Task.IsCancelled doesn't work

问题

I've got the following sample code:

static class Program
{
    static void Main()
    {
        var cts = new CancellationTokenSource();

        var task = Task.Factory.StartNew(
            () =>
                {
                    try
                    {
                        Console.WriteLine("Task: Running");
                        Thread.Sleep(5000);
                        Console.WriteLine("Task: ThrowIfCancellationRequested");
                        cts.Token.ThrowIfCancellationRequested();
                        Thread.Sleep(2000);
                        Console.WriteLine("Task: Completed");
                    }
                    catch (Exception exception)
                    {
                        Console.WriteLine("Task: " + exception.GetType().Name);
                        throw;
                    }
                }).ContinueWith(t => Console.WriteLine("ContinueWith: cts.IsCancellationRequested = {0}, task.IsCanceled = {1}, task.Exception = {2}", cts.IsCancellationRequested, t.IsCanceled, t.Exception == null ? "null" : t.Exception.GetType().Name));

        Thread.Sleep(1000);

        Console.WriteLine("Main: Cancel");
        cts.Cancel();

        try
        {
            Console.WriteLine("Main: Wait");
            task.Wait();
        }
        catch (Exception exception)
        {
            Console.WriteLine("Main: Catch " + exception.GetType().Name);
        }

        Console.WriteLine("Main: task.IsCanceled = {0}", task.IsCanceled);
        Console.WriteLine("Press any key to exit...");

        Console.ReadLine();
    }
}

The output is:

Task: Running
Main: Cancel
Main: Wait
Task: ThrowIfCancellationRequested
Task: OperationCanceledException
ContinueWith: cts.IsCancellationRequested = True, task.IsCanceled = False, task.Exception = AggregateException
Main: task.IsCanceled = False
Press any key to exit...

If I remove ContinueWith, then the output is:

Task: Running
Main: Cancel
Main: Wait
Task: ThrowIfCancellationRequested
Task: OperationCanceledException
Main: Catch AggregateException
Main: task.IsCanceled = False
Press any key to exit...

I don't understand, why task.IsCanceled returns false in both cases?

And why exception is rethrown only without ContinueWith?

What I'm trying to achieve is an uniform and simple way for waiting for task completion and a property that will indicate if task was cancelled or not.

回答1:

I think you are not cancelling the task itself, but just throwing an exception from a task. Try using StartNew(Action action,CancellationToken cancellationToken) instead of StartNew(Action action). You can also add cancellation token as a parameter to ContinueWith.

来源：https://stackoverflow.com/questions/15384237/task-iscancelled-doesnt-work

标签

task-parallel-library

task