Multithreading Multiple Producer and Consumer Threads Won't Sync BlockingCollection Race Condition

Question

I have multiple producers and multiple consumers. My shared resource is the BlockingCollection. However, my code only works if I have one consumer. I know it is a race condition since the output is different each time I run the code.

I thought BlockingCollection would take care of all the syncing etc. but it does not.

How can I sync my shared resource among all the producers and consumers then?

Here is my code:

/// <summary>
/// PURE PRODUCER TYPE
/// </summary>
class Caller
{

    private BlockingCollection<Call> incommingCalls;

    public Caller(BlockingCollection<Call> calls)
    {
        incommingCalls = calls;
        //start the producer thread
        Thread thread = new Thread(new ThreadStart(placeCall));
        thread.Start();
    }

    public void placeCall()
    {
            incommingCalls.Add(myCall);
    }

}



/// <summary>
/// CONSUMER
/// </summary>
class Fresher : Employee
{

    private BlockingCollection<Call> calls;

    public Fresher(BlockingCollection<Call> incalls)
    {
        calls = incalls;
        Thread thread = new Thread(new ThreadStart(HandleCalls));
        thread.Start();

    }

    /// <summary>
    /// 
    /// </summary>
    public void HandleCalls()
    {
        while (!incommingCalls.IsCompleted)
        {
            Call item;
            if (incommingCalls.TryTake(out item, 100000))
            {
                //do something with the call

            } //else do nothing - just wait

        }


}






/// <summary>
/// 
/// </summary>
class CallCenter
{

    private BlockingCollection<Call> fresherCalls;


    private List<Caller> myCallers;
    private List<Employee> myFreshers;


    public CallCenter() 
    {
        //initial incomming calls to the fresher queue
        fresherCalls = new BlockingCollection<Call>();

        myFreshers = new List<Employee>();
        myCallers = new List<Caller>();

        generate_freshers();

        //generate to start the producer
        generate_callers();

    }

    /// <summary>
    /// 
    /// </summary>
    private void generate_freshers() 
    {
        for (int i = 0; i < 1; i++ )
        {
            myFreshers.Add(new Fresher(fresherCalls, tlCalls, locker2));
        }
    }

    /// <summary>
    /// 
    /// </summary>
    private void generate_callers() 
    {
        for (int i = 0; i < 20; i++ )
        {
            myCallers.Add(new Caller(fresherCalls, locker));
        }

    }
}

Answer 1

I know it is a race condition since the output is different each time I run the code.

This is common with multithreading, and not necessarily due to a race condition (at least not a bad one). Order processing in multithreaded scenarios tends to not be deterministic - which would likely change the output.

That being said, with BlockingCollection<T>, it's typically easier to write your consumers as:

public void HandleCalls()
{
    foreach(var item in incommingCalls.GetConsumingEnumerable())
    {
        //do something with the call
    }
}

This will handle all of the synchronization and checking for you, for any number of consumers on the BlockingCollection<T>.

---

Edit: If you need to control the scheduling, and implement some form of Round-Robin Scheduling, you may want to take a look at the Parallel Extension Extras within the samples for the TPL. They provide a RoundRobinTaskScheduler which can be used to schedule Task<T> instances which work in a predictable manner.