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Parallel.ForEach<TSource, TLocal> Method (Partitioner<TSource>, Func<TLocal>, Func<TSource, ParallelLoopState, TLocal, TLocal>, Action<TLocal>)

Executes a foreach (For Each in Visual Basic) operation with thread-local data on a Partitioner 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 ForEach<TSource, TLocal>(
	Partitioner<TSource> source,
	Func<TLocal> localInit,
	Func<TSource, ParallelLoopState, TLocal, TLocal> body,
	Action<TLocal> localFinally
)

Type Parameters

TSource

The type of the elements in source.

TLocal

The type of the thread-local data.

Parameters

source
Type: System.Collections.Concurrent.Partitioner<TSource>

The partitioner that contains the original data source.

localInit
Type: System.Func<TLocal>

The function delegate that returns the initial state of the local data for each task.

body
Type: System.Func<TSource, ParallelLoopState, TLocal, TLocal>

The delegate that is invoked once per iteration.

localFinally
Type: System.Action<TLocal>

The delegate that performs a final action on the local state of each task.

Return Value

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

ExceptionCondition
ArgumentNullException

The source argument is null.

-or-

The body argument is null.

-or-

The localInit argument is null.

-or-

The localFinally argument is null.

InvalidOperationException

The SupportsDynamicPartitions property in the source Partitioner returns false or the partitioner returns null partitions.

AggregateException

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

This overload is provided for scenarios where you want to override the default partitioning scheme. For example, small loop bodies might benefit from partitioning the range. The ForEach method expects custom partitioners to support dynamic partitioning. For more information, see Custom Partitioners for PLINQ and TPL and How to: Implement Dynamic Partitions.

The localInit delegate is invoked once for each thread that participates in the loop's execution and returns the initial local state for each of those tasks. These initial states are passed to the first body invocations on each task. Then, every subsequent body invocation returns a possibly modified state value that is passed to the next body invocation. Finally, the last body invocation on each task returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per task to perform a final action on each task's local state. This delegate might be invoked concurrently on multiple tasks; therefore, you must synchronize access to any shared variables.

The Parallel.ForEach method may use more tasks than threads over the lifetime of its execution, as existing tasks complete and are replaced by new tasks. This gives the underlying TaskScheduler object the chance to add, change, or remove threads that service the loop.

.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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