Multithreading: When would I use a Join?

Let’s say you want to start some worker threads to perform some kind of calculation, and then do something afterwards with all the results.

List<Thread> workerThreads = new List<Thread>();
List<int> results = new List<int>();

for (int i = 0; i < 5; i++) {
    Thread thread = new Thread(() => {
        Thread.Sleep(new Random().Next(1000, 5000));
        lock (results) {
            results.Add(new Random().Next(1, 10));
        }
    });
    workerThreads.Add(thread);
    thread.Start();
}

// Wait for all the threads to finish so that the results list is populated.
// If a thread is already finished when Join is called, Join will return immediately.
foreach (Thread thread in workerThreads) {
    thread.Join();
}

Debug.WriteLine("Sum of results: " + results.Sum());

Oh yeah, and don’t use Random like that, I was just trying to write a minimal, easily understandable example. It ends up not really being random if you create new Random instances too close in time, since the seed is based on the clock.