Bibliothèque parallèle de tâches (TPL) et programmation asynchrone .NET FrameworkTPL and Traditional .NET Framework Asynchronous Programming

Le .NET Framework fournit les deux modèles standard suivants pour l’exécution d’opérations asynchrones liées aux E/S et orientées calculs :The .NET Framework provides the following two standard patterns for performing I/O-bound and compute-bound asynchronous operations:

  • Le modèle de programmation asynchrone, dans lequel les opérations asynchrones sont représentées par une paire de méthodes Begin/End telles que FileStream.BeginRead et Stream.EndRead.Asynchronous Programming Model (APM), in which asynchronous operations are represented by a pair of Begin/End methods such as FileStream.BeginRead and Stream.EndRead.

  • Le modèle asynchrone basé sur des événements, dans lequel les opérations asynchrones sont représentées par une paire méthode/événement et nommées OperationNameAsync et OperationName Completed, par exemple, WebClient.DownloadStringAsync et WebClient.DownloadStringCompleted.Event-based asynchronous pattern (EAP), in which asynchronous operations are represented by a method/event pair that are named OperationNameAsync and OperationNameCompleted, for example, WebClient.DownloadStringAsync and WebClient.DownloadStringCompleted. (Le modèle asynchrone basé sur des événements a été introduit avec le .NET Framework version 2.0).(EAP was introduced in the .NET Framework version 2.0.)

La bibliothèque parallèle de tâches peut être utilisée de plusieurs façons conjointement à l'un ou l'autre des modèles asynchrones.The Task Parallel Library (TPL) can be used in various ways in conjunction with either of the asynchronous patterns. Vous pouvez exposer à la fois les opérations de modèle de programmation asynchrone et de modèle asynchrone basé sur des événements en tant que Tâches aux consommateurs de la bibliothèque, ou bien exposer les modèles de programmation asynchrone et utiliser des objets Task pour les implémenter en interne.You can expose both APM and EAP operations as Tasks to library consumers, or you can expose the APM patterns but use Task objects to implement them internally. Dans les deux scénarios, en utilisant des objets Task, vous pouvez simplifier le code et bénéficier des fonctionnalités utiles suivantes :In both scenarios, by using Task objects, you can simplify the code and take advantage of the following useful functionality:

  • Enregistrer des rappels sous la forme de continuations de tâches, à tout moment après le lancement de la tâche.Register callbacks, in the form of task continuations, at any time after the task has started.

  • Coordonner plusieurs opérations qui s'exécutent en réponse à une méthode Begin_, à l'aide des méthodes ContinueWhenAll et ContinueWhenAny, ou des méthodes WaitAll ou WaitAny.Coordinate multiple operations that execute in response to a Begin_ method, by using the ContinueWhenAll and ContinueWhenAny methods, or the WaitAll method or the WaitAny method.

  • Encapsuler des opérations asynchrones liées aux E/S et orientées calcul dans le même objet Task.Encapsulate asynchronous I/O-bound and compute-bound operations in the same Task object.

  • Surveiller l'état de l'objet Task.Monitor the status of the Task object.

  • Marshaler le statut d'une opération sur un objet Task à l'aide de TaskCompletionSource<TResult>.Marshal the status of an operation to a Task object by using TaskCompletionSource<TResult>.

Encapsulation d’opérations de modèle de programmation asynchrone dans une tâcheWrapping APM Operations in a Task

Les classes System.Threading.Tasks.TaskFactory et System.Threading.Tasks.TaskFactory<TResult> fournissent plusieurs surcharges des méthodes TaskFactory.FromAsync et TaskFactory<TResult>.FromAsync qui vous permettent d'encapsuler une paire de méthodes de modèle de programmation asynchrone Begin/End dans une instance Task ou Task<TResult>.Both the System.Threading.Tasks.TaskFactory and System.Threading.Tasks.TaskFactory<TResult> classes provide several overloads of the TaskFactory.FromAsync and TaskFactory<TResult>.FromAsync methods that let you encapsulate an APM Begin/End method pair in one Task or Task<TResult> instance. Les différentes surcharges s'adaptent à toutes les paires de méthode Begin/End possédant de zéro à trois paramètres d'entrée.The various overloads accommodate any Begin/End method pair that have from zero to three input parameters.

