Waiting on a Task from multiple threads

Question

Is it possible to call Wait() on the same Task from two different threads at the same time?

For example, is the following code valid:

private BlockingCollection<Task> _queue = new BlockingCollection<Task>();
private Thread _taskPumpThread;
...
private void Run()
{
    _taskPumpThread= new Thread(() => TaskPump(...));
    _taskPumpThread.Start();

    DoNothing();
}

private void DoNothing()
{
    Task doNothing = new Task(() => Console.WriteLine("Doing nothing"));
    queue.Add(doNothing);
    doNothing.Wait(); // no try-catch for brevity
}

private void TaskPump(CancellationToken token)
{
    while (!token.IsCancellationRequested)
    {
         Task task = queue.Take();
         task.Start();
         task.Wait(); 
    }
}

Motivation:
I'm implementing a controller which ultimately serialized requests to a serial port, and handles notifications from the port. The requests can be sent from different threads, and they should be able to wait for a result (or an exception), but can opt to work asynchronously.

I'm considering the design above, as Task s nicely handle all cases (synchronous result, async calls and exceptions).

A related, though not identical, question, can be found here .

Answer 1

I'll limit myself to answering your very specific question.

If you run the following short program you'll see that the answer is yes , you can have multiple threads wait on the same task:

public static void Main(string[] args)
{
    Task t = Task.Delay(10000);
    new Thread(() => { ThreadRun(t); }).Start();
    new Thread(() => { ThreadRun(t); }).Start();
    Console.WriteLine("Main thread exits"); // this prints immediately.
}

private static void ThreadRun(Task t)
{
    t.Wait();
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " finished waiting"); // this prints about 10 seconds later.
}