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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


The type of the elements in source.


The type of the thread-local data.


Type: System.Collections.Concurrent.Partitioner<TSource>
The partitioner that contains the original data source.
Type: System.Func<TLocal>
The function delegate that returns the initial state of the local data for each task.
Type: System.Func<TSource, ParallelLoopState, TLocal, TLocal>
The delegate that is invoked once per iteration.
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.


The source argument is null.


The body argument is null.


The localInit argument is null.


The localFinally argument is null.


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


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

.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.