如何判断线程池里的线程全部执行完毕

CountdownEvent handler = new CountdownEvent(100);
for (int i = 0; i < 100; i++)
{
var j = i;
ThreadPool.QueueUserWorkItem(_ =>
{
Console.WriteLine("thread " + j);
Thread.Sleep(1000);   //wait 1 seconds to do something
handler.Signal();
});
}

handler.Wait();
Console.WriteLine("finished");