Question

I’m just getting to know some of the new .NET concurrent collections like ConcurrentDictionary and ConcurrentQueue and I was running some tests to see what happens when I write parallel to the Queue.

So I ran this:

    private static void ParallelWriteToQueue(Queue<int> queue)
    {
        Stopwatch sw = Stopwatch.StartNew();
        Parallel.For(1,1000001,(i) => queue.Enqueue(i));
        sw.Stop();
        Console.WriteLine("Regular int Queue - " + queue.Count + " time" + sw.ElapsedMilliseconds);
    }

And as I thought I got the next exception:

Source array was not long enough. Check srcIndex and length, and the array's lower bounds.

So this Queue can’t handle concurrent en-queues as predicted.

But, When I changed the type of the queue to string, there was no exception, and the result writes something like

Regular string Queue - 663209 time117

Which means that only about 663k were en-queued.

Why was there no exception?

What happened to all of the not en-queued items?

this is the same function with Queue

    private static void ParallelWriteToQueue(Queue<string> queue)
    {
        Stopwatch sw = Stopwatch.StartNew();
        Parallel.For(1, 100001, (i) => queue.Enqueue(i.ToString()));
        sw.Stop();
        Console.WriteLine("Regular string Queue - " + queue.Count + " time" + +sw.ElapsedMilliseconds);
    }

Answer 1

Whether or not you get an exception has nothing to do with the type you put in the queue. It is non-deterministic, I can reproduce the exception for both types and I can reproduce the case without exception also for both types – without changes to the code.

Running the following snippet shows this:

int exceptions = 0;
int noExceptions = 0;
for (int x = 0; x < 100; ++x)
{
    Queue<int> q = new Queue<int>();
    try
    {
        Parallel.For(1,1000001,(i) => q.Enqueue(i)); 
        ++noExceptions;
    }
    catch
    {
        ++exceptions;
    }
}

Console.WriteLine("Runs with exception: {0}. Runs without: {1}", exceptions, noExceptions);

The output is something like Runs with exception: 96. Runs without: 4

The reason is – as others already mentioned – that Queue is not thread safe. What happens here is called “Race condition”.