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Parallel.For Method (Int32, Int32, ParallelOptions, Action<Int32>)

Executes a for (For in Visual Basic) loop in which iterations may run in parallel and loop options can be configured.

Namespace:  System.Threading.Tasks
Assembly:  mscorlib (in mscorlib.dll)

public static ParallelLoopResult For(
	int fromInclusive,
	int toExclusive,
	ParallelOptions parallelOptions,
	Action<int> body


Type: System.Int32
The start index, inclusive.
Type: System.Int32
The end index, exclusive.
Type: System.Threading.Tasks.ParallelOptions
An object that configures the behavior of this operation.
Type: System.Action<Int32>
The delegate that is invoked once per iteration.

Return Value

Type: System.Threading.Tasks.ParallelLoopResult
A structure that contains information about which portion of the loop completed.


The CancellationToken in the parallelOptions argument is canceled.


The body argument is null.


The parallelOptions argument is null.


The exception that contains all the individual exceptions thrown on all threads.


The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.

The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the iteration count (Int32) as a parameter.

If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.

The following example shows how to cancel a parallel loop:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

class ParallelForCancellation
    // Demonstrated features:
    //		CancellationTokenSource
    // 		Parallel.For()
    //		ParallelOptions
    //		ParallelLoopResult
    // Expected results:
    // 		An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
    //		The order of execution of the iterations is undefined.
    //		The iteration when i=2 cancels the loop.
    //		Some iterations may bail out or not start at all; because they are temporally executed in unpredictable order, 
    //          it is impossible to say which will start/complete and which won't.
    //		At the end, an OperationCancelledException is surfaced.
    // Documentation:
    static void CancelDemo()
        CancellationTokenSource cancellationSource = new CancellationTokenSource();
        ParallelOptions options = new ParallelOptions();
        options.CancellationToken = cancellationSource.Token;

            ParallelLoopResult loopResult = Parallel.For(
                    (i, loopState) =>
                        Console.WriteLine("Start Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);

                        // Simulate a cancellation of the loop when i=2
                        if (i == 2)

                        // Simulates a long execution
                        for (int j = 0; j < 10; j++)
                            Thread.Sleep(1 * 200);

                            // check to see whether or not to continue
                            if (loopState.ShouldExitCurrentIteration) return;

                        Console.WriteLine("Finish Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);

            if (loopResult.IsCompleted)
                Console.WriteLine("All iterations completed successfully. THIS WAS NOT EXPECTED.");
        // No exception is expected in this example, but if one is still thrown from a task,
        // it will be wrapped in AggregateException and propagated to the main thread.
        catch (AggregateException e)
            Console.WriteLine("Parallel.For has thrown an AggregateException. THIS WAS NOT EXPECTED.\n{0}", e);
        // Catching the cancellation exception
        catch (OperationCanceledException e)
            Console.WriteLine("An iteration has triggered a cancellation. THIS WAS EXPECTED.\n{0}", e.ToString());

.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.