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Parallel.ForEach(Of TSource, TLocal) Method (OrderablePartitioner(Of TSource), ParallelOptions, Func(Of TLocal), Func(Of TSource, ParallelLoopState, Int64, TLocal, TLocal), Action(Of TLocal))

.NET Framework 4.6 and 4.5

Executes a foreach (For Each in Visual Basic) operation with 64-bit indexes and with thread-local data on a OrderablePartitioner(Of TSource) 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
Assemblies:   mscorlib (in mscorlib.dll)
  System.Threading.Tasks.Parallel (in System.Threading.Tasks.Parallel.dll)

Public Shared Function ForEach(Of TSource, TLocal) ( _
	source As OrderablePartitioner(Of TSource), _
	parallelOptions As ParallelOptions, _
	localInit As Func(Of TLocal), _
	body As Func(Of TSource, ParallelLoopState, Long, TLocal, TLocal), _
	localFinally As Action(Of TLocal) _
) As ParallelLoopResult

Type Parameters


The type of the elements in source.


The type of the thread-local data.


Type: System.Collections.Concurrent.OrderablePartitioner(Of TSource)

The orderable partitioner that contains the original data source.

Type: System.Threading.Tasks.ParallelOptions

An object that configures the behavior of this operation.

Type: System.Func(Of TLocal)

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

Type: System.Func(Of TSource, ParallelLoopState, Int64, TLocal, TLocal)

The delegate that is invoked once per iteration.

Type: System.Action(Of 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 Nothing.


The parallelOptions argument is Nothing.


The body argument is Nothing.


The localInit or localFinally argument is Nothing.


The SupportsDynamicPartitions property in the source Partitioner returns false or the partitioner returns Nothing partitions.


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


The CancellationToken in the parallelOptions argument is canceled.


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

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 task 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 thread 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: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

.NET for Windows Phone apps

Supported in: Windows Phone 8.1

Portable Class Library

Supported in: Portable Class Library
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