How to wait for thread to finish with .NET?

Question

I've never really used threading before in C# where I need to have two threads, as well as the main UI thread. Basically, I have the following.

public void StartTheActions()
{
//Starting thread 1....
Thread t1 = new Thread(new ThreadStart(action1));
t1.Start();

//Now, I want for the main thread (which is calling StartTheActions() ) to wait for t1 to finish (I have created an event in action1() for this) and then start t2...

//HOW DO I DO THIS?

Thread t2 = new Thread(new ThreadStart(action2));
t2.Start();

}

So, essentially, my question is how to have a thread wait for another one to finish. What is the best way to do this? Thanks!

Answer 1

Add

t1.Join();    // Wait until thread t1 finishes

after you start it, but that won't accomplish much as it's essentialy the same result as running on the main thread!

I can highly recommended reading Joe Albahari's Threading in C# free e-book, if you want to gain an understanding of threading in .NET.

Answer 2

You want the Thread.Join() method, or one of its overloads.

Answer 3

The previous two answers are great, and will work for simple scenarios. There are other ways to synchronize threads, however. The following will also work:

public void StartTheActions()
{
    ManualResetEvent syncEvent = new ManualResetEvent(false);

    Thread t1 = new Thread(
        () =>
        {
            // Do some work...
            syncEvent.Set();
        }
    );
    t1.Start();

    Thread t2 = new Thread(
        () =>
        {
            syncEvent.WaitOne();

            // Do some work...
        }
    );
    t2.Start();
}

ManualResetEvent is one of the various WaitHandle's that the .NET framework has to offer. They can provide much richer thread synchronization capabilities than the simple but very common tools like lock()/Monitor, Thread.Join, etc. They can also be used to synchronize more than two threads, allowing complex scenarios such as a 'master' thread that coordinates multiple 'child' threads, multiple concurrent processes that are dependent upon several stages of each other to be synchronized, etc.

Answer 4

I would have your main thread pass a callback method to your first thread, and when it's done, it will invoke the callback method on the mainthread, which can launch the second thread. This keeps your main thread from hanging while its waiting for a Join or Waithandle. Passing methods as delegates is a useful thing to learn with C# anyway.

Answer 5

I can see 5 options available:

1. Thread.Join

As with Mitch's answer. But this will block your UI thread, however you get a Timeout built in for you.

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2. Use a WaitHandle

ManualResetEvent is a WaitHandle as jrista suggested.

One thing to note is if you want to wait for multiple threads, WaitHandle.WaitAll() won't work by default, as it needs an MTA thread. You can get around this by marking your Main() method with MTAThread - however this blocks your message pump and isn't recommended from what I've read.

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3. Fire an event

See this page by Jon Skeet about events and multi-threading, it's possible that an event can become unsuscribed between the if and the EventName(this,EventArgs.Empty) - it's happened to me before.

(Hopefully these compile, I haven't tried)

public class Form1 : Form
{
    int _count;

    void ButtonClick(object sender, EventArgs e)
    {
        ThreadWorker worker = new ThreadWorker();
        worker.ThreadDone += HandleThreadDone;

        Thread thread1 = new Thread1(worker.Run);
        thread1.Start();

        _count = 1;
    }

    void HandleThreadDone(object sender, EventArgs e)
    {
        // You should get the idea this is just an example
        if (_count == 1)
        {
            ThreadWorker worker = new ThreadWorker();
            worker.ThreadDone += HandleThreadDone;

            Thread thread2 = new Thread1(worker.Run);
            thread2.Start();

            _count++;
        }
    }

    class ThreadWorker
    {
        public event EventHandler ThreadDone;

        public void Run()
        {
            // Do a task

            if (ThreadDone != null)
                ThreadDone(this, EventArgs.Empty);
        }
    }
}

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4. Use a delegate

public class Form1 : Form
{
    int _count;

    void ButtonClick(object sender, EventArgs e)
    {
        ThreadWorker worker = new ThreadWorker(HandleThreadDone);

        Thread thread1 = new Thread1(worker.Run);
        thread1.Start();

        _count = 1;
    }

    void HandleThreadDone()
    {
        // As before - just a simple example
        if (_count == 1)
        {
            ThreadWorker worker = new ThreadWorker();

            Thread thread2 = new Thread1(worker.Run);
            thread2.Start(HandleThreadDone);

            _count++;
        }
    }

    class ThreadWorker
    {
        // Switch to your favourite Action<T> or Func<T>
        public void Run(object state)
        {
            // Do a task

            Action completeAction = (Action)state;
            completeAction.Invoke();
        }
    }
}

If you do use the _count method, it might be an idea (to be safe) to increment it using

Interlocked.Increment(ref _count)

I'd be interested to know the difference between using delegates and events for thread notification, the only difference I know are events are called synchronously.

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5. Do it asynchronously instead

The answer to this question has a very clear description of your options with this method.

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Delegate/Events on the wrong thread

The event/delegate way of doing things will mean your event handler method is on Thread1/Thread2 not the main UI thread, so you will need to switch back right at the top of the HandleThreadDone methods:

// Delegate example
if (InvokeRequired)
{
    Invoke(new Action(HandleThreadDone));
    return;
}