Threads that return data in .NET

Question

I'm writing a program where I typically start five threads. The threads return in a non-determinate order. Each thread is calling a method which returns a List.

I'm doing this:

var masterList = List<string>();    
foreach (var threadParam in threadParams)
{
    var expression = threadParam ;
    ThreadStart sub = () => MyMethod(expressions);
    var thread = new Thread(sub)
    {
        Name = expression
    };

    listThreads.Add(thread);
    thread.Start();
}

var abort = true;
while (abort) //Wait until all threads finish
{
    var count = 0;
    foreach (var list in listThreads)
    {
        if (!list.IsAlive)
        {
            count++;
        }
    }

    if (count == listThreads.Count)
    {
        abort = false;
    }
}

So here is the problem:

Each thread when it terminates returns a list I would like to append the masterList declared earlier.

How would one go about this?

Also I KNOW there must be a better way than below to wait for all threads to finish

var abort = true;
while (abort) //Wait until all threads finish
{
    var count = 0;
    foreach (var list in listThreads)
    {
        if (!list.IsAlive)
        {
            count++;
        }
    }

    if (count == listThreads.Count)
    {
        abort = false;
    }
}

Answer 1

Use a WaitHandle

---

Here's an example:

using System;
using System.Threading;

class ThreadSleeper
{
    int seconds;
    AutoResetEvent napDone = new AutoResetEvent(false);

    private ThreadSleeper(int seconds)
    {
        this.seconds = seconds; 
    }

    public void Nap()
    {
        Console.WriteLine("Napping {0} seconds", seconds);
        Thread.Sleep(seconds * 1000);
        Console.WriteLine("{0} second nap finished", seconds);
        napDone.Set();
    }

    public static WaitHandle DoSleep(int seconds)
    {
        ThreadSleeper ts = new ThreadSleeper(seconds);
        Thread thread = new Thread(new ThreadStart(ts.Nap));
        thread.Start();
        return(ts.napDone);
    }
}

public class OperationsThreadsWaitingwithWaitHandle
{
    public static void Main()
    {
        WaitHandle[] waits = new WaitHandle[2];
        waits[0] = ThreadSleeper.DoSleep(8);
        waits[1] = ThreadSleeper.DoSleep(4);

        Console.WriteLine("Waiting for threads to finish");
        WaitHandle.WaitAll(waits);
        Console.WriteLine("Threads finished");
    }
}

---

Links to check out:

• Threads:Waiting with WaitHandle
• Jon Skeet's post
• Difference between Barrier in C# 4.0 and WaitHandle in C# 3.0?
• Novice C# threading: WaitHandles
• WaitHandle Exceptions and Work Arounds
• Multithreading with C#
• WaitHandle, AutoResetEvent and ManualResetEvent Classes in VB.Net