c# - What is "cancellationToken" in the TaskFactory.StartNew() used for?

Question

http://msdn.microsoft.com/en-us/library/dd988458.aspx

UPD:

so, let's discuss this article then: http://msdn.microsoft.com/en-us/library/dd997396.aspx

I've changed that code a little:

    static void Main()
    {

        var tokenSource2 = new CancellationTokenSource();
        CancellationToken ct = tokenSource2.Token;

        var task = Task.Factory.StartNew(() =>
        {

            // Were we already canceled?
            ct.ThrowIfCancellationRequested();

            bool moreToDo = true;
            Thread.Sleep(5000);
            while (moreToDo)
            {

                // Poll on this property if you have to do
                // other cleanup before throwing.
                if (ct.IsCancellationRequested)
                {
                    Console.WriteLine("exit");
                    // Clean up here, then...
                    ct.ThrowIfCancellationRequested();
                }

            }
        }, tokenSource2.Token); // this parameter useless

        Console.WriteLine("sleep");
        Thread.Sleep(2000);
        Console.WriteLine("cancel");

        tokenSource2.Cancel();

        // Just continue on this thread, or Wait/WaitAll with try-catch:
        try
        {
            task.Wait();
        }
        catch (AggregateException e)
        {
            foreach (var v in e.InnerExceptions)
            {
                Console.WriteLine(e.Message + " " + v.Message);
            }
        }

        Console.ReadKey();
    }

UPD: Well, this changes only task.IsCanceled, which is imho useless, due to I still ought to implement all manually.

Answer 1

Due to comments, I'm posting another answer.

Consider the following code:

var tokenSource = new CancellationTokenSource();
CancellationToken ct = tokenSource.Token;

tokenSource.Cancel(); 

var task = Task.Factory.StartNew(() =>
{    
  // Were we already canceled?
  ct.ThrowIfCancellationRequested();
  // do some processing
});

Even if the call tokenSource.Cancel() is issued before the task was actually started, you'll still allocate a worker thread from thread pool, so you'll waste some system resources.

But when you specify token argument in Task.Factory.StartNew, the task will be cancelled immediately, without allocating a worker thread.