Different methods to cancel a Task

From .NET 4.0, Task’s made our life easier. In comparison with threads, it is much simple to start a new thread execution. Because of this, you can create Task pretty simple, but when you want to make more complex things, you end up with a big headache.
In this post we will see how we should cancel a Task. Creating a task it is pretty simple thing:

Task task = new Tasl(()=> { Console.WriteLine(“New Task”); } );

But how we can cancel a task? To be able to cancel a task we need to use a cancellation token. Using this kind of object we will be able to transmit the cancelation request to our task. Don’t forget that you can use a cancellation token instance only once. This means that we cannot reuse the cancelation tokens more than once.
The task constructor accepts as second parameter a cancelation token. This token can be used in our task when we are in a point when we want to check if our task was cancelled. There are different methods to detect if the current task was cancelled or not.
The simplest way to check if our task was cancelled or not is with ThrowIfCancellationRequested method of CacellationToken. If our task was cancelled, than this method will throw an OperationCanceledException. Be aware that you should not cache this exception in your task. This exception should be catch in the code that call Wait method.

CancellationToken cancellationToken = cancellationTokenSource.Token;
…
cancellationToken.ThrowIfCancellationRequested();

Another way to check if a task was cancelled or not is by checking IsCancellationRquested property of CancellationToken. This property will return true if the task was cancelled. Like in the first option, if we have an infinite loop, we should stop it (this needs to be done from our code) – it will not be made automatically.

if (cancellationToken.IsCancellationRquested)
{
  // do custom action that needs to be done when the task is cancelled
}

There also cases when we cannot have this kind of checks in our code. For example if we call long running methods from external components. In this case we can register to an event of CancellationToken that will be called when our task is cancelled. In this event callback we can do any kind of action to cancel our task.

cancellationToken.Register( () =>
{
  // do custom action that needs to be done to cancel our task
});

When you create a new task, the CancellationToken instance can be obtained pretty easy from CancellationTokenSource. This is used to signal a cancellation action. Be aware of one important thing. If you wait a task with Wait method, than you should surround this call with a try cache. The OperationCanceledException exception needs to be catch. This method is thrown every time when a task is cancelled and ThrowIfCancellationRequested is used.