Why pass cancellation token to TaskFactory.StartNew?

Question

Besides the most common form of calling TaskFactory.StartNew with only the "action" parameter (1) https://msdn.microsoft.com/en-us/library/dd321439(v=vs.110).aspx

we alse have one method that accepts an extra parameter as the "Cancelation Token" (2) https://msdn.microsoft.com/en-us/library/dd988458.aspx

My question is, why should we use the call (2) instead of call (1)?

I mean, the example in MSDN for page (2) would also work if I don't pass the Cancellation Token as parameter (because the variable token is accessible from the delegate function. Something like:

var tokenSource = new CancellationTokenSource();
      var token = tokenSource.Token;
      var files = new List<Tuple<string, string, long, DateTime>>();

      var t = Task.Factory.StartNew( () => { string dir = "C:\\Windows\\System32\\";
                                object obj = new Object();
                                if (Directory.Exists(dir)) {
                                   Parallel.ForEach(Directory.GetFiles(dir),
                                   f => {
                                           if (token.IsCancellationRequested)
                                              token.ThrowIfCancellationRequested();
                                           var fi = new FileInfo(f);
                                           lock(obj) {
                                              files.Add(Tuple.Create(fi.Name, fi.DirectoryName, fi.Length, fi.LastWriteTimeUtc));          
                                           }
                                      });
                                 }
                              }
                        ); //note that I removed the ", token" from here
      tokenSource.Cancel();

So is there anything happening underneath when I pass cancellation token to Task.Factory.StartNew?

Thanks

Answer 1

Two things will happen.

1. If the token has been canceled before StartNew is called it will never start the thread and the task will be in the Canceled state.
2. If a OperationCanceledException is raised from inside the task and that exception was passed in the same token as StartNew it will cause the returned Task to enter the Cancelled state. If the token associated with the exception is a different token or you did not pass a token in the task will enter the Faulted state.

P.S. You should never call Task.Factory.StartNew without passing in a TaskScheduler because if you don't it can easily cause you to run code on the UI thread that you expected to run on a background thread. Use Task.Run( instead unless you absoultely need to use StartNew, Task.Run has the same CancellationToken behavior as StartNew.