Using the Task Parallel Library (TPL) for Events

The parallel tasks library was introduced with the .NET Framework 4.0 and is designed to simplify parallelism and concurrency. The API is very straightforward and usually involves passing in an Action to execute. Things get a little more interesting when you are dealing with asynchronous models such as events.

While the TPL has explicit wrappers for the asynchronous programming model (APM) that you can read about here: TPL APM Wrappers, there is no explicit way to manage events.

I usually hide the "muck" of subscribing and waiting for a completed action in events with a callback. For example, the following method generates a random number. I'm using a delay to simulate a service call and a thread task to make the call back asynchronous: you call into the method, then provide a delegate that is called once the information is available.

private static void _GenerateRandomNumber(Action<int> callback)
{
    var random = _random.Next(0, 2000) + 10;
    Console.WriteLine("Generated {0}", random);
    Task.Factory.StartNew(() =>
                                {
                                    Thread.Sleep(1000);
                                    callback(random);
                                }, TaskCreationOptions.None);
}

Now consider an algorithm that requires three separate calls to complete to provide the input values in order to compute a result. The calls are independent so they can be done in parallel. The TPL supports "parent" tasks that wait for their children to complete, and a first pass might look like this:

private static void _Incorrect()
{
           
    var start = DateTime.Now;

    int x = 0, y = 0, z = 0;

    Task.Factory.StartNew(
        () =>
            {
                Task.Factory.StartNew(() => _GenerateRandomNumber(result => x = result),
                                        TaskCreationOptions.AttachedToParent);
                Task.Factory.StartNew(() => _GenerateRandomNumber(result => y = result),
                                        TaskCreationOptions.AttachedToParent);
                Task.Factory.StartNew(() => _GenerateRandomNumber(result => z = result),
                                        TaskCreationOptions.AttachedToParent);
            }).ContinueWith(t =>
                                {
                                    var finish = DateTime.Now;
                                    Console.WriteLine("Bad Parallel: {0}+{1}+{2}={3} [{4}]",
                                        x, y, z,
                                        x+y+z,
                                        finish - start);
                                    _Parallel();                                           
                                });         
}

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