Explore the cancellation Task

We all know that Task.Runthe method can be passed a CancellationToken, for cancellation. But how many people really go to find out about when to call CancellationSource.Canceltime method, Taskwhether it was canceled

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We do an experiment

        public static async void Foo()
        {
            var source = new CancellationTokenSource();

            Task.Run(() =>
            {
                Thread.Sleep(TimeSpan.FromSeconds(5));
                Console.WriteLine("task 运行结束");
            }, source.Token);
            await Task.Delay(TimeSpan.FromSeconds(3));
            source.Cancel();
            Console.WriteLine("取消任务");
        }

Create the above program that allows a Task output "task to run over" after running five seconds, but were canceled after 3 seconds.

Surprisingly, even the successful implementation of the task is completed.

To further confirm the results, we add a follow-up mission, before the end of the state to view the next task

        public static async void Foo()
        {
            var source = new CancellationTokenSource();

            Task.Run(() =>
            {
                Thread.Sleep(TimeSpan.FromSeconds(5));
                Console.WriteLine("task 运行结束");
            }, source.Token).ContinueWith(task => Console.WriteLine(task.Status));
            await Task.Delay(TimeSpan.FromSeconds(3));
            source.Cancel();
            Console.WriteLine("取消任务");
        }

As a result RanToCompletioninstead Canceled, indicating that no task is canceled

If you do not cancel before we wait for three seconds, and add tasks to start running log

        public static async void Foo()
        {
            var source = new CancellationTokenSource();

            Task.Run(() =>
            {
                Console.WriteLine("task 运行开始");
                Thread.Sleep(TimeSpan.FromSeconds(5));
                Console.WriteLine("task 运行结束");
            }, source.Token).ContinueWith(task => Console.WriteLine(task.Status));
            source.Cancel();
            Console.WriteLine("取消任务");
        }

(…/media/1548577856869.png)]

Tasks are normally canceled

the reason

In fact, when Task.Runthe real task begins execution, calling CancellationSource.Cancelthe method does not cancel the task, or the end of the calling thread. Method Invocation still be carried out smoothly.

So with CancellationTokenmethod overloading what use is it?

1, as shown in the third example, the task before the start of operation, the call Cancelcan be directly cancel the task, avoid excessive consumption of a thread

2, when the corresponding token, in the execution thrown body OperationCanceledException, (i.e. calls CancellationToken.ThrowIfCancellationRequested（）) which can be captured, and the task state is set toCanceled

        public static async void Foo()
        {
            var source = new CancellationTokenSource();

            Task.Run(() =>
            {
                Console.WriteLine("task 运行开始");
                Thread.Sleep(TimeSpan.FromSeconds(5));
                source.Token.ThrowIfCancellationRequested();
                Console.WriteLine("task 运行结束");
            },source.Token).ContinueWith(task => Console.WriteLine(task.Status));
            await Task.Delay(TimeSpan.FromSeconds(3));
            source.Cancel();
            Console.WriteLine("取消任务");
        }

And without the use of this overload, the task will be terminated due to an uncaught exception, not canceled

        public static async void Foo()
        {
            var source = new CancellationTokenSource();

            Task.Run(() =>
            {
                Console.WriteLine("task 运行开始");
                Thread.Sleep(TimeSpan.FromSeconds(5));
                source.Token.ThrowIfCancellationRequested();
                Console.WriteLine("task 运行结束");
            }).ContinueWith(task => Console.WriteLine(task.Status));
            await Task.Delay(TimeSpan.FromSeconds(3));
            source.Cancel();
            Console.WriteLine("取消任务");
        }

So we Task.Runadd CancellationTokenstill need the manual to determine whether to cancel the token in the method of execution

Reference links:

• c# - How to cancel a running task? - Stack Overflow
• Task cancellation in C# and things you should know about it
• c# - Stop Task when task run - Stack Overflow
• Task.Run Method (System.Threading.Tasks) - Microsoft Docs
• Task Cancellation - Microsoft Docs
• How to: Cancel a Task and Its Children - Microsoft Docs
• How to: Use Parallel.Invoke to Execute Parallel Operations - Microsoft Docs

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