Pour les paires ayant des méthodes End qui retournent une valeur (Function en Visual Basic), utilisez les méthodes de TaskFactory<TResult> qui créent un Task<TResult>.For pairs that have End methods that return a value (Function in Visual Basic), use the methods in TaskFactory<TResult> that create a Task<TResult>. Pour les méthodes End qui retournent void (Sub en Visual Basic), utilisez les méthodes de TaskFactory qui créent un Task.For End methods that return void (Sub in Visual Basic), use the methods in TaskFactory that create a Task.

Pour les rares cas dans lesquels la méthode Begin a plus de trois paramètres ou contient les paramètres ref ou out, les surcharges FromAsync supplémentaires qui encapsulent uniquement la méthode End sont fournies.For those few cases in which the Begin method has more than three parameters or contains ref or out parameters, additional FromAsync overloads that encapsulate only the End method are provided.

L'exemple suivant montre la signature de la surcharge FromAsync qui correspond aux méthodes FileStream.BeginRead et FileStream.EndRead.The following example shows the signature for the FromAsync overload that matches the FileStream.BeginRead and FileStream.EndRead methods. Cette surcharge prend trois paramètres d'entrée, comme suit.This overload takes three input parameters, as follows.

public Task<TResult> FromAsync<TArg1, TArg2, TArg3>(
    Func<TArg1, TArg2, TArg3, AsyncCallback, object, IAsyncResult> beginMethod, //BeginRead
     Func<IAsyncResult, TResult> endMethod, //EndRead
     TArg1 arg1, // the byte[] buffer
     TArg2 arg2, // the offset in arg1 at which to start writing data
     TArg3 arg3, // the maximum number of bytes to read
     object state // optional state information
    ) 
Public Function FromAsync(Of TArg1, TArg2, TArg3)(
                ByVal beginMethod As Func(Of TArg1, TArg2, TArg3, AsyncCallback, Object, IAsyncResult),
                ByVal endMethod As Func(Of IAsyncResult, TResult),
                ByVal dataBuffer As TArg1,
                ByVal byteOffsetToStartAt As TArg2,
                ByVal maxBytesToRead As TArg3,
                ByVal stateInfo As Object)

Le premier paramètre est un délégué Func<T1,T2,T3,T4,T5,TResult> qui correspond à la signature de la méthode FileStream.BeginRead.The first parameter is a Func<T1,T2,T3,T4,T5,TResult> delegate that matches the signature of the FileStream.BeginRead method. Le deuxième paramètre est un délégué Func<T,TResult> qui prend un IAsyncResult et retourne un TResult.The second parameter is a Func<T,TResult> delegate that takes an IAsyncResult and returns a TResult. Étant donné que EndRead retourne un entier, le compilateur déduit le type de TResult en tant que Int32 et le type de la tâche en tant que Task.Because EndRead returns an integer, the compiler infers the type of TResult as Int32 and the type of the task as Task. Les quatre derniers paramètres sont identiques à ceux de la méthode FileStream.BeginRead :The last four parameters are identical to those in the FileStream.BeginRead method:

  • Tampon dans lequel stocker les données de fichiers.The buffer in which to store the file data.

  • Dans le tampon, offset à partir duquel commencer l'écriture des données.The offset in the buffer at which to begin writing data.

  • Quantité maximale de données à lire dans le fichier.The maximum amount of data to read from the file.

  • Objet facultatif qui stocke les données d'état définies par l'utilisateur à passer au rappel.An optional object that stores user-defined state data to pass to the callback.

Utilisation de ContinueWith pour les fonctionnalités de rappelUsing ContinueWith for the Callback Functionality

Si vous devez accéder aux données d'un fichier, et non juste au nombre d'octets, la méthode FromAsync n'est pas suffisante.If you require access to the data in the file, as opposed to just the number of bytes, the FromAsync method is not sufficient. Utilisez plutôt Task, dont la propriété Result contient les données de fichier.Instead, use Task, whose Result property contains the file data. Pour cela, ajoutez une continuation à la tâche d’origine.You can do this by adding a continuation to the original task. La continuation exécute le travail généralement effectué par le délégué AsyncCallback.The continuation performs the work that would typically be performed by the AsyncCallback delegate. Elle est appelée quand l'antécédent est terminé et que le tampon de données est rempli.It is invoked when the antecedent completes, and the data buffer has been filled. (L'objet FileStream doit être fermé avant le retour.)(The FileStream object should be closed before returning.)

