Cancelling a Task with a composite cancellation token in .NET C#

You cannot directly interrupt a Task in .NET while it’s running. You can do it indirectly through the CancellationTokenSource object. This object has a CancellationToken property which must be passed into the constructor of the Task:

CancellationTokenSource cancellationTokenSource	= new CancellationTokenSource();
CancellationToken cancellationToken = cancellationTokenSource.Token;

You can create a composite token that consists of several other tokens. The Task will then be cancelled if any of the underlying tokens has been cancelled. Here’s how you create a composite token:

CancellationTokenSource cancellationTokenSourceOne = new CancellationTokenSource();
CancellationTokenSource cancellationTokenSourceTwo = new CancellationTokenSource();
CancellationTokenSource cancellationTokenSourceThree = new CancellationTokenSource();
CancellationTokenSource compositeTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationTokenSourceOne.Token, cancellationTokenSourceTwo.Token, cancellationTokenSourceThree.Token);

Use this composite in the constructor of the Task:

Task task = Task.Factory.StartNew(() =>
{
	for (int i = 0; i < 100000; i++)
	{
		if (token.IsCancellationRequested)
	        {
		       Console.WriteLine("Task cancellation requested");
		       throw new OperationCanceledException(token);
	        }
        	else
        	{
         		Console.WriteLine(i);
        	}	 
	}
}, compositeTokenSource.Token);

You can cancel the task by calling the Cancel() method of any of the tokens in the composite:


cancellationTokenSourceTwo.Cancel();

Note that this method only signals the wish to cancel a task. .NET will not actively interrupt the task, you’ll have to monitor the status through the IsCancellationRequested property. It is your responsibility to stop the task. In this example we throw an OperationCanceledException which is a must in order to correctly acknowledge the cancellation. If you forget this step then the task status will not be set correctly. Once the task has been requested the stop it cannot be restarted.

If that’s all you want to do, i.e. throw an OperationCanceledException, then there’s a shorter version:

cancellationToken.ThrowIfCancellationRequested();

This will perform the cancellation check and throw the exception in one step. The loop can thus be simplified as follows:

Task task = Task.Factory.StartNew(() =>
{
	for (int i = 0; i < 100000; i++)
	{
		//shorthand
        	compositeTokenSource.ThrowIfCancellationRequested();
		Console.WriteLine(i);
	}
}, compositeTokenSource);

View the list of posts on the Task Parallel Library here.

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About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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