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

.NET Framework (current version)

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

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

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


Type: System.Collections.Generic.IEnumerable<TSource>

An enumerable 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 data in the source.

Exception Condition

The source argument is null.


The body 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 current element as a parameter.

The following example uses the ForEach<TSource>(IEnumerable<TSource>, Action<TSource>) method to count the number of vowels and non-whitespace characters in a text file. In this case, the ParallelLoopResult value returned by the method is ignored. Note that, because operations can run in parallel, you must ensure that incrementing the counter variables is an atomic operation, and that multiple threads do not attempt to access the counter variables simultaneously. For this purpose, the example uses the lock statement (in C#) and the SyncLock statement (in Visual Basic).

using System;
using System.IO;
using System.Threading.Tasks;

public class Example
   public static void Main()
      Task<String> task = ReadCharacters(@".\CallOfTheWild.txt");
      String text = task.Result;

      int nVowels = 0;
      int nNonWhiteSpace = 0;
      Object obj = new Object();

      ParallelLoopResult result = Parallel.ForEach(text, 
                                                   (ch) => {
                                                      Char uCh = Char.ToUpper(ch);
                                                      if ("AEIOUY".IndexOf(uCh) >= 0) {
                                                         lock (obj) {
                                                      if (! Char.IsWhiteSpace(uCh)) {
                                                         lock (obj) {
                                                   } );
      Console.WriteLine("Total characters:      {0,10:N0}", text.Length);
      Console.WriteLine("Total vowels:          {0,10:N0}", nVowels);
      Console.WriteLine("Total non-whitespace:  {0,10:N0}", nNonWhiteSpace);

   private static async Task<String> ReadCharacters(String fn)
      String text;
      using (StreamReader sr = new StreamReader(fn)) {
         text = await sr.ReadToEndAsync();
      return text;
// The example displays output like the following:
//       Total characters:         198,548
//       Total vowels:              58,421
//       Total non-whitespace:     159,461

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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