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Procédure : désencapsuler une tâche imbriquée

Vous pouvez retourner une tâche à partir d’une méthode, puis attendre ou poursuivre à partir de cette tâche, comme indiqué dans l’exemple suivant :

static Task<string> DoWorkAsync()
{
    return Task<String>.Factory.StartNew(() =>
    {
       //...
        return "Work completed.";
    });
}

static void StartTask()
{
    Task<String> t = DoWorkAsync();
    t.Wait();
    Console.WriteLine(t.Result);
}
Shared Function DoWorkAsync() As Task(Of String)

    Return Task(Of String).Run(Function()
                                   '...
                                   Return "Work completed."
                               End Function)
End Function

Shared Sub StartTask()

    Dim t As Task(Of String) = DoWorkAsync()
    t.Wait()
    Console.WriteLine(t.Result)
End Sub

Dans l’exemple précédent, la propriété Result est de type string (String en Visual Basic).

Toutefois, dans certains cas, il se peut que vous souhaitiez créer une tâche dans une autre tâche, puis retourner la tâche imbriquée. Dans ce cas, le TResult de la tâche englobante est lui-même une tâche. Dans l’exemple suivant, la propriété Result est une Task<Task<string>> en C# ou Task(Of Task(Of String)) en Visual Basic.

// Note the type of t and t2.
Task<Task<string>> t = Task.Factory.StartNew(() => DoWorkAsync());
Task<Task<string>> t2 = DoWorkAsync().ContinueWith((s) => DoMoreWorkAsync());

// Outputs: System.Threading.Tasks.Task`1[System.String]
Console.WriteLine(t.Result);
' Note the type of t and t2.
Dim t As Task(Of Task(Of String)) = Task.Run(Function() DoWorkAsync())
Dim t2 As Task(Of Task(Of String)) = DoWorkAsync().ContinueWith(Function(s) DoMoreWorkAsync())

' Outputs: System.Threading.Tasks.Task`1[System.String]
Console.WriteLine(t.Result)

Bien qu’il soit possible d’écrire du code pour désencapsuler la tâche externe et extraire la tâche d’origine et sa propriété Result, ce code n’est pas facile à écrire, car vous devez gérer des exceptions et des demandes d’annulation. Dans ce cas, nous vous recommandons d’utiliser l’une des méthodes d’extension Unwrap, comme indiqué dans l’exemple suivant.

// Unwrap the inner task.
Task<string> t3 = DoWorkAsync().ContinueWith((s) => DoMoreWorkAsync()).Unwrap();

// Outputs "More work completed."
Console.WriteLine(t.Result);
' Unwrap the inner task.
Dim t3 As Task(Of String) = DoWorkAsync().ContinueWith(Function(s) DoMoreWorkAsync()).Unwrap()

' Outputs "More work completed."
Console.WriteLine(t.Result)

Les méthodes Unwrap peuvent être utilisées pour transformer toute Task<Task> ou Task<Task<TResult>> (Task(Of Task) ou Task(Of Task(Of TResult)) en Visual Basic) en Task ou Task<TResult> (Task(Of TResult) en Visual Basic). La nouvelle tâche représente entièrement la tâche imbriquée interne et inclut l’état d’annulation ainsi que toutes les exceptions.

Exemple

L'exemple suivant décrit comment utiliser la méthode d’extension Unwrap.


namespace Unwrap
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading;
    using System.Threading.Tasks;
    // A program whose only use is to demonstrate Unwrap.
    class Program
    {
        static void Main()
        {
            // An arbitrary threshold value.
            byte threshold = 0x40;

            // data is a Task<byte[]>
            var data = Task<byte[]>.Factory.StartNew(() =>
                {
                    return GetData();
                });

            // We want to return a task so that we can
            // continue from it later in the program.
            // Without Unwrap: stepTwo is a Task<Task<byte[]>>
            // With Unwrap: stepTwo is a Task<byte[]>
            var stepTwo = data.ContinueWith((antecedent) =>
                {
                    return Task<byte>.Factory.StartNew( () => Compute(antecedent.Result));
                })
                .Unwrap();

