C# Threading/Lock confusion

Question

I have the following code:

var items = new List<string> {"1", "2", "3"}; // 200 items
foreach(var item in items) {
  ThreadPool.QueueUserWorkItem((DoWork), item);
}

private void DoWork(object obj)
{
  lock(this)
  {
    using(var sw = File.AppendText(@"C:\somepath.txt")
    {
      sw.WriteLine(obj);
    }
  }
}

Because of the threading, for some reason, I get a random number of the 200 items written out to the file. 60 or 127 or sometimes only 3. If I remove the ThreadPool and just write inside the original foreach loop, all 200 items are written successfully.

Not sure why this occurs?

Thanks for your help.

Answer 1

The following note from MSDN documentation on ThreadPool says it all:

The threads in the managed thread pool are background threads. That is, their IsBackground properties are true. This means that a ThreadPool thread will not keep an application running after all foreground threads have exited.

Your application simply exits (by reaching end of Main) before your threads finish running.

Answer 2

This is a simple version of what I was alluding to. It uses a single event and does not poll or spin, and it's written so as to be reusable as well as allowing multiple work sets at the same time. The lambda expressions could be factored out, if that's more convenient for debugging.

class Program
{
    static void Main(string[] args)
    {
        var items = new string[] { "1", "2", "3", "300" };
        using (var outfile = File.AppendText("file.txt"))
        {
            using (var ws = new WorkSet<string>(x =>
                    { lock (outfile) outfile.WriteLine(x); }))
                foreach (var item in items)
                    ws.Process(item);
        }
    }

    public class WorkSet<T> : IDisposable
    {
        #region Interface

        public WorkSet(Action<T> action)
        { _action = action; }

        public void Process(T item)
        {
            Interlocked.Increment(ref _workItems);
            ThreadPool.QueueUserWorkItem(o =>
                    { try { _action((T)o); } finally { Done(); } }, item);
        }

        #endregion
        #region Advanced
        public bool Done()
        {
            if (Interlocked.Decrement(ref _workItems) != 0)
                return false;

            _finished.Set();
            return true;
        }

        public ManualResetEvent Finished
        { get { return _finished; } }

        #endregion
        #region IDisposable

        public void Dispose()
        {
            Done();
            _finished.WaitOne();
        }

        #endregion
        #region Fields

        readonly Action<T> _action;
        readonly ManualResetEvent _finished = new ManualResetEvent(false);
        int _workItems = 1;

        #endregion
    }
}

Answer 3

How about short and sweet?

    static int wrkThreads = 0;
    static readonly EventWaitHandle exit = new ManualResetEvent(false);
    static readonly object syncLock = new object();

    static void Main( string[] items )
    {
        wrkThreads = items.Length;

        foreach ( var item in items )
            ThreadPool.QueueUserWorkItem(( DoWork ), item);

        exit.WaitOne();
    }

    static void DoWork( object obj )
    {
        lock ( syncLock ) {
            /* Do your file work here */
        }
        if ( Interlocked.Decrement(ref wrkThreads) == 0 )
            exit.Set();
    }