CancellationTokenSource.Cancel 方法
定義
重要
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傳達取消的要求。
多載
| 名稱 | Description |
|---|---|
| Cancel() |
傳達取消的要求。 |
| Cancel(Boolean) |
傳達取消的要求，並指定例外狀況發生時，是否應該處理剩餘的回呼和可取消的作業。 |
Cancel()
傳達取消的要求。
public:
void Cancel();public void Cancel();member this.Cancel : unit -> unitPublic Sub Cancel ()例外狀況
一個包含所有註冊回撥 CancellationToken拋出的異常的彙總例外。
範例
下列範例會使用隨機數產生器來模擬數據收集應用程式，以從11個不同儀器讀取10個整數值。 值為零表示一個儀器的測量失敗，在此情況下應該取消作業，而且不應計算整體平均數。
為了處理操作可能的消去，範例實例化一個 CancellationTokenSource 物件，產生一個消去標記，並傳達給物件 TaskFactory 。 TaskFactory物件再將消除標記傳遞給負責收集特定儀器讀數的任務。 TaskFactory.ContinueWhenAll<TAntecedentResult,TResult>(Task<TAntecedentResult>[], Func<Task<TAntecedentResult>[],TResult>, CancellationToken)此方法的使用是為了確保平均值僅在所有讀數成功收集後才計算。 如果任務因被取消而未完成，呼叫該 TaskFactory.ContinueWhenAll 方法時會拋出例外。
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
// Define the cancellation token.
CancellationTokenSource source = new CancellationTokenSource();
CancellationToken token = source.Token;
Random rnd = new Random();
Object lockObj = new Object();
List<Task<int[]>> tasks = new List<Task<int[]>>();
TaskFactory factory = new TaskFactory(token);
for (int taskCtr = 0; taskCtr <= 10; taskCtr++) {
int iteration = taskCtr + 1;
tasks.Add(factory.StartNew( () => {
int value;
int[] values = new int[10];
for (int ctr = 1; ctr <= 10; ctr++) {
lock (lockObj) {
value = rnd.Next(0,101);
}
if (value == 0) {
source.Cancel();
Console.WriteLine("Cancelling at task {0}", iteration);
break;
}
values[ctr-1] = value;
}
return values;
}, token));
}
try {
Task<double> fTask = factory.ContinueWhenAll(tasks.ToArray(),
(results) => {
Console.WriteLine("Calculating overall mean...");
long sum = 0;
int n = 0;
foreach (var t in results) {
foreach (var r in t.Result) {
sum += r;
n++;
}
}
return sum/(double) n;
} , token);
Console.WriteLine("The mean is {0}.", fTask.Result);
}
catch (AggregateException ae) {
foreach (Exception e in ae.InnerExceptions) {
if (e is TaskCanceledException)
Console.WriteLine("Unable to compute mean: {0}",
((TaskCanceledException) e).Message);
else
Console.WriteLine("Exception: " + e.GetType().Name);
}
}
finally {
source.Dispose();
}
}
}
// Repeated execution of the example produces output like the following:
// Cancelling at task 5
// Unable to compute mean: A task was canceled.
//
// Cancelling at task 10
// Unable to compute mean: A task was canceled.
//
// Calculating overall mean...
// The mean is 5.29545454545455.
//
// Cancelling at task 4
// Unable to compute mean: A task was canceled.
//
// Cancelling at task 5
// Unable to compute mean: A task was canceled.
//
// Cancelling at task 6
// Unable to compute mean: A task was canceled.
//
// Calculating overall mean...
// The mean is 4.97363636363636.
//
// Cancelling at task 4
// Unable to compute mean: A task was canceled.
//
// Cancelling at task 5
// Unable to compute mean: A task was canceled.
//
// Cancelling at task 4
// Unable to compute mean: A task was canceled.
//
// Calculating overall mean...
// The mean is 4.86545454545455.
Imports System.Collections.Generic
Imports System.Threading
Imports System.Threading.Tasks
Module Example
Public Sub Main()
' Define the cancellation token.
