The only thing I can think of is that at the moment that the backgroundworker if checking if there is a note waiting in the queue, that there is a second argument which is still missing at that point which is why he trows an error.

However, when I look at the contents of the queue, it's all there.

So let me show you the code:

public class queueClass
{ 
    public int note {get; set;}
    public int delay { get; set; }

    public queueClass(int selectedNote, int selectedDelay)
    {
        note = selectedNote;
        delay = selectedDelay;
    }
}

List<queueClass> midiQueue = new List<queueClass>();

public void playNote(int noteNumber, int delay)
    { 
        if (Enabled)
        {
            // ADD THE NOTE TO THE QUEUE 
            midiQueue.Add(new queueClass(noteNumber, delay));

            if (!bgwMidi.IsBusy)
            {
                bgwMidi.RunWorkerAsync();
                Console.WriteLine(DateTime.Now + " >>> BACKGROUNDWORKER MIDI STARTED");

            }
        }

        
    }

 private void bgwMidi_DoWork(object sender, DoWorkEventArgs e)
    {
        BackgroundWorker worker = sender as BackgroundWorker;

        while (worker.IsBusy)
        {
            if (worker.CancellationPending)
            {
                e.Cancel = true;
                break;
            }
            else
            {
                // CHECK IF THERE IS A MIDINOTE IN THE QUEUE
                if (midiQueue != null && midiQueue.Count > 0)
                {
                    if (midiQueue[0].note > -1 && midiQueue[0].delay > -1)
                    {
                        sendNote(midiQueue[0].note, midiQueue[0].delay);
                    }
                }
            }
        }
    }

The error is thrown on the first line of this code:

if (midiQueue[0].note > -1 && midiQueue[0].delay > -1)
{
   sendNote(midiQueue[0].note, midiQueue[0].delay);
}

and it says:

Object reference not set to an instance of an object

However when I look at my midiQueue it holds the values:

delay: 0

note: 5

For now I've tried to bypass the error by putting it into a try-catch block, but I would like to know what is causing this issue.

Does anybody have an idea on this?

Thanks!

what are the values of midiQueue? When you set breakpoints and step through the code, is it populated? Are the delay and note properties populated? — user1666620
– user1666620, Commented Jul 3, 2015 at 9:49
Your error is not with ints being null.ints cannot be null, their default value is 0. — Nathan
– Nathan, Commented Jul 3, 2015 at 9:50
You're using multithreading using an unthreadsafe object - List<queueClass>. What happens if you put a lock around each place that you use midiQueue? — Enigmativity
– Enigmativity, Commented Jul 3, 2015 at 9:53

Graffito · Accepted Answer · 2015-07-03 22:30:49Z

4

You should lock the midiqueue when Adding or removing items, if these operations are done in different threads.

When you push items:

lock (midiQueue) { midiQueue.Add(new queueClass(noteNumber, delay)); }

When you pop items:

lock (midiQueue) { /* the pop operation(s) */

When you use items:

lock (midiQueue)
{
   if (midiQueue != null && midiQueue.Count > 0)
   {
     if (midiQueue[0].note > -1 && midiQueue[0].delay > -1)
                    sendNote(midiQueue[0].note, midiQueue[0].delay);
   }
}

edited Jul 3, 2015 at 22:30

answered Jul 3, 2015 at 9:55

Graffito

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4 Comments

Cainnech Over a year ago

I did try this and added the lock before adding or removing items. However, that did not work. I'm still getting the exception.

Graffito Over a year ago

I updated my answer: you must also do a lock also when using the queue.

Cainnech Over a year ago

Thanks Graffito for the update, however I can't test it right now as the software is at work, and I won't be able to go there before tuesday, but I'll try it out then and keep you posted.

Cainnech Over a year ago

Well I've tested it and it still gives me issues. However, I don't think it's related to code, but I just think it's related to the Midi-output device. Because when I use the internal Synth-output, I don't have any issues. When I use my MidiSport output it occassionally crashes the entire system.

PMerlet · Accepted Answer · 2015-07-03 09:56:53Z

You check this :

midiQueue != null && midiQueue.Count > 0

So from there you know that you have an instanciated midiQueue and at least one element in midiQueue.

Then you do this :

midiQueue[0].note > -1 && midiQueue[0].delay > -1

And here is your problem. If midiQueue[0] is null then midiQueue[0].note will throw an "Object reference not set to an instance of an object" exception.

So you should modify your test :

if (midiQueue != null && midiQueue.Count > 0)
{
   if (midiQueue[0] != null && midiQueue[0].note > -1 && midiQueue[0].delay > -1)
   {
       sendNote(midiQueue[0].note, midiQueue[0].delay);
   }
}

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