How to cancel parallel loops in .NET C#

Cancelling parallel loops in C# is similar to cancelling “normal” tasks. You will need to supply a ParallelOptions object to the parallel loop assigning a cancellation token to its CancellationToken property. If you don’t know these objects then make sure to check out the following posts:

When you cancel the cancellation token then no new iterations will be started in a parallel loop. However, those already running will finish. Parallel.For and Parallel.ForEach will then throw an OperationCanceledException.

Declare the cancellation token:

CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();

Create a Task that will cancel the token after 5 seconds:

Task.Factory.StartNew(() =>
	Console.WriteLine("Token cancelled");

Define the parallel loop options and assign the cancellation token:

ParallelOptions parallelLoopOptions =
	new ParallelOptions()
		CancellationToken = cancellationTokenSource.Token

Perform some loop that is guaranteed to take more than 5 seconds:

	Parallel.For(0, Int64.MaxValue, parallelLoopOptions, index =>
		double result = Math.Pow(index, 3);
		Console.WriteLine("Index {0}, result {1}", index, result);
catch (OperationCanceledException)
	Console.WriteLine("Cancellation exception caught!");

Run the code and you’ll see that the parallel loop will likely run for slightly more than 5 seconds which is because loops running when the cancellation token is cancelled will be allowed to complete.

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


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

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