Parallel.For Method (Int64, Int64, ParallelOptions, Action<Int64, ParallelLoopState>)

.NET Framework (current version)

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Executes a for (For in Visual Basic) loop with 64-bit indexes in which iterations may run in parallel, loop options can be configured, and the state of the loop can be monitored and manipulated.

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

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


Type: System.Int64

The start index, inclusive.

Type: System.Int64

The end index, exclusive.

Type: System.Threading.Tasks.ParallelOptions

An object that configures the behavior of this operation.

Type: System.Action<Int64, ParallelLoopState>

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.

Exception Condition

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 following parameters: the iteration count (Int64), and a ParallelLoopState instance that may be used to break out of the loop prematurely.

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

The following example shows how to use the Parallel.For method with a ParallelOptions object:

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

class ParallelOptionsDemo
    // Demonstrated features:
    // 		Parallel.For()
    //		ParallelOptions
    // 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.
    //		Verify that no more than two threads have been used for the iterations.
    // Documentation:
    static void Main()
        ParallelOptions options = new ParallelOptions();
        options.MaxDegreeOfParallelism = 2; // -1 is for unlimited. 1 is for sequential.

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

        // 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 the following (unexpected) exception:\n{0}", e);


Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone
Available since 8.1
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