PARALLELIZATION AND THREADING ENHANCEMENTS in Visual Basic .NET

Printer Denso QR Bar Code in Visual Basic .NET PARALLELIZATION AND THREADING ENHANCEMENTS

Tasks are very easy to schedule and I think more intuitive than working with traditional threading and the thread pool. There are a number of ways to create a new task, but before you see them, you need to add the following using directive because all the task functionality is found in the System.Threading.Tasks namespace: using System.Threading.Tasks; The easiest way to create a task is with the Task.Factory.StartNew() method. This method accepts an Action delegate and immediately starts the task when created. Task task1 = Task.Factory.StartNew(() => Console.WriteLine("hello task 1")); Another way to create a task is to pass the code you want run into the task s constructor. The main difference with this method is that you have to explicitly start the task when using this method. This method could be useful for scenarios in which you don t want the task to run as soon as it is declared: Task task2 = new Task(() => Console.WriteLine("hello task 2")); task2.Start();

Task.Wait() and Task.WaitAll()

The Task.Wait() and Task.WaitAll() methods allow you to pause the flow of execution until the tasks you specify have completed their work. The following listing shows an example of using the Wait() method to ensure that task1 has completed and the WaitAll() method to ensure that task2, task3, and task4 have finished before exiting the application: Task Task Task Task task1 task2 task3 task4 = = = = Task.Factory.StartNew(() => Console.WriteLine("hello task 1")); new Task(() => Console.WriteLine("hello task 2")); Task.Factory.StartNew(() => Console.WriteLine("hello task 3")); Task.Factory.StartNew(() => Console.WriteLine("hello task 4"));

task2.Start(); task1.Wait(); Task.WaitAll(task2, task3, task4); Figure 5-4 illustrates the waiting process.

Task.WaitAny()

You can wait for any task to complete with the Task.WaitAny() method. It could be used, for example, if many tasks were retrieving the same data (e.g., the latest Microsoft stock price) from a number of different sources and you didn t care which individual source you received the information from. Task.WaitAny(task2, task3, task4);

You can see whether a task is completed by querying the IsCompleted property. It returns a Boolean value indicating whether the task has completed its work. while (task1.IsCompleted == false) { Console.WriteLine("Waiting on task 1"); }

ContinueWith()

It is often necessary to specify that work should be performed in a specific order. This can be declared in a fluent manner with the ContinueWith() method. In previous examples, the tasks occurred out of the order in which they were created. If you want to enforce this order one way, you could use the ContinueWith() method as follows: Task task3 = Task.Factory.StartNew(() => Console.WriteLine("hello task 1")) .ContinueWith((t)=> Console.WriteLine("hello task 2") ) .ContinueWith((t)=> Console.WriteLine("hello task 3") ) .ContinueWith((t)=> Console.WriteLine("hello task 4") ); The ContinueWith() method also accepts a TaskContinuationOptions enumeration that allows you to specify what should occur if a task fails, as well as a number of other situations. The following code calls the stock service with Stocks[1] as a parameter if the previous task failed to run: Task task3 = Task.Factory.StartNew(() => doSomethingBad()) .ContinueWith((t) => System.Diagnostics.Trace.Write("I will be run"), TaskContinuationOptions.OnlyOnFaulted);

The answer is maybe but not necessarily. Tasks are created in order to perform the work as quick as possible but it is up to the task manager and scheduler to decide the optimum means to achieve this.

You can retrieve a value that has been returned from a task by querying the result property: var data = Task.Factory.StartNew(() => GetResult()); Console.WriteLine("Parallel task returned with value of {0}", data.Result); An alternative method can be used if you are using Task<T> type: Task<string> t = new Task<string>(()=>GetResult()); t.Start(); Console.WriteLine("Parallel task returned with value of {0}", t.Result);

If you try and access the result of a task, and the task has completed its work, the value will be returned as you would expect. If, however, the task has not completed, execution will block until the task has completed. This could slow your application down as the common language runtime (CLR)) waits for a value to be returned. To minimize this, you probably want to run the task as soon as possible before you need access to the actual value.