Tasks with Timeouts – Contd.

As mentioned in the last post we can now have tasks with individual timeouts. The code looked a little heavy. Can we do better ? Yes we can !!


var tasks = new List<Task>();
try
{
//instead of checking for fault within the continutaion, 
//we can just use a TaskContinuationOption to tell communicate the right semantics

var t1 = Task.Factory.StartNew(_ => LongRunningTask(), TaskCreationOptions.AttachedToParent)
              .TimeoutAfter(1000)
              .ContinueWith(t => SomethingUsefulWithTheResult(), 
                                 TaskContinuationOptions.NotOnFaulted);

var t2 = Task.Factory.StartNew(_ => LongRunningTask(), TaskCreationOptions.AttachedToParent)
              .ContinueWith(t => SomethingUsefulWithTheResult());

var t3 = Task.Factory.StartNew(_ => LongRunningTask("Entering task3"), TaskCreationOptions.AttachedToParent)
              .ContinueWith(t => SomethingUsefulWithTheResult());
            
tasks.Add(t1);
tasks.Add(t2);
tasks.Add(t3);
                            
Task.WaitAll(tasks.ToArray());
}
catch (Exception ex)
{                
    Console.WriteLine("There was an exception");
    Console.WriteLine(ex.InnerException.Message);               
}

Tasks with Timeouts

So the task is to timeout a task. now, I never thought it would take me as long as it did. Turns out it is a really tricky problem problem. I was expecting something within the framework to make life easier, there isn’t anything by default but msdn to the rescue Tasks With Timeout.

So, the extension method is

    public static class TaskWithTimeout
    {
        internal struct VoidTypeStruct
        { }
        internal static void MarshalTaskResults<TResult>(Task source, TaskCompletionSource<TResult> proxy)
        {
            switch (source.Status)
            {
                case TaskStatus.Faulted:
                    proxy.TrySetException(source.Exception);
                    break;
                case TaskStatus.Canceled:
                    proxy.TrySetCanceled();
                    break;
                case TaskStatus.RanToCompletion:
                    Task<TResult> castedSource = source as Task<TResult>;
                    proxy.TrySetResult(
                        castedSource == null ? default(TResult) : // source is a Task
                            castedSource.Result); // source is a Task<TResult>
                    break;
            }
        }
        
        public static Task TimeoutAfter(this Task task, int millisecondsTimeout)
        {
            // Short-circuit #1: infinite timeout or task already completed
            if (task.IsCompleted || (millisecondsTimeout == Timeout.Infinite))
            {
                // Either the task has already completed or timeout will never occur.
                // No proxy necessary.
                return task;
            }

            // tcs.Task will be returned as a proxy to the caller
            TaskCompletionSource<VoidTypeStruct> tcs = new TaskCompletionSource<VoidTypeStruct>();

            // Short-circuit #2: zero timeout
            if (millisecondsTimeout == 0)
            {
                // We've already timed out.
                tcs.SetException(new TimeoutException());
                return tcs.Task;
            }

            // Set up a timer to complete after the specified timeout period
            Timer timer = new Timer(state =>
            {
                // Recover your state information
                var myTcs = (TaskCompletionSource<VoidTypeStruct>)state;
                // Fault our proxy with a TimeoutException
                myTcs.TrySetException(new TimeoutException());
            }, tcs, millisecondsTimeout, Timeout.Infinite);

            // Wire up the logic for what happens when source task completes
            task.ContinueWith(antecedent =>
                                {
                                    timer.Dispose(); // Cancel the timer
                                    MarshalTaskResults(antecedent, tcs); // Marshal results to proxy
                                },
                                CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);

            return tcs.Task;
        }

        public static Task<TResult> TimeoutAfter<TResult>(this Task<TResult> task, int millisecondsTimeout)
        {
            // Short-circuit #1: infinite timeout or task already completed
            if (task.IsCompleted || (millisecondsTimeout == Timeout.Infinite))
            {
                // Either the task has already completed or timeout will never occur.
                // No proxy necessary.
                return task;
            }

