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

.NET Framework (current version)

The .NET API Reference documentation has a new home. Visit the .NET API Browser on to see the new experience.

Executes a foreach (For Each in Visual Basic) operation on a Partitioner in which iterations may run in parallel.

Namespace:   System.Threading.Tasks
Assembly:  mscorlib (in mscorlib.dll)

public static ParallelLoopResult ForEach<TSource>(
	Partitioner<TSource> source,
	Action<TSource> body


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

The partitioner that contains the original data source.

Type: System.Action<TSource>

The delegate that is invoked once per iteration.

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 elements in source.

Exception Condition

The source argument is null.


The body argument is null.


The SupportsDynamicPartitions property in the source partitioner returns false.


The exception that is thrown when any methods in the source partitioner return null.


The GetPartitions method in the source partitioner does not return the correct number of partitions.

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 Parallel.ForEach<TSource> 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 following example shows how to implement a range partitioner for use with Parallel.ForEach<TSource>:

using System;
using System.Collections.Concurrent;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

class RangePartitionerDemo 
        static void Main()
            Stopwatch sw = null;

            long sum = 0;
            long SUMTOP = 10000000;

            // Try sequential for
            sw = Stopwatch.StartNew();
            for (long i = 0; i < SUMTOP; i++) sum += i;
            Console.WriteLine("sequential for result = {0}, time = {1} ms", sum, sw.ElapsedMilliseconds);

            // Try parallel for -- this is slow!
            //sum = 0;
            //sw = Stopwatch.StartNew();
            //Parallel.For(0L, SUMTOP, (item) => Interlocked.Add(ref sum, item));
            //Console.WriteLine("parallel for  result = {0}, time = {1} ms", sum, sw.ElapsedMilliseconds);

            // Try parallel for with locals
            sum = 0;
            sw = Stopwatch.StartNew();
            Parallel.For(0L, SUMTOP, () => 0L, (item, state, prevLocal) => prevLocal + item, local => Interlocked.Add(ref sum, local));
            Console.WriteLine("parallel for w/locals result = {0}, time = {1} ms", sum, sw.ElapsedMilliseconds);

            // Try range partitioner
            sum = 0;
            sw = Stopwatch.StartNew();
            Parallel.ForEach(Partitioner.Create(0L, SUMTOP), (range) =>
                long local = 0;
                for (long i = range.Item1; i < range.Item2; i++) local += i;
                Interlocked.Add(ref sum, local);
            Console.WriteLine("range partitioner result = {0}, time = {1} ms", sum, sw.ElapsedMilliseconds);

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
Return to top