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

.NET Framework 4.6 and 4.5

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

Namespace:  System.Threading.Tasks
Assemblies:   mscorlib (in mscorlib.dll)
  System.Threading.Tasks.Parallel (in System.Threading.Tasks.Parallel.dll)

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

Type Parameters


The type of the data in the source.


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.


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

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1
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