如何停止异步任务？

Question

Suppose that I have a List<Task> which help me to manage all the task based on index:

private List<Task> tasks = new List<Task>(new Task[3]);

When the application start a menu is displayed, and the user can select which task can start, simply:

public void Menu()
{
    Console.WriteLine("1. Task");
    Console.WriteLine("2. Task");
    Console.WriteLine("3. Task");
    Console.WriteLine("0. Exit");

    ConsoleKeyInfo input;

    do
    {
        input = Console.ReadKey();
        Console.WriteLine();

        switch (input.Key)
        {
            case ConsoleKey.NumPad1:
                 StartTask(1);
                 break;
            case ConsoleKey.NumPad2:
                 StartTask(2);
                 break;
            case ConsoleKey.NumPad3:
                 StartTask(3);
                 break;
        }
     } while (input.Key != ConsoleKey.NumPad0);
}

inside the method StartTask I pass the number of Task to execute, this is the logic:

public void StartTask(int instance)
{
    int index = instance - 1;

    //Task not yet instantiated, create new one
    if(tasks.ElementAtOrDefault(index) == null)
    {
        //Make a new cancellation token
        cts.Insert(index, new CancellationTokenSource());

        tasks.Insert(index, new Task(async () => 
        {
            await foo.DoWork(cts[index].Token);
        }, cts[index].Token));

        if(!tasks[index].Status.Equals(TaskStatus.Running))
        {
            tasks[index].Start();
        }
    }
}

as you can see I also have a List<CancellationTokenSource> cts which help me to terminate the task. The Cancel method is placed in another menu like above.

Now the problem is pretty clear: I can't do that because the checking is executed before the await operation of DoWork(); that contains this:

public class Foo
{
     private async Task DoWorkAsync(CancellationToken token)
     {
         while (!token.IsCancellationRequested)
         {
             string newData = DateTime.Now.ToLongTimeString();

             Console.WriteLine(newData);
              await Task.Delay(200);
         }
     }
}

essentially: how can I stop the Task that execute the loop with the CancellationToken ?

The token passed as parameter cannot change the value if the stop is executed later the start. Essentially the value of IsCancellationRequested is changing only in the Task class context, not in the argument passed.

Answer 1

Pass cancellation token to DoWorkAsync. And use it in 'while' loop as a condition.

while(!token.IsCancellationRequested)
{ 
   await Task.Delay(200, token); 
}

'token' should be passed as a parameter. To be honest, I think there is too much redundant code in your solution ...

How to cancel task using cancellation token source:

void CancelTask(int taskNumber)
{
   int index = taskNumber - 1;
   cts[index].Cancel();
}

This code will set 'IsCancellationRequested' flag of 'token' you use in the loop into 'true'.