Procedura: annullare un'attività e i relativi figli
In questi esempi viene mostrato come eseguire le attività seguenti:
Creazione e avvio di un'attività annullabile.
Passaggio di un token di annullamento al delegato dell'utente e facoltativamente all'istanza dell'attività.
Rilevamento della richiesta di annullamento nel delegato dell'utente e risposta a tale richiesta.
Rilevamento facoltativo dell'annullamento dell'attività nel thread chiamante.
Il thread chiamante non termina forzatamente l'attività, bensì si limita a segnalare la richiesta di annullamento. Se l'attività è già in esecuzione, è responsabilità del delegato dell'utente rilevare la richiesta e rispondere in modo appropriato. Se l'annullamento viene richiesto prima dell'esecuzione dell'attività, il delegato dell'utente non viene eseguito e l'oggetto attività passa allo stato Canceled.
Esempio
In questo esempio viene mostrato come terminare un oggetto Task e i relativi figli in risposta a una richiesta di annullamento. Viene inoltre mostrato che quando un delegato dell'utente viene terminato mediante la generazione di un oggetto OperationCanceledException, il thread chiamante può utilizzare facoltativamente il metodo Wait o il metodo WaitAll per attendere il completamento delle attività. In questo caso, il delegato deve utilizzare un blocco try-catch per gestire le eccezioni nel thread chiamante.
' How to: Cancel a Task and Its Children
Imports System.Threading
Imports System.Threading.Tasks
Module CancelATask
Sub Main()
Console.WriteLine("Press any key to start. Press 'c' to cancel.")
Console.ReadKey()
Dim tokenSource As New CancellationTokenSource()
Dim token As CancellationToken = tokenSource.Token
' Store references to the tasks so that we can wait on them and
' observe their status after cancellation.
Dim tasks(10) As Task
' Request cancellation of a single task when the token source is canceled.
' Pass the token to the user delegate, and also to the task so it can
' handle the exception correctly.
tasks(0) = Task.Factory.StartNew(Sub() DoSomeWork(1, token), token)
' Request cancellation of a task and its children. Note the token is passed
' to (1) the user delegate and (2) as the second argument to StartNew, so
' that the task instance can correctly handle the OperationCanceledException.
tasks(1) = Task.Factory.StartNew(Sub()
' Create some cancelable child tasks.
For i As Integer = 2 To 10
' For each child task, pass the same token
' to each user delegate and to StartNew.
tasks(i) = Task.Factory.StartNew(Sub(iteration) DoSomeWork(iteration, token), i, token)
' Passing the same token again to do work on the parent task.
' All will be signaled by the call to tokenSource.Cancel below.
DoSomeWork(2, token)
Next
End Sub _
, token)
' Give the tasks a second to start.
Thread.Sleep(1000)
' Request cancellation from the UI thread.
If Console.ReadKey().KeyChar = "c"c Then
tokenSource.Cancel()
Console.WriteLine("\nTask cancellation requested.")
' Optional: Observe the change in the Status property on the task.
' It is not necessary to wait on tasks that have canceled. However,
' if you do wait, you must enclose the call in a try-catch block to
' catch the OperationCanceledExceptions that are thrown. If you do
' not wait, no OCE is thrown if the token that was passed to the
' StartNew method is the same token that requested the cancellation.
End If
Try
Task.WaitAll(tasks)
Catch e As AggregateException
' For demonstration purposes, show the OCE message.
For Each v In e.InnerExceptions
Console.WriteLine("msg: " + v.Message)
Next
End Try
' Prove that the tasks are now all in a canceled state.
For i As Integer = 0 To tasks.Length
Console.WriteLine("task(0) status is now 1", i, tasks(i).Status)
Next
' Keep the console window open while the
' task completes its output.
Console.ReadLine()
End Sub
Sub DoSomeWork(ByVal taskNum As Integer, ByVal ct As CancellationToken)
' Was cancellation already requested?
If ct.IsCancellationRequested = True Then
Console.WriteLine("We were cancelled before we got started.")
Console.WriteLine("Press Enter to quit.")
ct.ThrowIfCancellationRequested()
End If
Dim maxIterations As Integer = 1000
' NOTE!!! An "OperationCanceledException was unhandled
' by user code" error will be raised here if "Just My Code"
' is enabled on your computer. On Express editions JMC is
' enabled and cannot be disabled. The exception is benign.
