Question

Work on this small test application to learn threading/locking. I have the following code, I would think that the line should only write to console once. However it doesn’t seem to be working as expected. Any thoughts on why? What I’m trying to do is add this Lot object to a List, then if any other threads try and hit that list, it would block. Am i completely misusing lock here?

class Program
{
    static void Main(string[] args)
    {
        int threadCount = 10;

        //spin up x number of test threads
        Thread[] threads = new Thread[threadCount];
        Work w = new Work();
        for (int i = 0; i < threadCount; i++)
        {
            threads[i] = new Thread(new ThreadStart(w.DoWork));
        }
        for (int i = 0; i < threadCount; i++)
        {
            threads[i].Start();
        }

        // don't let the console close
        Console.ReadLine();
    }
}

public class Work
{
    List<Lot> lots = new List<Lot>();
    private static readonly object thisLock = new object();

    public void DoWork()
    {
        Lot lot = new Lot() { LotID = 1, LotNumber = "100" };
        LockLot(lot);
    }

    private void LockLot(Lot lot)
    {
        // i would think that "Lot has been added" should only print once?
        lock (thisLock)
        {
            if(!lots.Contains(lot))
            {
                lots.Add(lot);
                Console.WriteLine("Lot has been added");
            }
        }
    }
}

Answer 1

The lock statement ensures that two pieces of code will not execute simultaneously.

If two threads enter a lock block at once, the seconbd thread will wait until the first one finishes, then continue and execute the block.

In your code, lots.Contains(lot) is always false because the DoWork method creates a different Lot object in each thread. Therefore, eah thread adds another Lot object after acquiring the lock.

You probably want to override Equals and GetHashCode in your Lot class and make it compare by value, so that lots.Contains(lot) will return true for different Lot objects with the same values.