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.