HOW TO：取消工作及其子系
這些範例將說明如何執行下列工作：
建立及啟動可取消的工作。
將取消語彙基元傳遞至您的使用者委派，並選擇性地傳遞至工作執行個體。
注意使用者委派中的取消要求，並予以回應。
(選擇性) 注意已取消工作的呼叫端執行緒。
呼叫端執行緒不會強制結束工作；它只會指出有人要求取消。 如果工作已在執行中，則應由使用者委派注意要求並予以適當回應。 如果是在工作執行前要求取消，則使用者委派未曾執行，而工作物件會轉換成「已取消」狀態。
範例
此範例說明如何在回應取消要求時結束 Task 及其子系。 此範例也說明在使用者委派藉由擲回 OperationCanceledException 而結束時，呼叫端執行緒可以選擇性地使用 Wait 方法或 WaitAll 方法等候工作完成。 在這種情況下，委派必須使用 try catch 區塊來處理呼叫端執行緒的例外狀況。
' 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();
}
}
}
}
}
System.Threading.Tasks.Task 類別已與以 System.Threading.CancellationTokenSource 和 System.Threading.CancellationToken 型別為基礎的取消模型完全整合。 如需詳細資訊，請參閱取消和工作取消。
請參閱
參考
System.Threading.CancellationTokenSource
System.Threading.CancellationToken
System.Threading.Tasks.Task<TResult>