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

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 Parameters

TSource

The type of the data in the source.

Parameters

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

An enumerable data source.

body
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.

ExceptionCondition
ArgumentNullException

The source argument is null.

-or-

The body argument is null.

AggregateException

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) {
                                                            nVowels++;
                                                         }
                                                      }
                                                      if (! Char.IsWhiteSpace(uCh)) {
                                                         lock (obj) {
                                                            nNonWhiteSpace++;
                                                         }   
                                                      }
                                                   } );
      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: 4.5.2, 4.5.1, 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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