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How to: Cancel a Parallel.For or ForEach Loop

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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.

Example

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, 10_000_000).ToArray();
            CancellationTokenSource cts = new();

            // Use ParallelOptions instance to store the CancellationToken
            ParallelOptions options = new()
            {
                CancellationToken = cts.Token,
                MaxDegreeOfParallelism = 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 is 'c')
                    cts.Cancel();
                Console.WriteLine("press any key to exit");
            });

            try
            {
                Parallel.ForEach(nums, options, (num) =>
                {
                    double d = Math.Sqrt(num);
                    Console.WriteLine("{0} on {1}", d, Environment.CurrentManagedThreadId);
                });
            }
            catch (OperationCanceledException e)
            {
                Console.WriteLine(e.Message);
            }
            finally
            {
                cts.Dispose();
            }

            Console.ReadKey();
        }
    }
}

' How to: Cancel a Parallel.For or ForEach Loop

Imports System.Threading
Imports System.Threading.Tasks

Module CancelParallelLoops
    Sub Main()
        Dim nums() As Integer = Enumerable.Range(0, 10000000).ToArray()
        Dim cts As New CancellationTokenSource

        ' Use ParallelOptions instance to store the CancellationToken
        Dim po As 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.
        Dim t As Task = Task.Factory.StartNew(Sub()
                                                  If Console.ReadKey().KeyChar = "c"c Then
                                                      cts.Cancel()
                                                  End If
                                                  Console.WriteLine(vbCrLf & "Press any key to exit.")
                                              End Sub)

        Try

            ' The error "Exception is unhandled by user code" will appear if "Just My Code" 
            ' is enabled. This error is benign. You can press F5 to continue, or disable Just My Code.
            Parallel.ForEach(nums, po, Sub(num)
                                           Dim d As Double = Math.Sqrt(num)
                                           Console.CursorLeft = 0
                                           Console.Write("{0:##.##} on {1}", d, Thread.CurrentThread.ManagedThreadId)
                                       End Sub)

        Catch e As OperationCanceledException
            Console.WriteLine(e.Message)
        Finally
            cts.Dispose()
        End Try

        Console.ReadKey()

    End Sub
End Module

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. This immediately stops all iterations from executing as the exception is thrown. If some other token causes cancellation, the loop will throw an AggregateException with an OperationCanceledException as an InnerException.

See also