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TaskFactory.ContinueWhenAll Method (Task[], Action<Task[]>, CancellationToken)

Creates a continuation task that starts when a set of specified tasks has completed.

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

public Task ContinueWhenAll(
	Task[] tasks,
	Action<Task[]> continuationAction,
	CancellationToken cancellationToken


Type: System.Threading.Tasks.Task[]

The array of tasks from which to continue.

Type: System.Action<Task[]>

The action delegate to execute when all tasks in the tasks array have completed.

Type: System.Threading.CancellationToken

The cancellation token to assign to the new continuation task.

Return Value

Type: System.Threading.Tasks.Task
The new continuation task.


An element in the tasks array has been disposed.


The CancellationTokenSource that created cancellationToken has already been disposed.


The tasks array is null.


The continuationAction argument is null.


The tasks array is empty or contains a null value.

The ContinueWhenAll(Task[], Action<Task[]>, CancellationToken) method executes the continuationAction delegate when all tasks in the tasks array have completed, regardless of their completion status.

The following example creates a cancellation token, which it passes to separate tasks that use a regular expression to count the number of words in a set of text files. The cancellation token is set if a file cannot be found. The ContinueWhenAll method is used to launch a task that displays the total word count when all the antecedent tasks have completed. If the cancellation token is set, which indicates that one or more tasks have been cancelled, it handles the AggregateException exception and displays an error message.

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

public class Example
   public static void Main()
      string[] filenames = { "chapter1.txt", "chapter2.txt", 
                             "chapter3.txt", "chapter4.txt",
                             "chapter5.txt" };
      string pattern = @"\b\w+\b";
      var tasks = new List<Task>();  
      CancellationTokenSource source = new CancellationTokenSource();
      CancellationToken token = source.Token;
      int totalWords = 0;

      // Determine the number of words in each file. 
      foreach (var filename in filenames)
         tasks.Add( Task.Factory.StartNew( fn => { token.ThrowIfCancellationRequested(); 

                                                   if (! File.Exists(fn.ToString())) {

                                                   StreamReader sr = new StreamReader(fn.ToString());
                                                   String content = sr.ReadToEnd();
                                                   int words = Regex.Matches(content, pattern).Count;
                                                   Interlocked.Add(ref totalWords, words); 
                                                   Console.WriteLine("{0,-25} {1,6:N0} words", fn, words); }, 
                                           filename, token));

      var finalTask = Task.Factory.ContinueWhenAll(tasks.ToArray(), wordCountTasks => {
                                                    if (! token.IsCancellationRequested) 
                                                       Console.WriteLine("\n{0,-25} {1,6} total words\n", 
                                                                         String.Format("{0} files", wordCountTasks.Length), 
                                                   }, token); 
      try {                                                   
      catch (AggregateException ae) {
         foreach (Exception inner in ae.InnerExceptions)
            if (inner is TaskCanceledException)
               Console.WriteLine("\nFailure to determine total word count: a task was cancelled.");
               Console.WriteLine("\nFailure caused by {0}", inner.GetType().Name);      
      finally {
// The example displays output like the following: 
//       chapter2.txt               1,585 words 
//       chapter1.txt               4,012 words 
//       Failure to determine total word count: a task was cancelled.

.NET Framework

Supported in: 4.6, 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

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, 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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