L'exemple suivant montre comment retourner un Task qui encapsule la paire BeginRead/EndRead de la classe FileStream.The following example shows how to return a Task that encapsulates the BeginRead/EndRead pair of the FileStream class.

const int MAX_FILE_SIZE = 14000000;
public static Task<string> GetFileStringAsync(string path)
{
    FileInfo fi = new FileInfo(path);
    byte[] data = null;
    data = new byte[fi.Length];

    FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);

    //Task<int> returns the number of bytes read
    Task<int> task = Task<int>.Factory.FromAsync(
            fs.BeginRead, fs.EndRead, data, 0, data.Length, null);

    // It is possible to do other work here while waiting
    // for the antecedent task to complete.
    // ...

    // Add the continuation, which returns a Task<string>. 
    return task.ContinueWith((antecedent) =>
    {
        fs.Close();

        // Result = "number of bytes read" (if we need it.)
        if (antecedent.Result < 100)
        {
            return "Data is too small to bother with.";
        }
        else
        {
            // If we did not receive the entire file, the end of the
            // data buffer will contain garbage.
            if (antecedent.Result < data.Length)
                Array.Resize(ref data, antecedent.Result);

            // Will be returned in the Result property of the Task<string>
            // at some future point after the asynchronous file I/O operation completes.
            return new UTF8Encoding().GetString(data);
        }
    });
}
Const MAX_FILE_SIZE As Integer = 14000000
Shared Function GetFileStringAsync(ByVal path As String) As Task(Of String)
    Dim fi As New FileInfo(path)
    Dim data(fi.Length) As Byte

    Dim fs As FileStream = New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, True)

    ' Task(Of Integer) returns the number of bytes read
    Dim myTask As Task(Of Integer) = Task(Of Integer).Factory.FromAsync(
        AddressOf fs.BeginRead, AddressOf fs.EndRead, data, 0, data.Length, Nothing)

    ' It is possible to do other work here while waiting
    ' for the antecedent task to complete.
    ' ...

    ' Add the continuation, which returns a Task<string>. 
    Return myTask.ContinueWith(Function(antecedent)
                                   fs.Close()
                                   If (antecedent.Result < 100) Then
                                       Return "Data is too small to bother with."
                                   End If
                                   ' If we did not receive the entire file, the end of the
                                   ' data buffer will contain garbage.
                                   If (antecedent.Result < data.Length) Then
                                       Array.Resize(data, antecedent.Result)
                                   End If

                                   ' Will be returned in the Result property of the Task<string>
                                   ' at some future point after the asynchronous file I/O operation completes.
                                   Return New UTF8Encoding().GetString(data)
                               End Function)

End Function

La méthode peut ensuite être appelée, comme suit.The method can then be called, as follows.


 Task<string> t = GetFileStringAsync(path);          

 // Do some other work:
 // ...

 try
 {
      Console.WriteLine(t.Result.Substring(0, 500));
 }
 catch (AggregateException ae)
 {
     Console.WriteLine(ae.InnerException.Message);
 }            
Dim myTask As Task(Of String) = GetFileStringAsync(path)

' Do some other work
' ...

Try
    Console.WriteLine(myTask.Result.Substring(0, 500))
Catch ex As AggregateException
    Console.WriteLine(ex.InnerException.Message)
End Try

Fourniture de données d'état personnaliséesProviding Custom State Data

Dans les opérations IAsyncResult typiques, si votre délégué AsyncCallback a besoin de certaines données d'état personnalisées, vous devez les passer via le dernier paramètre de la méthode Begin, afin que les données puissent être empaquetées dans l'objet IAsyncResult qui est finalement passé à la méthode de rappel.In typical IAsyncResult operations, if your AsyncCallback delegate requires some custom state data, you have to pass it in through the last parameter in the Begin method, so that the data can be packaged into the IAsyncResult object that is eventually passed to the callback method. Cela n'est en général pas obligatoire quand les méthodes FromAsync sont utilisées.This is typically not required when the FromAsync methods are used. Si les données personnalisées sont connues de la continuation, elles peuvent être capturées directement dans le délégué de continuation.If the custom data is known to the continuation, then it can be captured directly in the continuation delegate. L'exemple suivant ressemble au précédent, mais au lieu d'examiner la propriété Result de la tâche antérieure, la continuation examine les données d'état personnalisées directement accessibles au délégué utilisateur de la continuation.The following example resembles the previous example, but instead of examining the Result property of the antecedent, the continuation examines the custom state data that is directly accessible to the user delegate of the continuation.