' Just press F5 to continue executing your code.
For i As Integer = 0 To maxIterations
' Do a bit of work. Not too much.
Dim sw As New SpinWait()
For j As Integer = 0 To 3000
sw.SpinOnce()
Next
Console.WriteLine("...0 ", taskNum)
If ct.IsCancellationRequested Then
Console.WriteLine("bye from 0.", taskNum)
Console.WriteLine("\nPress Enter to quit.")
ct.ThrowIfCancellationRequested()
End If
Next
End Sub
End Module
namespace CancellationWithOCE
{
using System;
using System.Threading;
using System.Threading.Tasks;
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Press any key to start. Press 'c' to cancel.");
Console.ReadKey();
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
// Store references to the tasks so that we can wait on them and
// observe their status after cancellation.
Task[] tasks = new Task[10];
// Request cancellation of a single task when the token source is canceled.
// Pass the token to the user delegate, and also to the task so it can
// handle the exception correctly.
tasks[0] = Task.Factory.StartNew(() => DoSomeWork(1, token), token);
// Request cancellation of a task and its children. Note the token is passed
// to (1) the user delegate and (2) as the second argument to StartNew, so
// that the task instance can correctly handle the OperationCanceledException.
tasks[1] = Task.Factory.StartNew(() =>
{
// Create some cancelable child tasks.
for (int i = 2; i < 10; i++)
{
// For each child task, pass the same token
// to each user delegate and to StartNew.
tasks[i] = Task.Factory.StartNew(iteration =>
DoSomeWork((int)iteration, token), i, token);
}
// Passing the same token again to do work on the parent task.
// All will be signaled by the call to tokenSource.Cancel below.
DoSomeWork(2, token);
}, token);
// Give the tasks a second to start.
Thread.Sleep(1000);
// Request cancellation from the UI thread.
if (Console.ReadKey().KeyChar == 'c')
{
tokenSource.Cancel();
Console.WriteLine("\nTask cancellation requested.");
// Optional: Observe the change in the Status property on the task.
// It is not necessary to wait on tasks that have canceled. However,
// if you do wait, you must enclose the call in a try-catch block to
// catch the OperationCanceledExceptions that are thrown. If you do
// not wait, no OCE is thrown if the token that was passed to the
// StartNew method is the same token that requested the cancellation.
#region Optional_WaitOnTasksToComplete
try
{
Task.WaitAll(tasks);
}
catch (AggregateException e)
{
// For demonstration purposes, show the OCE message.
foreach (var v in e.InnerExceptions)
Console.WriteLine("msg: " + v.Message);
}
// Prove that the tasks are now all in a canceled state.
for (int i = 0; i < tasks.Length; i++)
Console.WriteLine("task[{0}] status is now {1}", i, tasks[i].Status);
#endregion
}
// Keep the console window open while the
// task completes its output.
Console.ReadLine();
}
static void DoSomeWork(int taskNum, CancellationToken ct)
{
// Was cancellation already requested?
if (ct.IsCancellationRequested)
{
Console.WriteLine("We were cancelled before we got started.");
Console.WriteLine("Press Enter to quit.");
ct.ThrowIfCancellationRequested();
}
int maxIterations = 1000;
// NOTE!!! A benign "OperationCanceledException was unhandled
// by user code" error might be raised here. Press F5 to continue. Or,
// to avoid the error, uncheck the "Enable Just My Code"
// option under Tools > Options > Debugging.
for (int i = 0; i < maxIterations; i++)
{
// Do a bit of work. Not too much.
var sw = new SpinWait();
for (int j = 0; j < 3000; j++) sw.SpinOnce();
Console.WriteLine("...{0} ", taskNum);
if (ct.IsCancellationRequested)
{
Console.WriteLine("bye from {0}.", taskNum);
Console.WriteLine("\nPress Enter to quit.");
ct.ThrowIfCancellationRequested();
}
}
}
}
}
La classe System.Threading.Tasks.Task è completamente integrata con il modello di annullamento basato sui tipi System.Threading.CancellationTokenSource e System.Threading.CancellationToken. Per ulteriori informazioni, vedere Annullamento e Annullamento delle attività.
Vedere anche
Riferimenti
System.Threading.CancellationTokenSource
System.Threading.CancellationToken
System.Threading.Tasks.Task<TResult>
Concetti
Attività annidate e attività figlio
Espressioni lambda in PLINQ e TPL