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Parallel.ForEach(TSource, TLocal) Method (IEnumerable(TSource), Func(TLocal), Func(TSource, ParallelLoopState, TLocal, TLocal), Action(TLocal))

Executes a foreach (For Each in Visual Basic) operation with thread-local data on an IEnumerable 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)

static member ForEach : 
        source:IEnumerable<'TSource> * 
        localInit:Func<'TLocal> * 
        body:Func<'TSource, ParallelLoopState, 'TLocal, 'TLocal> * 
        localFinally:Action<'TLocal> -> ParallelLoopResult

Type Parameters


The type of the data in the source.


The type of the thread-local data.


Type: System.Collections.Generic.IEnumerable(TSource)

An enumerable 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 a null reference (Nothing in Visual Basic).


The body argument is a null reference (Nothing in Visual Basic).


The localInit argument is a null reference (Nothing in Visual Basic).


The localFinally argument is a null reference (Nothing in Visual Basic).


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

The body delegate is invoked once for each element in the source enumerable. It is provided with the following parameters: the current element, a ParallelLoopState instance that may be used to break out of the loop prematurely, and some local state that may be shared amongst iterations that execute on the same thread.

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 task returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per thread 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.

The following example shows how to use a ForEach method with local state:

No code example is currently available or this language may not be supported.

.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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