public Task<string> GetFileStringAsync2(string path)
{             
    FileInfo fi = new FileInfo(path);
    byte[] data = new byte[fi.Length];                       
    MyCustomState state = GetCustomState();
    FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);
    // We still pass null for the last parameter because
    // the state variable is visible to the continuation delegate.
    Task<int> task = Task<int>.Factory.FromAsync(
            fs.BeginRead, fs.EndRead, data, 0, data.Length, null);

    return task.ContinueWith((antecedent) =>
    {
        // It is safe to close the filestream now.
        fs.Close();

        // Capture custom state data directly in the user delegate.
        // No need to pass it through the FromAsync method.
        if (state.StateData.Contains("New York, New York"))
        {
            return "Start spreading the news!";
        }
        else
        {
            // If we did not receive the entire file, the end of the
            // data buffer will contain garbage.
            if (antecedent.Result < data.Length)
                Array.Resize(ref data, antecedent.Result);

            // Will be returned in the Result property of the Task<string>
            // at some future point after the asynchronous file I/O operation completes.
            return new UTF8Encoding().GetString(data);
        }
    });
    
}
Public Function GetFileStringAsync2(ByVal path As String) As Task(Of String)
    Dim fi = New FileInfo(path)
    Dim data(fi.Length) As Byte
    Dim state As New MyCustomState()

    Dim fs As New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, True)
    ' We still pass null for the last parameter because
    ' the state variable is visible to the continuation delegate.
    Dim myTask As Task(Of Integer) = Task(Of Integer).Factory.FromAsync(
            AddressOf fs.BeginRead, AddressOf fs.EndRead, data, 0, data.Length, Nothing)

    Return myTask.ContinueWith(Function(antecedent)
                                   fs.Close()
                                   ' Capture custom state data directly in the user delegate.
                                   ' No need to pass it through the FromAsync method.
                                   If (state.StateData.Contains("New York, New York")) Then
                                       Return "Start spreading the news!"
                                   End If

                                   ' If we did not receive the entire file, the end of the
                                   ' data buffer will contain garbage.
                                   If (antecedent.Result < data.Length) Then
                                       Array.Resize(data, antecedent.Result)
                                   End If
                                   '/ Will be returned in the Result property of the Task<string>
                                   '/ at some future point after the asynchronous file I/O operation completes.
                                   Return New UTF8Encoding().GetString(data)
                               End Function)

End Function

Synchronisation de plusieurs tâches FromAsyncSynchronizing Multiple FromAsync Tasks

Les méthodes ContinueWhenAll et ContinueWhenAny statiques offrent une flexibilité supplémentaire en cas d'utilisation avec les méthodes FromAsync.The static ContinueWhenAll and ContinueWhenAny methods provide added flexibility when used in conjunction with the FromAsync methods. L'exemple suivant montre comment initialiser plusieurs opérations d'E/S asynchrones et attendre qu'elles soient toutes terminées avant d'exécuter la continuation.The following example shows how to initiate multiple asynchronous I/O operations, and then wait for all of them to complete before you execute the continuation.

public Task<string> GetMultiFileData(string[] filesToRead)
{
    FileStream fs;
    Task<string>[] tasks = new Task<string>[filesToRead.Length];
    byte[] fileData = null;
    for (int i = 0; i < filesToRead.Length; i++)
    {
        fileData = new byte[0x1000];
        fs = new FileStream(filesToRead[i], FileMode.Open, FileAccess.Read, FileShare.Read, fileData.Length, true);

        // By adding the continuation here, the 
        // Result of each task will be a string.
        tasks[i] = Task<int>.Factory.FromAsync(
                 fs.BeginRead, fs.EndRead, fileData, 0, fileData.Length, null)
                 .ContinueWith((antecedent) =>
                     {
                         fs.Close();

                         // If we did not receive the entire file, the end of the
                         // data buffer will contain garbage.
                         if (antecedent.Result < fileData.Length)
                             Array.Resize(ref fileData, antecedent.Result);

                         // Will be returned in the Result property of the Task<string>
                         // at some future point after the asynchronous file I/O operation completes.
                         return new UTF8Encoding().GetString(fileData);
                     });
    }

