Understanding the .NET Task Parallel Library TaskScheduler


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The most talented orchestra will sound uninspired without an equally talented conductor. Similar conclusions can be drawn to a Parallel Programming workload conductor. The .NET Task Parallel Library (TPL) Parallel programming conductor is the TaskScheduler. TaskScheduler is arguably one of the more enigmatic TPL classes. It's hardly noticeable; yet it's critical to a smoothly executing Parallel workload.

Compounding its mystery is the appearance of multiple TPL TaskSchedulers; the Default and specialized TaskSchedulers that, for example, execute Tasks on the Windows Presentation Foundation (WPF) user interface threads. The best way to understand TaskScheduler is to implement a custom TaskScheduler, run a parallel workload on the custom TaskScheduler, and observe what happens. That's exactly what this article will do, beginning with an overview of the sample workload.


A demonstration Parallel workload appears below.

static void Main(string[] args)
    var tasks = new Task[4];
    var scheduler = new SimpleScheduler();
    using (scheduler)//Automatically invoke dispose when you exit using.
        Task taskS1 = new Task(() => 
        { Write("Running 1 seconds"); Thread.Sleep(1000); return "String value 1.."; });
        tasks[0] = taskS1;
        Task taskS2 = new Task(() => 
        { Write("Running 2 seconds"); Thread.Sleep(2000); return "String value 2.."; });
        tasks[1] = taskS2;
        Task taskS3 = new Task(() => 
        { Write("Running 3 seconds"); Thread.Sleep(3000); return "String value 3.."; });
        tasks[2] = taskS3;
        Task taskS4 = new Task(() => 
        { Write("Running 4 seconds"); Thread.Sleep(4000); return "String value 4.."; });
        tasks[3] = taskS4;
        foreach (var t in tasks)
        Write("Press any key to quit..");
static void Write(string msg)
    Console.WriteLine(DateTime.Now.ToString() + " on Thread " + Thread.CurrentThread.ManagedThreadId.ToString() + " -- " + msg);

The code executes 4 Tasks on TPL. A complete introduction to Tasks is beyond the scope of this article, but Understanding Tasks in .NET Framework 4.0 Task Parallel Library is a good introduction.

Tasks simulate a workload executing Thread.Sleeps inside Llamda expressions. The running code produces output resembling what you see below.

Starting Thread 10
5/3/2011 8:27:45 PM on Thread 9 -- Press any key to quit..
5/3/2011 8:27:45 PM on Thread 10 -- Running 1 seconds
5/3/2011 8:27:46 PM on Thread 10 -- Running 2 seconds
5/3/2011 8:27:48 PM on Thread 10 -- Running 3 seconds
5/3/2011 8:27:51 PM on Thread 10 -- Running 4 seconds

Part of the output indicates the Thread Id of the code executing the Write statement. Thread Id 9 is the application's main Thread. Thread 10 is a Thread created by the SimpleScheduler TaskScheduler class. Later in the article I'll explain where this Thread is created.

As stated earlier the article will walk through a custom TaskScheduler. SimpleScheduler is a custom TaskScheduler implementation.

SimpleScheduler Architecture

TaskScheduler is an Abstract class. So, building a custom Scheduler requires some Overrides. A list of Overridable functions appears below.

public virtual int MaximumConcurrencyLevel { get; }
protected abstract IEnumerable GetScheduledTasks();

protected internal abstract void QueueTask(Task task);

protected internal virtual bool TryDequeue(Task task);
protected abstract bool TryExecuteTaskInline(Task task, bool 

As you may have noticed all the overrides are protected. Like other .NET components TPL is as much a Runtime environment as it is a collection of classes. When a Task invokes Start, the Runtime funnels the Task to the QueueTask and the TryExecuteInLine methods on the selected TaskScheduler.

Overriding MaximumConcurrencyLevel and TryDequeue are optional. Every class must Override QueueTask, TryExecuteInLine, and GetScheduledTasks.

Overriding GetScheduledTasks is required for debugger support.

For later reference, the full source code for SimpleScheduler appears below.

public sealed class SimpleScheduler : TaskScheduler, IDisposable
    private BlockingCollection<Task> _tasks = new BlockingCollection<Task>();
    private Thread _main = null;
    public SimpleScheduler()
        _main = new Thread(new ThreadStart(this.Main));
    private void Main()
        Console.WriteLine("Starting Thread " + Thread.CurrentThread.ManagedThreadId.ToString());
        foreach (var t in _tasks.GetConsumingEnumerable())
    /// <summary>
    /// Used by the Debugger
    /// </summary>
    /// <returns></returns>
    protected override IEnumerable<Task> GetScheduledTasks()
        return _tasks.ToArray<Task>();
    protected override void QueueTask(Task task)
        if (!_main.IsAlive) { _main.Start(); }//Start thread if not done so already
    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
        return false;
    #region IDisposable Members
    public void Dispose()
        _tasks.CompleteAdding(); //Drops you out of the thread

QueueTask is the heart of the SimpleScheduler.


The QueueTask implementation appears below.

    protected override void QueueTask(Task task)
        if (!_main.IsAlive) { _main.Start(); }//Start thread if not done so already

As stated earlier, the TPL runtime funnels Tasks to the QueueTask method. QueueTask does two things. First it adds the incoming Task to a BlockingCollection. A complete introduction to BlockingCollection is beyond the scope of this article, but Introducing the .NET Framework 4.0 Task Parallel Library BlockingCollection is a helpful introduction.

After adding to the BlockingCollection; QueueTask starts a Thread that removes Tasks from the BlockingCollection and executes the Task.

Executing a Task

The Main method runs inside of an executing Thread. As you may recall in the output displayed earlier in the article; this is Thread Id #10. Code for the Main method appears below.

    private void Main()
        Console.WriteLine("Starting Thread " + Thread.CurrentThread.ManagedThreadId.ToString());
        foreach (var t in _tasks.GetConsumingEnumerable())

GetConsumingEnumerable returns a Task each time a Task is added to the underlying BlockingCollection. The Foreach loop breaks when CompleteAdding is invoked on the BlockingCollection. In the sample; CompleteAdding is invoked inside the Dispose method. Failing to break out of the loop will keep the Thread alive and waiting for more Tasks.

After the Task executes; the underlying Task populates its Result value. Had there been Wait statements or Continuations these statements would have executed just like any other code attached to the Task.


Aside from the samples, there is not a lot of guidance for building TaskSchedulers. Much of what is written here comes from tinkering with the samples. In fact much of the Microsoft documentation recommends using the default TaskScheduler unless a developer has some really unique scenarios.


TaskSchedulers are an important component of .NET Framework Task Parallel Library. However TaskSchedulers are classes few developers will ever need to implement. A simple understanding of the TaskScheduler role is adequate for leveraging TPL.


Task Schedulers

Task Schedulers and Synchronization Context

About the Author

Jeffrey Juday

Jeff is a software developer specializing in enterprise application integration solutions utilizing BizTalk, SharePoint, WCF, WF, and SQL Server. Jeff has been developing software with Microsoft tools for more than 15 years in a variety of industries including: military, manufacturing, financial services, management consulting, and computer security. Jeff is a Microsoft BizTalk MVP. Jeff spends his spare time with his wife Sherrill and daughter Alexandra.

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