Can you spot the Deadlock 4: the solution!

As most of you correctly highlighted, the last exercise exhibits a deadlock situation. The main thread here simply creates and starts another thread and waits for its completion, but this expectation is never fulfilled.

class Program
{
  static void Main(string[] args)
  {
    var workerThread = new Thread(Worker);
    workerThread.Start();
    workerThread.Join();
  }
...

When one looks at the worker thread code, you realize that the stopping condition is never going to be met: Thread.CurrentThread.IsAlive just cannot be false! The current thread is alive, otherwise it will be dead, hence not executing code.

...
  static void Worker()
  {
    while (Thread.CurrentThread.IsAlive)
    {
      Console.Write("Blah");
    }
  }
}

Based on a true story, this example shows a very common misconception, that the framework provides facilities to control the life cycle of threads, and for that matter that Join will trigger a graceful stop. Well, sorry to break this to you guys, this is just not happening. It is your responsibility to implement an appropriate mechanism.

Here is a valid implementation using a shared flag:

    public class Solution
    {
        private readonly object synchro = new object();
        private bool run;

        static void Main(string[] args) {
            var workerThread = new Thread(Worker);
            run = true;
            workerThread.Start();
            lock (this.synchro) {
                run = false;
            }
            // wait for the thread to start
            workerThread.Join();
        }

        private void Worker() {
            for (;;) {
                lock (this.synchro) {
                    if (!run) {
                        break;
                    }
                }
                Console.Write("Blah ");
            }
        }
    }

New! Available on GitHub: for the curious, the critics and the laziest, I have but the code on GitHub as a VS 2012 solution.

Related articles

• Can you spot the deadlock 4 ?! (dupdob.wordpress.com)