取消等待本身

Question

我有下面的代碼示例：

public static async Task Async() 
{ 
    CancellationTokenSource source = new CancellationTokenSource(); 
    source.CancelAfter(500); 
    Stopwatch sw = Stopwatch.StartNew(); 
    await RunThread(ExpensiveOperation, source.Token); 
    sw.Stop(); 
    Console.WriteLine(sw.Elapsed); 
} 

public static async Task RunThread(Action act, CancellationToken token) 
{ //modify this method to handle cancelling the token during the following await 
    await Task.Run(act); //Task.Run(act, token) doesn't help 
} 

public static void ExpensiveOperation() 
{ 
    Thread.Sleep(1000); //simulates CPU expensive operation 
} 

現在，我怎麼能修改RunThread方法真正停止被取消，等待已久的任務，通過註冊任務，之後那些那麼回事500毫秒，不等待ExpensiveOperation的實際完成？

Answer 1

您應該通過令牌操作本身，並且從時間檢查時間：

public static async Task RunThread(Action<CancellationToken> act, CancellationToken token) 
{ 
    await Task.Run(() => act(token), token); 
} 

public static void ExpensiveOperation(CancellationToken token) 
{ 
    for (int i = 0; i < 10; i++) 
    { 
     token.ThrowIfCancellationRequested(); 
     Thread.Sleep(100); 
    } 
} 

也傳遞令牌Task.Run因此返回Task會知道它被取消而不是僅僅出現故障。

如果不能從ExpensiveOperation內取消（無論是不能更改的代碼，或者它實際上是一個異步操作，而不是一個同步），然後使用該WithCancellation擴展方法：

static Task WithCancellation(this Task task, CancellationToken cancellationToken) 
{ 
    return task.IsCompleted 
     ? task 
     : task.ContinueWith(
      completedTask => completedTask.GetAwaiter().GetResult(), 
      cancellationToken, 
      TaskContinuationOptions.ExecuteSynchronously, 
      TaskScheduler.Default); 
} 

public static async Task RunThread(Action act, CancellationToken token) 
{ 
    await Task.Run(act).WithCancellation(token); 
} 

請注意，這種方法實際上並不取消操作，它只是讓您的代碼流動就像它的行爲一樣。