    // Wait for all tasks to complete. 
    return Task<string>.Factory.ContinueWhenAll(tasks, (data) =>
    {
        // Propagate all exceptions and mark all faulted tasks as observed.
        Task.WaitAll(data);

        // Combine the results from all tasks.
        StringBuilder sb = new StringBuilder();
        foreach (var t in data)
        {
            sb.Append(t.Result);
        }
        // Final result to be returned eventually on the calling thread.
        return sb.ToString();
    });

}
Public Function GetMultiFileData(ByVal filesToRead As String()) As Task(Of String)
    Dim fs As FileStream
    Dim tasks(filesToRead.Length) As Task(Of String)
    Dim fileData() As Byte = Nothing
    For i As Integer = 0 To filesToRead.Length
        fileData(&H1000) = New Byte()
        fs = New FileStream(filesToRead(i), FileMode.Open, FileAccess.Read, FileShare.Read, fileData.Length, True)

        ' By adding the continuation here, the 
        ' Result of each task will be a string.
        tasks(i) = Task(Of Integer).Factory.FromAsync(AddressOf fs.BeginRead,
                                                      AddressOf fs.EndRead,
                                                      fileData,
                                                      0,
                                                      fileData.Length,
                                                      Nothing).
                                                  ContinueWith(Function(antecedent)
                                                                   fs.Close()
                                                                   'If we did not receive the entire file, the end of the
                                                                   ' data buffer will contain garbage.
                                                                   If (antecedent.Result < fileData.Length) Then
                                                                       ReDim Preserve fileData(antecedent.Result)
                                                                   End If

                                                                   'Will be returned in the Result property of the Task<string>
                                                                   ' at some future point after the asynchronous file I/O operation completes.
                                                                   Return New UTF8Encoding().GetString(fileData)
                                                               End Function)
    Next

    Return Task(Of String).Factory.ContinueWhenAll(tasks, Function(data)

                                                              ' Propagate all exceptions and mark all faulted tasks as observed.
                                                              Task.WaitAll(data)

                                                              ' Combine the results from all tasks.
                                                              Dim sb As New StringBuilder()
                                                              For Each t As Task(Of String) In data
                                                                  sb.Append(t.Result)
                                                              Next
                                                              ' Final result to be returned eventually on the calling thread.
                                                              Return sb.ToString()
                                                          End Function)
End Function

Tâches FromAsync pour la méthode End uniquementFromAsync Tasks For Only the End Method

Pour les rares cas dans lesquels la méthode Begin nécessite plus de trois paramètres d'entrée, ou a des paramètres ref ou out, vous pouvez utiliser les surcharges FromAsync, par exemple, TaskFactory<TResult>.FromAsync(IAsyncResult, Func<IAsyncResult,TResult>), qui représentent uniquement la méthode End.For those few cases in which the Begin method requires more than three input parameters, or has ref or out parameters, you can use the FromAsync overloads, for example, TaskFactory<TResult>.FromAsync(IAsyncResult, Func<IAsyncResult,TResult>), that represent only the End method. Ces méthodes peuvent également être utilisées pour tous les scénarios dans lesquels vous est passé un IAsyncResult que vous voulez encapsuler dans une tâche.These methods can also be used in any scenario in which you are passed an IAsyncResult and want to encapsulate it in a Task.

static Task<String> ReturnTaskFromAsyncResult()
{
    IAsyncResult ar = DoSomethingAsynchronously();
    Task<String> t = Task<string>.Factory.FromAsync(ar, _ =>
        {
            return (string)ar.AsyncState;
        });

    return t;
}
Shared Function ReturnTaskFromAsyncResult() As Task(Of String)
    Dim ar As IAsyncResult = DoSomethingAsynchronously()
    Dim t As Task(Of String) = Task(Of String).Factory.FromAsync(ar, Function(res) CStr(res.AsyncState))
    Return t
End Function

Lancement et annulation de tâches FromAsyncStarting and Canceling FromAsync Tasks

La tâche retournée par une méthode FromAsync a l’état WaitingForActivation et sera lancée à un moment donné par le système après la création de la tâche.The task returned by a FromAsync method has a status of WaitingForActivation and will be started by the system at some point after the task is created. Si vous essayez d’appeler Start pour une telle tâche, une exception sera levée.If you attempt to call Start on such a task, an exception will be raised.

