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

Executes a for (For in Visual Basic) loop in which iterations may run in parallel and the state of the loop can be monitored and manipulated.

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

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


Type: System.Int32

The start index, inclusive.

Type: System.Int32

The end index, exclusive.

Type: System.Action<Int32, 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.


The body argument is null.


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

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 (Int32), and a ParallelLoopState instance that may be used to break out of the loop prematurely.

Calling the Break method informs the for operation that iterations after the current one don't have to execute. However, all iterations before the current one will still have to be executed if they haven't already.

Therefore, calling Break is similar to using a break operation within a conventional for loop in a language like C#, but it is not a perfect substitute: For example, there is no guarantee that iterations after the current one will definitely not execute.

If executing all iterations before the current one is not necessary, use the Stop method instead of using Break. Calling Stop informs the for loop that it may abandon all remaining iterations, regardless of whether they're before or after the current iteration, because all required work will have already been completed. However, as with Break, there are no guarantees regarding which other iterations will not execute.

If a loop is ended prematurely, the ParallelLoopResult structure that is returned will contain relevant information about the loop's completion.

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

This example demonstrates how to use the ParallelLoopState instance, which is passed to the body delegate of some Parallel.For overloads, to stop a loop's execution before all iterations have completed executing.

using System;
using System.Threading.Tasks;

class Demo
    int N = 1000;

    void TestMethod()
        Parallel.For(0, N, (i, loopState) =>
            if (i == 100)

.NET Framework

Supported in: 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1

Windows Phone 8.1, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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