This example showed what happens when tasks are cancelled. In this example, some tasks will be able to run to completion, others will be cancelled and other won’t even get a chance to start because the cancellation token was signalled before the task gets a chance to start.
Here is the code for the cancellation example shown in the talk
class Program
{
static void Main(string[] args)
{
const int numTasks = 9;
// Set up the cancellation source and get the token.
CancellationTokenSource tokenSource = new CancellationTokenSource();
CancellationToken token = tokenSource.Token;
// Set up the tasks
Task[] tasks = new Task[numTasks];
for (int i = 0; i < numTasks; i++)
tasks[i] = Task.Factory.StartNew(() => PerformTask(token), token);
// Now the tasks are all set up, show the state.
// Most will be WaitingToRun, some will be Running
foreach (Task t in tasks.OrderBy(t => t.Id))
Console.WriteLine("Tasks {0} state: {1}", t.Id, t.Status);
// Give some of the tasks a chance to do something.
Thread.Sleep(1500);
// Cancel the tasks
Console.WriteLine("Cancelling tasks");
tokenSource.Cancel();
Console.WriteLine("Cancellation Signalled");
try
{
// Wait for the tasks to cancel if they've not already completed
Task.WaitAll(tasks);
}
catch (AggregateException aex)
{
aex.Handle(ex =>
{
// Handle the cancelled tasks
TaskCanceledException tcex = ex as TaskCanceledException;
if (tcex != null)
{
Console.WriteLine("Handling cancellation of task {0}", tcex.Task.Id);
return true;
}
// Not handling any other types of exception.
return false;
});
}
// Show the state of each of the tasks.
// Some will be RanToCompletion, others will be Cancelled.
foreach(Task t in tasks.OrderBy(t => t.Id))
Console.WriteLine("Tasks {0} state: {1}", t.Id, t.Status);
Console.WriteLine("Program End");
Console.ReadLine();
}
static void PerformTask(CancellationToken token)
{
try
{
// The loop simulates work that can be cooperatively cancelled.
Console.WriteLine("Task {0}: Starting", Task.CurrentId);
for (int i = 0; i < 4; i++)
{
// Check for the cancellation to be signalled
token.ThrowIfCancellationRequested();
// Write out a little bit showing the progress of the task
Console.WriteLine("Task {0}: {1}/4 In progress", Task.CurrentId, i + 1);
Thread.Sleep(500); // Simulate doing some work
}
// By getting here the task will RunToCompletion even if the
// token has been signalled.
Console.WriteLine("Task {0}: Finished", Task.CurrentId);
}
catch (OperationCanceledException)
{
// Any clean up code goes here.
Console.WriteLine("Task {0}: Cancelling", Task.CurrentId);
throw; // To ensure that the calling code knows the task was cancelled.
}
catch(Exception)
{
// Clean up other stuff
throw; // If the calling code also needs to know.
}
}
}
Here is the output of the program (your results may vary):
Task 1: Starting Task 1: 1/4 In progress Task 2: Starting Task 2: 1/4 In progress Tasks 1 state: Running Task 3: Starting Task 3: 1/4 In progress Tasks 2 state: Running Task 4: Starting Task 4: 1/4 In progress Tasks 3 state: Running Tasks 4 state: Running Tasks 5 state: WaitingToRun Tasks 6 state: WaitingToRun Tasks 7 state: WaitingToRun Tasks 8 state: WaitingToRun Tasks 9 state: WaitingToRun Task 1: 2/4 In progress Task 2: 2/4 In progress Task 3: 2/4 In progress Task 4: 2/4 In progress Task 1: 3/4 In progress Task 2: 3/4 In progress Task 4: 3/4 In progress Task 3: 3/4 In progress Task 1: 4/4 In progress Task 2: 4/4 In progress Task 4: 4/4 In progress Task 3: 4/4 In progress Task 5: Starting Task 5: 1/4 In progress
To this point the tasks have been given a chance to operate normally. The tasks that have started are outputing to the console their progress. The main thread reports on the state of the tasks and shows tasks 1 to 4 are Running while the remainder are WaitingToRun. After a while the scheduler decides to start task 5.
Next the tasks are going to be cancelled.
Cancelling tasks Cancellation Signalled Task 1: Finished Task 2: Finished Task 4: Finished Task 3: Finished Task 5: Cancelling
When the cancellation token is signalled the tasks have to cooperate. Tasks 1 to 4 are too far gone and will run to completion. Task 5, which was only just started, cooperates with the cancellation request and writes that it is cancelling. No waiting tasks are started.
In the main thread, the control is blocked until all the tasks have either finished or cooperate with the cancellation request. Once the WaitAll unblocks the program handles any cancelled tasks in the catch block.
Handling cancellation of task 9 Handling cancellation of task 8 Handling cancellation of task 7 Handling cancellation of task 6 Handling cancellation of task 5
Tasks 6 to 9 never got a chance to start. Task 5 was started, but was cancelled. Therefore task 5’s cancellation can be handled inside the task and outside it. Different clean up may be required in each place.
Finally, the program lists the end state (See also: Task state transitions) of each of the tasks:
Tasks 1 state: RanToCompletion Tasks 2 state: RanToCompletion Tasks 3 state: RanToCompletion Tasks 4 state: RanToCompletion Tasks 5 state: Canceled Tasks 6 state: Canceled Tasks 7 state: Canceled Tasks 8 state: Canceled Tasks 9 state: Canceled Program End
When writing code to handle cancelled tasks, watch out for this gotcha that can trip you up if you are not careful.
