Question

If I add jobs to the thread pool with QueueUserWorkItem… how do I keep my program from going forward until all jobs are completed?

I know I could add some logic to keep the app from running until all jobs are completed, but I want to know if there is something like Thread.Join() or if there’s any way to retrieve each thread that is being assigned a job.

Answer 1

You could use events to sync. Like this:

private static ManualResetEvent resetEvent = new ManualResetEvent(false);

public static void Main()
{
    ThreadPool.QueueUserWorkItem(arg => DoWork());
    resetEvent.WaitOne();
}

public static void DoWork()
{
    Thread.Sleep(5000);
    resetEvent.Set();
}

If you don’t want to embed event set into your method, you could do something like this:

var resetEvent = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(
    arg => 
    {
        DoWork();
        resetEvent.Set();
    });
resetEvent.WaitOne();

For multiple items:

var events = new List<ManualResetEvent>();

foreach(var job in jobs)
{   
    var resetEvent = new ManualResetEvent(false);
    ThreadPool.QueueUserWorkItem(
        arg =>
        {
            DoWork(job);
            resetEvent.Set();
        });
    events.Add(resetEvent);
}
WaitHandle.WaitAll(events.ToArray());