Cancellation token in Task constructor: why?
.net-4.0, c#, cancellation-token, task-parallel-library
Solution
Passing a `CancellationToken` into the `Task` constructor associates it with the task.
Quoting Stephen Toub's answer from MSDN:
This has two primary benefits:
- 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.
- 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).
Problem
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?