Vous ne pouvez pas annuler une tâche FromAsync, car les API .NET Framework sous-jacentes ne prennent actuellement pas en charge l’annulation d’opérations d’E/S réseau ou de fichier en cours.You cannot cancel a FromAsync task, because the underlying .NET Framework APIs currently do not support in-progress cancellation of file or network I/O. Vous pouvez ajouter des fonctionnalités d’annulation à une méthode qui encapsule un appel FromAsync, mais vous pouvez répondre uniquement à l’annulation avant que FromAsync soit appelé ou terminé (par exemple, dans une tâche de continuation).You can add cancellation functionality to a method that encapsulates a FromAsync call, but you can only respond to the cancellation before FromAsync is called or after it completed (for example, in a continuation task).

Certaines classes prenant en charge le modèle asynchrone basé sur des événements, comme WebClient, prennent en charge l'annulation et vous pouvez intégrer ces fonctionnalités d'annulation native à l'aide de jetons d'annulation.Some classes that support EAP, for example, WebClient, do support cancellation, and you can integrate that native cancellation functionality by using cancellation tokens.

Exposition d’opérations complexes de modèle asynchrone basé sur des événements en tant que tâchesExposing Complex EAP Operations As Tasks

La bibliothèque parallèle de tâches ne fournit pas de méthodes conçues spécifiquement pour encapsuler une opération asynchrone basée sur des événements de la même façon que la famille FromAsync de méthodes encapsule le modèle IAsyncResult.The TPL does not provide any methods that are specifically designed to encapsulate an event-based asynchronous operation in the same way that the FromAsync family of methods wrap the IAsyncResult pattern. Toutefois, la bibliothèque parallèle de tâches fournit la classe System.Threading.Tasks.TaskCompletionSource<TResult>, qui peut être utilisée pour représenter tout ensemble d'opérations arbitraire tel qu'un Task<TResult>.However, the TPL does provide the System.Threading.Tasks.TaskCompletionSource<TResult> class, which can be used to represent any arbitrary set of operations as a Task<TResult>. Les opérations peuvent être synchrones ou asynchrones et peuvent être liées aux E/S ou orientées calcul, ou les deux.The operations may be synchronous or asynchronous, and may be I/O bound or compute-bound, or both.

L'exemple suivant montre comment utiliser un TaskCompletionSource<TResult> pour exposer un ensemble d'opérations WebClient asynchrones à du code client en tant que Task<TResult> de base.The following example shows how to use a TaskCompletionSource<TResult> to expose a set of asynchronous WebClient operations to client code as a basic Task<TResult>. Cette méthode vous permet d'entrer un tableau d'URL web et un terme ou nom à rechercher, puis retourne le nombre d'occurrences de ce terme ou nom sur chaque site.The method lets you enter an array of Web URLs, and a term or name to search for, and then returns the number of times the search term occurs on each site.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

public class SimpleWebExample
{
  public Task<string[]> GetWordCountsSimplified(string[] urls, string name,
                                                CancellationToken token)
  {
      TaskCompletionSource<string[]> tcs = new TaskCompletionSource<string[]>();
      WebClient[] webClients = new WebClient[urls.Length];
      object m_lock = new object();
      int count = 0;
      List<string> results = new List<string>();

      // If the user cancels the CancellationToken, then we can use the
      // WebClient's ability to cancel its own async operations.
      token.Register(() =>
      {
          foreach (var wc in webClients)
          {
              if (wc != null)
                  wc.CancelAsync();
          }
      });


      for (int i = 0; i < urls.Length; i++)
      {
          webClients[i] = new WebClient();

          #region callback
          // Specify the callback for the DownloadStringCompleted
          // event that will be raised by this WebClient instance.
          webClients[i].DownloadStringCompleted += (obj, args) =>
          {

              // Argument validation and exception handling omitted for brevity.

              // Split the string into an array of words,
              // then count the number of elements that match
              // the search term.
              string[] words = args.Result.Split(' ');
              string NAME = name.ToUpper();
              int nameCount = (from word in words.AsParallel()
                               where word.ToUpper().Contains(NAME)
                               select word)
                              .Count();

              // Associate the results with the url, and add new string to the array that
              // the underlying Task object will return in its Result property.
              lock (m_lock)
              {
                 results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, name));

                 // If this is the last async operation to complete,
                 // then set the Result property on the underlying Task.
                 count++;
                 if (count == urls.Length)
                 {
                    tcs.TrySetResult(results.ToArray());
                 }
              }
          };
          #endregion

