Question

I am very new to threading, as a beginner in C#. I have a program that will be firing multiple threads inside of a windows app. My aim is to start a new thread for every item within a list. The items in this list are workstation names on anetwork. Each thread that is created will look to do repairs on each machine, when the thread has finished it will write to a log file of any errors found etc. But what i want to be able to determine is when all threads have finished. So if i have 100 machines, 100 threads, how do i determine when all have closed?

Heres my method below :-

private void repairClientsToolStripMenuItem_Click(object sender, EventArgs e)
{
    if (machineList.Count() != 0)
    {
        foreach (string ws in machineList)
        {
            new Thread(new ParameterizedThreadStart(fixClient), stack).Start(ws);
        }
    }
    else
    {
         MessageBox.Show("Please import data before attempting this procedure");
    }
}

Answer 1

The way to do this would be to keep a reference to all the threads and then Join on them. This basically means that the current thread will block until the joined thread completes.

Change your loop to something like:

foreach (string ws in machineList)
{
   var thread = new Thread(new ParameterizedThreadStart(fixClient), stack);
   _machineThreads.Add(thread)
   thread.Start();
}

(where _machineThreads is a list of System.Thread)

You can then block until all are complete with something like:

private void WaitUntilAllThreadsComplete()
{
   foreach (Thread machineThread in _machineThreads)
   {
      machineThread.Join();
   } 
}

HOWEVER – you almost certainly don’t want to be doing this for the scenario you describe:

• You shouldn’t create a large number of threads – explicitly creating hundreds of threads is not a great idea
• You should prefer other approaches – try looking at Parallel.ForEach and System.Threading.Task. These days, .Net gives you lots of help when working with threading and asynchronous tasks – I really would advise you read up on it rather than trying to “roll your own” with explicit threads.
• This looks like a click handler. Is it ASP.NET web forms, or a desktop application? If the former, I certainly wouldn’t advise spawning lots of threads to perform background tasks from the request. In either case, do you really want your web page or GUI to block while waiting for the threads to complete?