Is it possible to get a null reference error from the ListBox.Items property?

Question

EDIT 11-20-2009: This question was posted quite some time ago, but the problem just crept up again this morning; so I'm hoping somebody else can provide some insight (though the answers provided already have been helpful).

Once in a blue moon in our production environment we get a NullReferenceException from referencing the Items property of a ListBox control. I've included some example code below.

The parent form of the ListBox in question holds a private Queue<string> called QueuedMessages. This queue receives new messages on events. On a timer that goes off every 500 ms, the following method gets executed:

void DisplayQueuedMessages() {
    lock (QueuedMessages) {
        while (QueuedMessages.Count > 0) {
            string msg = QueuedMessages.Dequeue();
            this.lbxMessages.Items.Insert(0, msg); // NullReferenceException
            if (this.lbxMessages.Items.Count > MAX_LBX_ITEMS) {
                this.lbxMessages.Items.RemoveAt(Me.lbxMessages.Items.Length - 1);
            }
        }
    }
}

Again, as I've mentioned, this only throws NullReferenceException very rarely. In several months of using the application it has happened three or four times.

Furthermore, the few times this has happened, it seems either the ListBox.Items property or just the ListBox itself is mysteriously gone for good: all subsequent methods that add items to the ListBox throw exceptions. The only way to recover is to close the application and bring it back up.

Unfortunately, constantly distracted with a million other things to do, I never got around to adding logging before the insertions. I've added the logging now, but it could be a month or more before we see this problem again. In the meantime, any more ideas? What are some possible explanations for this?

I guess my real question is: Has anyone else ever seen this happen -- accessing a ListBox that did exist and suddenly getting a NullReferenceException -- and were you ever able to figure out why/how to fix the problem?

Answer 1

How sure are you that Me.ListBox1 hasn't become null? That would be my first guess.

Answer 2

Look deeply in all the code, its very likely that Me.ListBox1 is null.

I once worked on an app built by a third party that code like this that was called only in a specific case and caused an exception during asp.net rendering code ...

void ClearItems()
{
   SomeField.Text = "";
   ...
   AnotherField = null; 
   ...
}

Also pay attention to the stack trace in the other failures, it would be different if it blows inside the control's method than if it blows in your code.