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

.NET Framework (current version)

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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)

Public Shared Function ForEach(Of TSource, TLocal) (
	source As IEnumerable(Of TSource),
	localInit As Func(Of TLocal),
	body As Func(Of TSource, ParallelLoopState, TLocal, TLocal),
	localFinally As Action(Of TLocal)
) As ParallelLoopResult


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

An enumerable data source.

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, 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.

Type Parameters


The type of the data in the source.


The type of the thread-local data.

Exception Condition

The source argument is null.


The body argument is null.


The localInit argument is null.


The localFinally argument is null.


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(Of TSource) 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(Of TSource) method with local state:

Imports System.Threading
Imports System.Threading.Tasks
Module ForEachDemo

    ' Demonstrated features:
    '   Parallel.ForEach()
    '   Thread-local state
    ' Expected results:
    '   This example sums up the elements of an int[] in parallel.
    '   Each thread maintains a local sum. When a thread is initialized, that local sum is set to 0.
    '   On every iteration the current element is added to the local sum.
    '   When a thread is done, it safely adds its local sum to the global sum.
    '   After the loop is complete, the global sum is printed out.
    ' Documentation:
    Private Sub ForEachDemo()
        ' The sum of these elements is 40.
        Dim input As Integer() = {4, 1, 6, 2, 9, 5, _
        10, 3}
        Dim sum As Integer = 0

            ' source collection
                                 ' thread local initializer
                                 Return 0
                             End Function,
                             Function(n, loopState, localSum)
                                 ' body
                                 localSum += n
                                 Console.WriteLine("Thread={0}, n={1}, localSum={2}", Thread.CurrentThread.ManagedThreadId, n, localSum)
                                 Return localSum
                             End Function,
                                 ' thread local aggregator
                                 Interlocked.Add(sum, localSum)
                             End Sub)

            Console.WriteLine(vbLf & "Sum={0}", sum)
        Catch e As AggregateException
            ' No exception is expected in this example, but if one is still thrown from a task,
            ' it will be wrapped in AggregateException and propagated to the main thread.
            Console.WriteLine("Parallel.ForEach has thrown an exception. THIS WAS NOT EXPECTED." & vbLf & "{0}", e)
        End Try
    End Sub

End Module

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone
Available since 8.1
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