Monday, July 26, 2010

Running Parallel Tasks with The Task Parallel Library

Down at home with Conjunctivitis, was boring at home, so was listening to some old classics and then thought of writing a post on how you can run tasks with the Task Parallel Library (TPL).

Going forward, Microsoft encourages developers to use TPL for concurrent programming. In my previous post I talked about data parallelism , where I showed how blocks of work running inside a loop can be scheduled to run on different threads.

In previous versions of .Net if I want to execute a task in another thread I had do this.
Thread thread = new Thread(
() =>
{
//Do some work
Console.WriteLine("Starting thread");
}
);
thread.Start();

With TPL I only do this..

Parallel.Invoke(
() =>
{
//Do some work
Console.WriteLine("Starting thread");
}
);
The static Invoke method of the Parallel class has 2 overloads, the one that we use takes in a number of varying void and parameter less delegates.

If you want more control over, what you pass into the thread and if you also need the return value, you could use the Task class within the System.Threading.Tasks namespace.

.Net 2.0 introduced the Thread class with another constructor that takes in a ParameterizedThreadStart delegate that does not have a return type but takes in an object as a parameter.

With TPL this can be achieved much more easily with a Task class, which will take care of scheduling this work in another thread.
Lets take a look at some code...

Task <int> task = new Task <int>(obj =>
{
return ((int)obj + 10);
},
14);

task.Start();
Console.WriteLine(task.Result);

Line number 1, we create a task object, the generic integer specifies that the return value from the thread is an integer.

Next, the first parameter into the .Ctor of the Task object is a delegate that takes in object and returns a value of the type specified as the generic, in our case it is an integer.

The next parameter is the state object, basically this is the input parameter into the thread. Finally the 3rd parameter takes the actual value of the parameter that we pass into the thread, in this case I am passing 14.

Inside the lambda function, I just add the input value with 10 and return, now I can access the Task.Result property and would see 24.

Accessing the Result property of the Task object before the execution of the thread will cause the calling (main) thread to halt and will return once the value for result is available.

Another efficient way of running this task in TPL is like this....

Task<int>t = Task.Factory.StartNew<int>(
obj =>
{
return ((int)obj + 10);
},
14);

Console.WriteLine(t.Result);

I like the above if I don't need the flexibility of creating the Task separately and the starting it separately.

No comments:

Post a Comment