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    <p>This is the simplest, naive, thread-pool implementation for educational purposes I could come up with (C# / .NET 3.5). It is not using the .NET's thread pool implementation in any way.</p> <pre><code>using System; using System.Collections.Generic; using System.Threading; namespace SimpleThreadPool { public sealed class Pool : IDisposable { public Pool(int size) { this._workers = new LinkedList&lt;Thread&gt;(); for (var i = 0; i &lt; size; ++i) { var worker = new Thread(this.Worker) { Name = string.Concat("Worker ", i) }; worker.Start(); this._workers.AddLast(worker); } } public void Dispose() { var waitForThreads = false; lock (this._tasks) { if (!this._disposed) { GC.SuppressFinalize(this); this._disallowAdd = true; // wait for all tasks to finish processing while not allowing any more new tasks while (this._tasks.Count &gt; 0) { Monitor.Wait(this._tasks); } this._disposed = true; Monitor.PulseAll(this._tasks); // wake all workers (none of them will be active at this point; disposed flag will cause then to finish so that we can join them) waitForThreads = true; } } if (waitForThreads) { foreach (var worker in this._workers) { worker.Join(); } } } public void QueueTask(Action task) { lock (this._tasks) { if (this._disallowAdd) { throw new InvalidOperationException("This Pool instance is in the process of being disposed, can't add anymore"); } if (this._disposed) { throw new ObjectDisposedException("This Pool instance has already been disposed"); } this._tasks.AddLast(task); Monitor.PulseAll(this._tasks); // pulse because tasks count changed } } private void Worker() { Action task = null; while (true) // loop until threadpool is disposed { lock (this._tasks) // finding a task needs to be atomic { while (true) // wait for our turn in _workers queue and an available task { if (this._disposed) { return; } if (null != this._workers.First &amp;&amp; object.ReferenceEquals(Thread.CurrentThread, this._workers.First.Value) &amp;&amp; this._tasks.Count &gt; 0) // we can only claim a task if its our turn (this worker thread is the first entry in _worker queue) and there is a task available { task = this._tasks.First.Value; this._tasks.RemoveFirst(); this._workers.RemoveFirst(); Monitor.PulseAll(this._tasks); // pulse because current (First) worker changed (so that next available sleeping worker will pick up its task) break; // we found a task to process, break out from the above 'while (true)' loop } Monitor.Wait(this._tasks); // go to sleep, either not our turn or no task to process } } task(); // process the found task lock(this._tasks) { this._workers.AddLast(Thread.CurrentThread); } task = null; } } private readonly LinkedList&lt;Thread&gt; _workers; // queue of worker threads ready to process actions private readonly LinkedList&lt;Action&gt; _tasks = new LinkedList&lt;Action&gt;(); // actions to be processed by worker threads private bool _disallowAdd; // set to true when disposing queue but there are still tasks pending private bool _disposed; // set to true when disposing queue and no more tasks are pending } public static class Program { static void Main() { using (var pool = new Pool(5)) { var random = new Random(); Action&lt;int&gt; randomizer = (index =&gt; { Console.WriteLine("{0}: Working on index {1}", Thread.CurrentThread.Name, index); Thread.Sleep(random.Next(20, 400)); Console.WriteLine("{0}: Ending {1}", Thread.CurrentThread.Name, index); }); for (var i = 0; i &lt; 40; ++i) { var i1 = i; pool.QueueTask(() =&gt; randomizer(i1)); } } } } } </code></pre>
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    1. CO+1 Thank you. I was using this snippet but after an extremely long period of time, I encountered an error: `Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. at System.Collections.Generic.LinkedList'1.InternalInsertNodeBefore(LinkedListNode>'` node, LinkedListNode'1 newNode) at System.Collections.Generic.LinkedList'1.AddLast(T value) at Prog.Pool.Worker()`. Any idea what is causing this?
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    2. CO@Legend not sure what the problem might be, but if I had to guess I'd say it is related to the fact that `_workers` linked list is being accessed outside of lock. If using .NET 4, you could try using `ConcurrentQueue<Action>` instead.
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    3. CO+1 Thank you. You are right. I asked a question here: http://stackoverflow.com/questions/16763626/nullreferenceexception-when-creating-a-thread It seems that the problem was indeed due to the missing lock. Thanks for your time. I'm currently using .NET 3.5 and this works like a charm.
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