Several threads in the same lock

Question

I'm trying to implement a thread-safe wrapper for a regular Queue class. But it seems that several threads may acquire the same lock. So my questions are:
1. How it could be?
2. How to avoid it?
3. Any useful advice are pleased

This is my queue:

public class SafeQueue<T> : SimpleQueue<T>
{
    private readonly object sync = new object();
    private readonly Queue<T> queue = new Queue<T>();

    public override T Dequeue()
    {
        T item;
        lock (sync)
        {
            Debug.Print("{1:mm ss ffff} {0} locked in dequeue", Thread.CurrentThread.ManagedThreadId, DateTime.Now);
            item = queue.Dequeue();
        }
        Debug.Print("{1:mm ss ffff} {0} unlocked dequeue", Thread.CurrentThread.ManagedThreadId, DateTime.Now);
        return item;

    }

    public override void Enqueue(T item)
    {
        lock (sync)
        {
            queue.Enqueue( item );
        }
    }

    public override IEnumerator<T> GetEnumerator()
    {
        Queue<T> q;
        lock (sync)
        {
            q = new Queue<T>(queue);
        }

        return ((IEnumerable<T>) q).GetEnumerator();
    }

    public override int Count
    {
        get
        {
            int c;
            lock (sync)
            {
                Debug.Print("{1:mm ss ffff} {0} locked in count", Thread.CurrentThread.ManagedThreadId, DateTime.Now);
                c = queue.Count;
            }
            Debug.Print("{1:mm ss ffff} {0} unlocked count. Ret {2}", Thread.CurrentThread.ManagedThreadId, DateTime.Now,c);
            return c;
        }
    }
}

And this is a piece from where I use it (Work is a thread function):

private readonly SafeQueue<string> taskQueue = new SafeQueue<string>();
private volatile bool stop = false;
...
 private void Work()
    {
        while ( !stop )
        {
            string dir = null;
            if (taskQueue.Count > 0)
            {
                dir = taskQueue.Dequeue();
            }
...

So, the problem basically is: two threads are acquire the lock on Count property and return a positive value. Since that they both try to dequeue an element.

Answer 1

One obvious bug is that an item might be dequeue between you checking the count, and the associated Dequeue. For this reason threadsafe queues typically have an atomic TryDequeue operations.

With such an operation, your code becomes:

while ( !stop )
{
    string dir;
    if(taskQueue.TryDequeue(out dir))
    {
    }

Your debug prints can be misleading too. You only print the "unlocked" line some time after the actual unlock occurs. So between the actual unlock and your debug output a different thread might enter the lock and print its "locked" line, leading to confusing output.

.net has two built in thread safe queues: ConcurrentQueue<T>, which is non blocking, BlockingCollection<T> which is blocking.