Dim source As New CancellationTokenSource()
Dim token As CancellationToken = source.Token
Dim lockObj As New Object()
Dim rnd As New Random
Dim tasks As New List(Of Task(Of Integer()))
Dim factory As New TaskFactory(token)
For taskCtr As Integer = 0 To 10
Dim iteration As Integer = taskCtr + 1
tasks.Add(factory.StartNew(Function()
Dim value, values(9) As Integer
For ctr As Integer = 1 To 10
SyncLock lockObj
value = rnd.Next(0,101)
End SyncLock
If value = 0 Then
source.Cancel
Console.WriteLine("Cancelling at task {0}", iteration)
Exit For
End If
values(ctr-1) = value
Next
Return values
End Function, token))
Next
Try
Dim fTask As Task(Of Double) = factory.ContinueWhenAll(tasks.ToArray(),
Function(results)
Console.WriteLine("Calculating overall mean...")
Dim sum As Long
Dim n As Integer
For Each t In results
For Each r In t.Result
sum += r
n+= 1
Next
Next
Return sum/n
End Function, token)
Console.WriteLine("The mean is {0}.", fTask.Result)
Catch ae As AggregateException
For Each e In ae.InnerExceptions
If TypeOf e Is TaskCanceledException
Console.WriteLine("Unable to compute mean: {0}",
CType(e, TaskCanceledException).Message)
Else
Console.WriteLine("Exception: " + e.GetType().Name)
End If
Next
Finally
source.Dispose()
End Try
End Sub
End Module
' Repeated execution of the example produces output like the following:
' Cancelling at task 5
' Unable to compute mean: A task was canceled.
'
' Cancelling at task 10
' Unable to compute mean: A task was canceled.
'
' Calculating overall mean...
' The mean is 5.29545454545455.
'
' Cancelling at task 4
' Unable to compute mean: A task was canceled.
'
' Cancelling at task 5
' Unable to compute mean: A task was canceled.
'
' Cancelling at task 6
' Unable to compute mean: A task was canceled.
'
' Calculating overall mean...
' The mean is 4.97363636363636.
'
' Cancelling at task 4
' Unable to compute mean: A task was canceled.
'
' Cancelling at task 5
' Unable to compute mean: A task was canceled.
'
' Cancelling at task 4
' Unable to compute mean: A task was canceled.
'
' Calculating overall mean...
' The mean is 4.86545454545455.
備註
關聯的 Associated CancellationToken 會收到取消通知，並轉換到一個回 IsCancellationRequested 傳 true 的狀態。
任何在 CancellationToken 中註冊的回調或可取消操作，如果之前尚未被 的呼叫 Cancel()執行，則會被執行。 後續呼叫 Cancel() 不會再次執行相同的回撥，除非重新註冊。 （避免多次呼叫 Cancel()，因為此類程式碼的意圖通常不明確。）
回調以 LIFO 順序同步執行。
我們建議可取消操作與回撥註冊 CancellationToken 時不要拋出例外。
這種 Cancel 的超載會彙整拋入 的 AggregateException異常，使得一個回撥拋出例外不會阻止其他註冊回撥的執行。
呼叫此方法與呼叫 Cancel(false)的效果相同。
另請參閱
適用於
Cancel(Boolean)
傳達取消的要求，並指定例外狀況發生時，是否應該處理剩餘的回呼和可取消的作業。
public:
void Cancel(bool throwOnFirstException);public void Cancel(bool throwOnFirstException);member this.Cancel : bool -> unitPublic Sub Cancel (throwOnFirstException As Boolean)參數
- throwOnFirstException
- Boolean
true若例外應立即傳播;否則，。 false
例外狀況
一個包含所有註冊回撥 CancellationToken拋出的異常的彙總例外。
備註
相關 CancellationToken 被通知取消，並轉換到一個狀態，返回 IsCancellationRequestedtrue。
任何在 CancellationToken 中註冊的回調或可取消操作，如果之前尚未被 的呼叫 Cancel()執行，則會被執行。 後續呼叫 Cancel() 不會再次執行相同的回撥，除非重新註冊。 （避免多次呼叫 Cancel()，因為此類程式碼的意圖通常不明確。）
回調以 LIFO 順序同步執行。
我們建議可取消操作與回撥註冊 CancellationToken 時不要拋出例外。
若 throwOnFirstException 為 true，例外將立即從呼叫中傳播至 Cancel，阻止剩餘回調及可取消操作被處理。
若 throwOnFirstException 是 false，則此過載會彙整拋入 AggregateException的異常，使得一個回調拋出例外不會阻止其他註冊回撥的執行。
