My name
is
Jon Skeet

Queue of action delegates not invoking correctly, why?

I was trying to make a Queue<Action>. When I went to dequeue and invoke each action, it kept stopping at 4. I walked through the code and it was processing everything correctly. What am I doing wrong here?

public class Program
{
    static void Main(string[] args)
    {
        SomeClass test = new SomeClass();
        test.EnqueuePrint("1");
        test.EnqueuePrint("2");
        test.EnqueuePrint("3");
        test.EnqueuePrint("4");
        test.EnqueuePrint("5");
        test.EnqueuePrint("6");
        test.EnqueuePrint("7");
        test.EnqueuePrint("8");
        Console.WriteLine("Process the queue!");
        test.ProcessQueue();
    }
}

public class SomeClass
{
    private Queue<Action> _actionQueue;

    public SomeClass()
    {
        _actionQueue = new Queue<Action>();
    }
    public void EnqueuePrint(string message)
    {
        _actionQueue.Enqueue(() => Print(message));
    }

    private void Print(string message)
    {
        Console.WriteLine(message);
    }

    public void ProcessQueue()
    {
        for (int i = 0; i < _actionQueue.Count; i++)
        {
            _actionQueue.Dequeue().Invoke();
        }
    }
}

Output

Process the queue!
1
2
3
4

The problem is that the count decreases as you dequeue items. So by the time i is 4, the count is also 4, so the loop stops.

If you want to keep dequeuing until the queue is empty, you could use:

while (_actionQueue.Count > 0)
{
    _actionQueue.Dequeue().Invoke();
}

Or if the action might (in other cases) add more items to the queue and you only want to process as many items as there were originally, you could use:

int originalCount = _actionQueue.Dequeue();
for (int i = 0; i < originalCount; i++)
{
    _actionQueue.Dequeue().Invoke();
}

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