          // Call DownloadStringAsync for each URL.
          Uri address = null;
          address = new Uri(urls[i]);
          webClients[i].DownloadStringAsync(address, address);

      } // end for

      // Return the underlying Task. The client code
      // waits on the Result property, and handles exceptions
      // in the try-catch block there.
      return tcs.Task;
   }
}
Imports System.Collections.Generic
Imports System.Net
Imports System.Threading
Imports System.Threading.Tasks

Public Class SimpleWebExample
   Dim tcs As New TaskCompletionSource(Of String())
   Dim token As CancellationToken
   Dim results As New List(Of String)
   Dim m_lock As New Object()
   Dim count As Integer
   Dim addresses() As String
   Dim nameToSearch As String
   
   Public Function GetWordCountsSimplified(ByVal urls() As String, ByVal str As String,
                                           ByVal token As CancellationToken) As Task(Of String())
      addresses = urls
      nameToSearch = str
      
      Dim webClients(urls.Length - 1) As WebClient

      ' If the user cancels the CancellationToken, then we can use the
      ' WebClient's ability to cancel its own async operations.
      token.Register(Sub()
                        For Each wc As WebClient In webClients
                           If wc IsNot Nothing Then
                              wc.CancelAsync()
                           End If
                        Next
                     End Sub)

      For i As Integer = 0 To urls.Length - 1
         webClients(i) = New WebClient()

         ' Specify the callback for the DownloadStringCompleted
         ' event that will be raised by this WebClient instance.
         AddHandler webClients(i).DownloadStringCompleted, AddressOf WebEventHandler

         Dim address As New Uri(urls(i))
         ' Pass the address, and also use it for the userToken
         ' to identify the page when the delegate is invoked.
         webClients(i).DownloadStringAsync(address, address)
      Next

      ' Return the underlying Task. The client code
      ' waits on the Result property, and handles exceptions
      ' in the try-catch block there.
      Return tcs.Task
   End Function

   Public Sub WebEventHandler(ByVal sender As Object, ByVal args As DownloadStringCompletedEventArgs)

      If args.Cancelled = True Then
         tcs.TrySetCanceled()
         Return
      ElseIf args.Error IsNot Nothing Then
         tcs.TrySetException(args.Error)
         Return
      Else
         ' Split the string into an array of words,
         ' then count the number of elements that match
         ' the search term.
         Dim words() As String = args.Result.Split(" "c)

         Dim name As String = nameToSearch.ToUpper()
         Dim nameCount = (From word In words.AsParallel()
                          Where word.ToUpper().Contains(name)
                          Select word).Count()

         ' Associate the results with the url, and add new string to the array that
         ' the underlying Task object will return in its Result property.
         SyncLock (m_lock)
            results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, nameToSearch))
            count = count + 1
            If (count = addresses.Length) Then
               tcs.TrySetResult(results.ToArray())
            End If
         End SyncLock
      End If
   End Sub
End Class

Pour obtenir un exemple plus complet incluant une gestion supplémentaire des exceptions et indiquant comment appeler la méthode depuis le code client, consultez Guide pratique : Inclure dans un wrapper des modèles EAP dans une tâche.For a more complete example, which includes additional exception handling and shows how to call the method from client code, see How to: Wrap EAP Patterns in a Task.

Souvenez-vous que toute tâche créée par un TaskCompletionSource<TResult> sera lancée par ce TaskCompletionSource et que le code utilisateur ne doit donc pas appeler la méthode Start dans cette tâche.Remember that any task that is created by a TaskCompletionSource<TResult> will be started by that TaskCompletionSource and, therefore, user code should not call the Start method on that task.

Implémentation du modèle de programmation asynchrone à l’aide de tâchesImplementing the APM Pattern By Using Tasks

Dans certains scénarios, il peut être souhaitable d’exposer directement le modèle IAsyncResult à l’aide de paires de méthodes Begin/End dans une API.In some scenarios, it may be desirable to directly expose the IAsyncResult pattern by using Begin/End method pairs in an API. Vous pouvez, par exemple, maintenir la cohérence avec les API existantes ou posséder des outils automatisés qui nécessitent ce modèle.For example, you may want to maintain consistency with existing APIs, or you may have automated tools that require this pattern. Dans de tels cas, vous pouvez utiliser des tâches pour simplifier l'implémentation en interne du modèle de programmation asynchrone.In such cases, you can use Tasks to simplify how the APM pattern is implemented internally.