            // Without Unwrap: antecedent.Result = Task<byte>
            // and the following method will not compile.
            // With Unwrap: antecedent.Result = byte and
            // we can work directly with the result of the Compute method.
            var lastStep = stepTwo.ContinueWith( (antecedent) =>
                {
                    if (antecedent.Result >= threshold)
                    {
                      return Task.Factory.StartNew( () =>  Console.WriteLine("Program complete. Final = 0x{0:x} threshold = 0x{1:x}", stepTwo.Result, threshold));
                    }
                    else
                    {
                        return DoSomeOtherAsynchronousWork(stepTwo.Result, threshold);
                    }
                });

            lastStep.Wait();
            Console.WriteLine("Press any key");
            Console.ReadKey();
        }

        #region Dummy_Methods
        private static byte[] GetData()
        {
            Random rand = new Random();
            byte[] bytes = new byte[64];
            rand.NextBytes(bytes);
            return bytes;
        }

        static Task DoSomeOtherAsynchronousWork(int i, byte b2)
        {
            return Task.Factory.StartNew(() =>
                {
                    Thread.SpinWait(500000);
                    Console.WriteLine("Doing more work. Value was <= threshold");
                });
        }
        static byte Compute(byte[] data)
        {

            byte final = 0;
            foreach (byte item in data)
            {
                final ^= item;
                Console.WriteLine("{0:x}", final);
            }
            Console.WriteLine("Done computing");
            return final;
        }
        #endregion
    }
}
'How to: Unwrap a Task
Imports System.Threading
Imports System.Threading.Tasks

Module UnwrapATask2

    Sub Main()
        ' An arbitrary threshold value.
        Dim threshold As Byte = &H40

        ' myData is a Task(Of Byte())

        Dim myData As Task(Of Byte()) = Task.Factory.StartNew(Function()
                                                                  Return GetData()
                                                              End Function)
        ' We want to return a task so that we can
        ' continue from it later in the program.
        ' Without Unwrap: stepTwo is a Task(Of Task(Of Byte))
        ' With Unwrap: stepTwo is a Task(Of Byte)

        Dim stepTwo = myData.ContinueWith(Function(antecedent)
                                              Return Task.Factory.StartNew(Function()
                                                                               Return Compute(antecedent.Result)
                                                                           End Function)
                                          End Function).Unwrap()

        Dim lastStep = stepTwo.ContinueWith(Function(antecedent)
                                                Console.WriteLine("Result = {0}", antecedent.Result)
                                                If antecedent.Result >= threshold Then
                                                    Return Task.Factory.StartNew(Sub()
                                                                                     Console.WriteLine("Program complete. Final = &H{1:x} threshold = &H{1:x}",
                                                                                                       stepTwo.Result, threshold)
                                                                                 End Sub)
                                                Else
                                                    Return DoSomeOtherAsynchronousWork(stepTwo.Result, threshold)
                                                End If
                                            End Function)
        Try
            lastStep.Wait()
        Catch ae As AggregateException
            For Each ex As Exception In ae.InnerExceptions
                Console.WriteLine(ex.Message & ex.StackTrace & ex.GetBaseException.ToString())
            Next
        End Try

        Console.WriteLine("Press any key")
        Console.ReadKey()
    End Sub

#Region "Dummy_Methods"
    Function GetData() As Byte()
        Dim rand As Random = New Random()
        Dim bytes(64) As Byte
        rand.NextBytes(bytes)
        Return bytes
    End Function

    Function DoSomeOtherAsynchronousWork(ByVal i As Integer, ByVal b2 As Byte) As Task
        Return Task.Factory.StartNew(Sub()
                                         Thread.SpinWait(500000)
                                         Console.WriteLine("Doing more work. Value was <= threshold.")
                                     End Sub)
    End Function

    Function Compute(ByVal d As Byte()) As Byte
        Dim final As Byte = 0
        For Each item As Byte In d
            final = final Xor item
            Console.WriteLine("{0:x}", final)
        Next
        Console.WriteLine("Done computing")
        Return final
    End Function
#End Region
End Module

Voir aussi