How would I wait for multiple threads to stop?

Question

I have a Main thread that spawns around 20 worker threads.
I need to stop the Main thread until all the other threads are finished.
I know about (thread).Join. But that only works for one thread.

and multiple Joins hurt performance like this.

t1.Join()
t2.Join()
...
t20.Join()

as the program waits one by one for each to stop.

How would I make it such that the main thread waits for all of a set of threads to end?

Answer 1

You should really look into Task Parallelism (Task Parallel Library). It uses a thread-pool, but also manage task-stealing etc.

Quote: "The TPL scales the degree of concurrency dynamically to most efficiently use all the processors that are available. In addition, the TPL handles the partitioning of the work, the scheduling of threads on the ThreadPool, cancellation support, state management, and other low-level details." on Task Parallel Library

You can use it like this:

Task[] tasks = new Task[3]
{
    Task.Factory.StartNew(() => MethodA()),
    Task.Factory.StartNew(() => MethodB()),
    Task.Factory.StartNew(() => MethodC())
};

//Block until all tasks complete.
Task.WaitAll(tasks);

Or if you use some kind of a loop to spawn your threads:

Data Parallelism (Task Parallel Library)

Answer 2

The joins are fine if that's what you want it to do. The main thread still has to wait for all the worker threads to terminate. Check out this website which is a sample chapter from C# in a Nutshell. It just so happens to be the threading chapter: http://www.albahari.com/threading/part4.aspx.