            // tcs.Task will be returned as a proxy to the caller
            TaskCompletionSource<TResult> tcs = new TaskCompletionSource<TResult>();

            // Short-circuit #2: zero timeout
            if (millisecondsTimeout == 0)
            {
                // We've already timed out.
                tcs.SetException(new TimeoutException());
                return tcs.Task;
            }

            // Set up a timer to complete after the specified timeout period
            Timer timer = new Timer(state =>
                                    {
                                        // Recover your state information
                                        var myTcs = (TaskCompletionSource<TResult>)state;
                                        // Fault our proxy with a TimeoutException
                                        myTcs.TrySetException(new TimeoutException());
                                    }, tcs, millisecondsTimeout, Timeout.Infinite);

            // Wire up the logic for what happens when source task completes
            task.ContinueWith(antecedent =>
                                {
                                    timer.Dispose(); // Cancel the timer
                                    MarshalTaskResults(antecedent, tcs); // Marshal results to proxy
                                }, 
                                CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously,TaskScheduler.Default);

            return tcs.Task;
        }
    }

A lot of code for doing this, and the msdn article remains the better source of explanation.

Now using this.

public class Program
	{
		private static List<int> Output = new List<int>();
		private static Random _random = new Random();
		
		public static int LongRunningTask(string message)
		{
			Console.WriteLine(message);
			Console.WriteLine("Managed thread Id " + Thread.CurrentThread.ManagedThreadId);
			//Simulate a long running task
            Thread.Sleep(TimeSpan.FromSeconds(2));
			var number = _random.Next();
			Console.WriteLine("Adding " + number + " From thread  - " + Thread.CurrentThread.ManagedThreadId);
			return number;
		}

		public static void Main(string[] args)
		{
			Console.WriteLine("In Main");
			Console.WriteLine("Managed thread Id " + Thread.CurrentThread.ManagedThreadId);
			var cts = new CancellationTokenSource();
			var tasks = new List<Task>();
			try
			{
//In the continuation check for the condition of fault (or something more if you so need) and perform the //continuation
				var t1 = Task.Factory.StartNew(_ =>	LongRunningTask("Entering task1"),
												    TaskCreationOptions.AttachedToParent)
                                     .TimeoutAfter(1000)
                                     .ContinueWith(antecedent => {
						if(!(antecedent.IsCanceled || antecedent.IsFaulted))
                                                         Output.Add(antecedent.Result);
								}
								, cts.Token);
				var t2 = Task.Factory.StartNew(_ => LongRunningTask("Entering task2"),
													TaskCreationOptions.AttachedToParent)
                                     .ContinueWith(_ => Output.Add(_.Result));
                var t3 = Task.Factory.StartNew(_ => LongRunningTask("Entering task3"), 
													TaskCreationOptions.AttachedToParent)
                                     .ContinueWith(_ => Output.Add(_.Result));
            
                tasks.Add(t1);
                tasks.Add(t2);
                tasks.Add(t3);
                            
                Task.WaitAll(tasks.ToArray());
            }
            catch (Exception ex)
            {                
                Console.WriteLine("There was an exception");
                Console.WriteLine(ex.InnerException.Message);               
            }

            Console.WriteLine("Output :");
            Output.ForEach(_ => Console.WriteLine(_));

            Console.ReadLine();
        }
    }

The important part being that the continuation is applied after the timeout and it won’t work with the other way around.
The output therefore basically looks like this :

In Main
Managed thread Id 9
Entering task1
Managed thread Id 10
Entering task2
Managed thread Id 11
Entering task3
Managed thread Id 14
Adding 194443354 From thread  - 10
Adding 792426557 From thread  - 11
Adding 230130793 From thread  - 14
Output :
792426557
230130793

Next, time I will try and write about some other things on Flirting with Tasks.