L’exemple suivant montre comment utiliser des tâches pour implémenter une paire de méthodes de modèle de programmation asynchrone Begin/End pour une méthode orientée calcul de longue durée.The following example shows how to use tasks to implement an APM Begin/End method pair for a long-running compute-bound method.

class Calculator
{
    public IAsyncResult BeginCalculate(int decimalPlaces, AsyncCallback ac, object state)
    {
        Console.WriteLine("Calling BeginCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId);
        Task<string> f = Task<string>.Factory.StartNew(_ => Compute(decimalPlaces), state);
        if (ac != null) f.ContinueWith((res) => ac(f));
        return f;
    }

    public string Compute(int numPlaces)
    {
        Console.WriteLine("Calling compute on thread {0}", Thread.CurrentThread.ManagedThreadId);

        // Simulating some heavy work.
        Thread.SpinWait(500000000);

        // Actual implemenation left as exercise for the reader.
        // Several examples are available on the Web.
        return "3.14159265358979323846264338327950288";
    }

    public string EndCalculate(IAsyncResult ar)
    {
        Console.WriteLine("Calling EndCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId);
        return ((Task<string>)ar).Result;
    }
}

public class CalculatorClient
{
    static int decimalPlaces = 12;
    public static void Main()
    {
        Calculator calc = new Calculator();
        int places = 35;

        AsyncCallback callBack = new AsyncCallback(PrintResult);
        IAsyncResult ar = calc.BeginCalculate(places, callBack, calc);

        // Do some work on this thread while the calulator is busy.
        Console.WriteLine("Working...");
        Thread.SpinWait(500000);
        Console.ReadLine();
    }

    public static void PrintResult(IAsyncResult result)
    {
        Calculator c = (Calculator)result.AsyncState;
        string piString = c.EndCalculate(result);
        Console.WriteLine("Calling PrintResult on thread {0}; result = {1}",
                    Thread.CurrentThread.ManagedThreadId, piString);
    }
}
Class Calculator
    Public Function BeginCalculate(ByVal decimalPlaces As Integer, ByVal ac As AsyncCallback, ByVal state As Object) As IAsyncResult
        Console.WriteLine("Calling BeginCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId)
        Dim myTask = Task(Of String).Factory.StartNew(Function(obj) Compute(decimalPlaces), state)
        myTask.ContinueWith(Sub(antedecent) ac(myTask))

    End Function
    Private Function Compute(ByVal decimalPlaces As Integer)
        Console.WriteLine("Calling compute on thread {0}", Thread.CurrentThread.ManagedThreadId)

        ' Simulating some heavy work.
        Thread.SpinWait(500000000)

        ' Actual implemenation left as exercise for the reader.
        ' Several examples are available on the Web.
        Return "3.14159265358979323846264338327950288"
    End Function

    Public Function EndCalculate(ByVal ar As IAsyncResult) As String
        Console.WriteLine("Calling EndCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId)
        Return CType(ar, Task(Of String)).Result
    End Function
End Class

Class CalculatorClient
    Shared decimalPlaces As Integer
    Shared Sub Main()
        Dim calc As New Calculator
        Dim places As Integer = 35
        Dim callback As New AsyncCallback(AddressOf PrintResult)
        Dim ar As IAsyncResult = calc.BeginCalculate(places, callback, calc)

        ' Do some work on this thread while the calulator is busy.
        Console.WriteLine("Working...")
        Thread.SpinWait(500000)
        Console.ReadLine()
    End Sub

    Public Shared Sub PrintResult(ByVal result As IAsyncResult)
        Dim c As Calculator = CType(result.AsyncState, Calculator)
        Dim piString As String = c.EndCalculate(result)
        Console.WriteLine("Calling PrintResult on thread {0}; result = {1}",
                   Thread.CurrentThread.ManagedThreadId, piString)
    End Sub

End Class

Utilisation de l'exemple de code StreamExtensionsUsing the StreamExtensions Sample Code

Le fichier Streamextensions.cs, dans Exemples de programmation parallèle avec .NET Framework 4, contient plusieurs implémentations de référence qui utilisent des objets Task pour des opérations d’E/S réseau ou de fichier asynchrones.The Streamextensions.cs file, in Samples for Parallel Programming with the .NET Framework 4, contains several reference implementations that use Task objects for asynchronous file and network I/O.

Voir aussiSee also