How to: Cancel a Parallel.For or ForEach Loop
The Parallel.For and Parallel.ForEach methods support cancellation through the use of cancellation tokens. For more information about cancellation in general, see Cancellation. In a parallel loop, you supply the CancellationToken to the method in the ParallelOptions parameter and then enclose the parallel call in a try-catch block.
The following example shows how to cancel a call to Parallel.ForEach. You can apply the same approach to a Parallel.For call.
namespace CancelParallelLoops { using System; using System.Linq; using System.Threading; using System.Threading.Tasks; class Program { static void Main() { int[] nums = Enumerable.Range(0, 10000000).ToArray(); CancellationTokenSource cts = new CancellationTokenSource(); // Use ParallelOptions instance to store the CancellationToken ParallelOptions po = new ParallelOptions(); po.CancellationToken = cts.Token; po.MaxDegreeOfParallelism = System.Environment.ProcessorCount; Console.WriteLine("Press any key to start. Press 'c' to cancel."); Console.ReadKey(); // Run a task so that we can cancel from another thread. Task.Factory.StartNew(() => { if (Console.ReadKey().KeyChar == 'c') cts.Cancel(); Console.WriteLine("press any key to exit"); }); try { Parallel.ForEach(nums, po, (num) => { double d = Math.Sqrt(num); Console.WriteLine("{0} on {1}", d, Thread.CurrentThread.ManagedThreadId); po.CancellationToken.ThrowIfCancellationRequested(); }); } catch (OperationCanceledException e) { Console.WriteLine(e.Message); } Console.ReadKey(); } } }
If the token that signals the cancellation is the same token that is specified in the ParallelOptions instance, then the parallel loop will throw a single OperationCanceledException on cancellation. If some other token causes cancellation, the loop will throw an AggregateException with an OperationCanceledException as an InnerException.
