CODEWITHSUNDEEP
c#multithreading
user3200174
user3200174

Reputation: 81

Are threads automatically queued in order to access locked code?

I have a code that manages a large queue of data, it's locked witch lock statement to ensure only a single thread is working on it at a time.

The order of data in queue is really important, and each thread with its parameters can either add or take from it.

How do I ensure threads are queued to start in order of FIFO like my queue? Does the lock statement guarantee this?

 var t = new Thread(() => parse(params));      //This is how I start my threads.
 t.Start();

Upvotes: 0

Views: 506

Answers (2)

Douglas
Douglas

Reputation: 54887

No, the lock statement does not guarantee FIFO ordering. Per Albahari:

If more than one thread contends the lock, they are queued on a “ready queue” and granted the lock on a first-come, first-served basis (a caveat is that nuances in the behavior of Windows and the CLR mean that the fairness of the queue can sometimes be violated).

If you want to ensure that your items are retrieved in a FIFO order, you should use the ConcurrentQueue<T> collection instead.

Edit: If you're targeting .NET 2.0, you could use a custom implementation for a concurrent thread-safe queue. Here's a trivial one:

public class ThreadSafeQueue<T>
{
    private readonly object syncLock = new object();
    private readonly Queue<T> innerQueue = new Queue<T>();

    public void Enqueue(T item)
    {
        lock (syncLock)
            innerQueue.Enqueue(item);
    }

    public bool TryDequeue(out T item)
    {
        lock (syncLock)
        {
            if (innerQueue.Count == 0)
            {
                item = default(T);
                return false;
            }

            item = innerQueue.Dequeue();
            return true;
        }
    }
}

Upvotes: 3

S.N
S.N

Reputation: 5140

Lock does't guarantee First In First Out access. An alternate approach would be Queue if you are limited with .NET 2.0. Keep in mind that, Queue is not thread safe hence you should synchronize the access.

Upvotes: 1

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