what is the correct way to cancel multiple tasks i

问题:

I have a button thats spawns 4 tasks. The same button changes to a cancel button and clicking this should cancel all 4 tasks. Should I pass the same cancel token to all 4 tasks and have them poll on the same token for IsCancelRequested ? I am confused after reading the msdn doc on createlinkedtokensource. How is this normally done ? thank you

Update: Task.WaitAll() waits tills all tasks complete execution. Similarly how to know when all tasks have been canceled once the shared cancel token source is set to cancel.

回答1:

Yeah, what you said about using a single CancellationToken is correct. You can create a single CancellationTokenSource and use its CancellationToken for all of the tasks. Your tasks should check the token regularly for cancellation.

For example:

const int NUM_TASKS = 4;

CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken ct = cts.Token;

Task[] tasks = new Task[NUM_TASKS];
for (int i = 0; i < NUM_TASKS; i++)
{
    tasks[i] = Task.Factory.StartNew(() =>
    {
        while (true)
        {
            Thread.Sleep(1000);
            if (ct.IsCancellationRequested)
                break;
        }
    }, ct);
}

Task.WaitAll(tasks);

Your button can call cts.Cancel(); to cancel the tasks.

Update for question update:

There are a few ways to do what you ask. One way is to use ct.IsCancellationRequested to check cancellation without throwing, then allow your task to complete. Then Task.WaitAll(tasks) will complete when all of the tasks have been cancelled.

I've updated the code to reflect that change.

回答2:

Yes you should pass the same token and use this to cancel all the tasks in one go, if that's your intent.

回答3:

const int NUM_TASKS = 4;

CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken ct = cts.Token;

Task[] tasks = new Task[NUM_TASKS];
for (int i = 0; i < NUM_TASKS; i++)
{
    tasks[i] = Task.Factory.StartNew(() =>
    {
        while (true)
        {
            Thread.Sleep(1000);
            if (ct.IsCancellationRequested)
                break;
        }
    }, ct);
}

Task.WaitAll(tasks);

回答4:

Use BackroundWorker class, set property WorkerSupportsCancellation, start tasks by calling RunWorkerAsync() and stop them using CancelAsync()

You do not neet to sync your code with the UI.