Cancellation token in Task constructor: why?

Question

Certain System.Threading.Tasks.Task constructors take a CancellationToken as a parameter:

CancellationTokenSource source = new CancellationTokenSource();
Task t = new Task (/* method */, source.Token);

What baffles me about this is that there is no way from inside the method body to actually get at the token passed in (e.g., nothing like Task.CurrentTask.CancellationToken). The token has to be provided through some other mechanism, such as the state object or captured in a lambda.

So what purpose does providing the cancellation token in the constructor serve?

Answer 1

Passing a CancellationToken into the Task constructor associates it with the task.

Quoting Stephen Toub's answer from MSDN:

This has two primary benefits:

1. If the token has cancellation requested prior to the Task starting to execute, the Task won't execute. Rather than transitioning to Running, it'll immediately transition to Canceled. This avoids the costs of running the task if it would just be canceled while running anyway.
2. If the body of the task is also monitoring the cancellation token and throws an OperationCanceledException containing that token (which is what ThrowIfCancellationRequested does), then when the task sees that OperationCanceledException, it checks whether the OperationCanceledException's token matches the Task's token. If it does, that exception is viewed as an acknowledgement of cooperative cancellation and the Task transitions to the Canceled state (rather than the Faulted state).