[RESOLVED] how to make main thread wait for all threads before continuing?

I have this code:

Code:

---

for (int i = 0; i < 4; i++)
{
  Thread t = new Thread(new ThreadStart(Proc));
  t.Start();
}

//I want main thread to wait (in this case, for 4 threads to complete) before calling the codes below
Console.Write("Complete");

---

Any ideas?

You probably could do a while loop checking the Thread.IsAlive or Thread.ThreadState every 5 seconds or something:

Code:

---

while (t.IsAlive == true)
{
    Thread.Sleep(5000)
}

---

Sorry if the code doesn't work, coded it without VS.

EDIT: The task is yours to make that code work with 4 threads and not just one ;)

Don't do thread synchronisation by hand. Polling for completion of a background thread is an utterly horrible way of doing things. Spinning up new threads by hand is almost always the wrong thing to do as well. If you have C#4, you can use the new TPL to handle all of this for you:

csharp Code:

List<Task> tasks = new List<Task>();
for (int i = 0; i < 4; i++)
{
    tasks.Add(Task.Factory.StartNew(Proc));
}
 
Task.WaitAll(tasks.ToArray());
Console.WriteLine("Complete");

The WaitAll method will block until all the Tasks passed to it have finished.

If you don't have C#4, I would suggest decorating your Proc call with a WaitHandle that you use to wait on:

csharp Code:

List<WaitHandle> waitHandles = new List<WaitHandle>();
for (int i = 0; i < 4; i++)
{
    ManualResetEvent waitHandle = new ManualResetEvent(false);
    waitHandles.Add(waitHandle);
    Thread t = new Thread(new ThreadStart(() =>
                                                {
                                                    Proc();
                                                    waitHandle.Set();
                                                }));
    t.Start();
}
WaitHandle.WaitAll(waitHandles.ToArray());
Console.WriteLine("Complete");

Of course, this doesn't stop you starting threads by hand, I'd suggest if you need to use this route to use the ThreadPool to stop the overhead of thread creation/destruction.

Just out of curiosity, what is the content of your Proc method? You will usually find that if it's something that causes blocking (such as disk or network I/O) then there is an asynchronous API as well that let's you do things in parallel without needing to multithread your app.

Quote:

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Originally Posted by Evil_Giraffe

Just out of curiosity, what is the content of your Proc method? You will usually find that if it's something that causes blocking (such as disk or network I/O) then there is an asynchronous API as well that let's you do things in parallel without needing to multithread your app.

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Thanks for all the replies. I'm trying to create my own web service stress tester, so I need a bunch of threads to make multiple requests. Will using the waitall method cause the form to hang? I'd like to be able to move the windows form even while the rest of the threads are doing their job.

Yes, the form will hang if you call WaitAll from the UI thread. That blocks the thread that makes the call until everything it is waiting for completes.

The web request API should give you an async version of its methods (BeginRequest/EndRequest). Learning how to use these methods is probably the better way for you to go here.

Alternatively, if you are using the Task Parallel Library (first code snippet I posted) you can do the same thing with continuations. These are bits of code that get scheduled once a task has completed. You can schedule them on the UI thread if they need to update the UI.

I'd suggest reading up on those two areas (bit hard to give you everything you need to know in a single post) and coming back if you have more questions.

I was able to figure out a solution. I just created another thread (that will be in a loop) to basically call the control.invoke() method, which allows the main thread to do it's own updating of the controls (ie. labels).

Quote:

---

Originally Posted by Evil_Giraffe

Don't do thread synchronisation by hand. Polling for completion of a background thread is an utterly horrible way of doing things. Spinning up new threads by hand is almost always the wrong thing to do as well. If you have C#4, you can use the new TPL to handle all of this for you:

csharp Code:

List<Task> tasks = new List<Task>();
for (int i = 0; i < 4; i++)
{
    tasks.Add(Task.Factory.StartNew(Proc));
}
 
Task.WaitAll(tasks.ToArray());
Console.WriteLine("Complete");

The WaitAll method will block until all the Tasks passed to it have finished.

If you don't have C#4, I would suggest decorating your Proc call with a WaitHandle that you use to wait on:

csharp Code:

List<WaitHandle> waitHandles = new List<WaitHandle>();
for (int i = 0; i < 4; i++)
{
    ManualResetEvent waitHandle = new ManualResetEvent(false);
    waitHandles.Add(waitHandle);
    Thread t = new Thread(new ThreadStart(() =>
                                                {
                                                    Proc();
                                                    waitHandle.Set();
                                                }));
    t.Start();
}
WaitHandle.WaitAll(waitHandles.ToArray());
Console.WriteLine("Complete");

Of course, this doesn't stop you starting threads by hand, I'd suggest if you need to use this route to use the ThreadPool to stop the overhead of thread creation/destruction.

---

this codes will freeze the UI.

we've known that since Jul 13th, 2011, 06:04 PM. There's another option available that could be used. The Parallel.For and Parallel.ForEach do all the threading for you. You'll still have ui issue though.

You can use async/await to allow you to await multiple tasks without freezing the UI. Not sure that that was available when this thread was originally active.

Quote:

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Originally Posted by Lord Orwell

we've known that since Jul 13th, 2011, 06:04 PM. There's another option available that could be used. The Parallel.For and Parallel.ForEach do all the threading for you. You'll still have ui issue though.

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Parallel does not solve the UI freeze.

Quote:

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Originally Posted by jmcilhinney

You can use async/await to allow you to await multiple tasks without freezing the UI. Not sure that that was available when this thread was originally active.

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I can use Task or async/await but I don't know which one is better?