What's Wrong With This Code (#10)

Tuesday, January 9, 2007

This time, Joe Developer thinks he found a bug in the .NET framework. Joe read that the volatile modifier is "usually used for a field that is accessed by multiple threads without using the lock Statement". Joe wants to try this out, and writes the following class.

class Worker

{

    public void Start()

    {

        queue = new Queue<string>();

        Thread[] threads = new Thread[maxThreads];

        for (int i = 0; i < threads.Length; i++)

            threads[i] = new Thread(PopulateQueue);

        Array.ForEach(threads, delegate(Thread t) { t.Start(); });

        Array.ForEach(threads, delegate(Thread t) { t.Join(); });

        Debug.Assert(queue.Count == maxThreads * maxIterations);

    }

    void PopulateQueue()

    {

        for (int i = 0; i < maxIterations; i++)

        {

            queue.Enqueue("foo");

        }

    }

    volatile Queue<string> queue;

    const int maxThreads = 5;

    const int maxIterations = 1000000;

}

This code ran successfully at least a dozen times, then suddenly blew up with the following exception:

System.ArgumentException was unhandled
  Message="Destination array was not long enough. 
           Check destIndex and length, and the array's lower bounds."
  Source="mscorlib"
  ParamName=""
  StackTrace:
       at System.Array.Copy ...
       at System.Collections.Generic.Queue`1.SetCapacity ...
       at System.Collections.Generic.Queue`1.Enqueue ...
       at Worker.PopulateQueue ...
       ...

Joe thinks something has gone terribly wrong in the CLR, and for once, Joe would like to show his boss a problem in someone else's software. What should his boss think?

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