Spawn Multiple Threads for work then wait until all finished

My preference for this is to handle this via a single WaitHandle, and use Interlocked to avoid locking on a counter:

class Program{    static void Main(string[] args)    {        int numThreads = 10;        ManualResetEvent resetEvent = new ManualResetEvent(false);        int toProcess = numThreads;        // Start workers.        for (int i = 0; i < numThreads; i++)        {            new Thread(delegate()            {                Console.WriteLine(Thread.CurrentThread.ManagedThreadId);                // If we're the last thread, signal                if (Interlocked.Decrement(ref toProcess) == 0)                    resetEvent.Set();            }).Start();        }        // Wait for workers.        resetEvent.WaitOne();        Console.WriteLine("Finished.");    }}

This works well, and scales to any number of threads processing, without introducing locking.

I like @Reed's solution. Another way to accomplish the same in .NET 4.0 would be to use a CountdownEvent.

class Program{    static void Main(string[] args)    {        var numThreads = 10;        var countdownEvent = new CountdownEvent(numThreads);        // Start workers.        for (var i = 0; i < numThreads; i++)        {            new Thread(delegate()            {                Console.WriteLine(Thread.CurrentThread.ManagedThreadId);                // Signal the CountdownEvent.                countdownEvent.Signal();            }).Start();        }        // Wait for workers.        countdownEvent.Wait();        Console.WriteLine("Finished.");    }}

If you have more than 64 wait handles for an STA Thread as Mark says. you could create a list with your threads and wait for all to complete in a second loop.

//check that all threads have completed.foreach (Thread thread in threadList){     thread.Join();}