Skip to main content

Task Parallel Library - Refresher - Stop an Iteration

I have been talking about the Task Parallel Library (TPL) 2 years back , when it was in CTP, I was taking a class on High Performance Computing yesterday, and I just remembered that I have forgotten all about this library :).

This library has now been officially released with .Net 4.0 and Microsoft recommend you to actually use this library if possible when writing concurrent programs, so that your program can take maximum advantage on the numbers of processes that you have.

I thought of posting a sample code as a refresher,.
Here is the example, I have a list of Customer objects and I need to get the object that matches a specific criteria, lets say the Name property should be "F".

Here is how my Customer object looks like.

public class Customer
public int ID { get; set; }
public string Name { get; set; }
public int Age { get; set; }

Lets assume that the Name property is unique.

If I was to write the algorithm for this in .Net 1.1 or .Net 2.0 my logic would look like this.

foreach (Customer c in dataSource)

if (c.Name == "F")
result = c;

This code will run in the same thread, unless you want to write a partition algorithm and then give crunches of the data source to different threads.

This is where TPL comes in to play, if I was using TPL I would write this code like this.

Customer result = null;
IList dataSource = GetMockDataSource(); //Get the data

Parallel.For(0, dataSource.Count, (i, state) =>
if (!state.IsStopped)
Customer c = dataSource[i];
if (c.Name == "F")
result = c;



This is what happens under the cover, the TPL runtime would partition the array (in our case we are using mere numbers and accessing the Customer object through the index) and create threads and give crunches of the indexes to each and every thread. The runtime can actually now spawn thread on different cores according to resource availability. By comparison this will increase performance as we are dividing the Customer list into crunches and each crunch is processed by different threads managed by the runtime.

Lets examine the code,
Line number 1 and 2 says it all, line number 3 is the place where we are using the TPL library.
The Parallel class is within the System.Threading.Tasks namespace, the static method For has many overloads, in the one that we used, the first parameter specifies the index the loop should start from and the second parameter specifies where the loop should end.

The 3 parameter, takes in an Action Delegate of type , for simplicity I have implemented it as a lambda function.
Within the lambda function, I check if the current Customer object satisfies our criteria, if so I use the ParallelLoopState object to signal to the runtime that we should now stop all iterations as we have found what we have been looking for by signaling ParallelLoopState.Stop().

When you call the Stop method on the ParallelLoopState object, the runtime will not create any more iteration , however, it cannot stop the iteration that have already started, so we explicitly check if the some other thread has signaled to stop by checking the IsStopped property of the ParallelLoopState object.

Although this example could have been done more efficiently using PLINQ, I chose the task library to show the underlining basics.


Popular posts from this blog

Hosting WCF services on IIS or Windows Services?

There came one of those questions from the client whether to use II7 hosting or windows service hosting for WCF services. I tried recollecting a few points and thought of writing it down.
WCF applications can be hosted in 2 main ways- In a Windows service- On IIS 7 and aboveWhen WCF was first released, IIS 6 did not support hosting WCF applications that support Non-HTTP communication like Net.TCP or Net.MSMQ and developers had to rely on hosting these services on Windows Services.With the release of IIS 7, it was possible to deploy these Non-Http based applications also on IIS 7. Following are the benefits of using IIS 7 to host WCF applications
Less development effort
Hosting on Windows service, mandates the creating of a Windows service installer project on windows service and writing code to instantiate the service, whereas the service could just be hosted on IIS by creating an application on IIS, no further development is needed, just the service implementation is n…

Task based Asynchronous pattern, Async & Await and .NET 4.5

One of the key features in .Net 4.5 is to write asynchronous programs much easier. So if I was to write asynchronous programs in .Net 2.0/3.5, I would either follow the event based model or the callback based model. For an example, a synchronous method that does intensive work (say the DoWork()) can be made asynchronous by using the following patterns
1) Implementing the IAsyncResult pattern. in this implementation, 2 methods are exposed for the DoWork() synchronous method, the BeginDoWork() and the EndDoWork() method. The user will call the BeingDoWork() passing in the required parameters and a callback of the delegate type AsyncCallback(IAsyncResult). The BeginDoWork() will spawn a new thread a return control back to the user. Once work is completed in the spawned method, as a last step, it will call the inform the AsyncResult implementation, which in turns will call the EndDoWork() (which is the callback that was passed in to the BeginDoWork()).
2) Implementing the event pattern. Her…

MEF (Managed Extensibility Framework), .NET 4, Dependency Injection and Plug-in Development

Almost after .Net 4 was released I remember reading about MEF (Managed Extensibility Framework), this was a framework for developers to compose their application with required dependencies. At first this looks like the Unity Container used for dependency injection, but MEF is much more than a dependency container but there is nothing stopping you from using MEF as a dependency injector.I remember around 5 years back when I was in a project that created a framework that allows developers to plug-in there modules as WinForm screens. The developer would create a set of screens with the intended functionalities and the drop this component in the bin folder of the framework and then will move on to do some painful configurations for the framework to pick up the module. Dropping the component into the bin folder and doing a bit of configuration is all that s needed for the framework to pick up display the screens. Typically, the configurations would also contain metadata about the screen.Al…