Monitoring a synchronous method for timeout

Question

I'm looking for an efficient way to throw a timeout exception if a synchronous method takes too long to execute. I've seen some samples but nothing that quite does what I want.

What I need to do is

1. Check that the sync method does exceed its SLA
2. If it does throw a timeout exception

I do not have to terminate the sync method if it executes for too long. (Multiple failures will trip a circuit breaker and prevent cascading failure)

My solution so far is show below. Note that I do pass a CancellationToken to the sync method in the hope that it will honor a cancellation request on timeout. Also my solution returns a task that can then be awaited on etc as desired by my calling code.

My concern is that this code creates two tasks per method being monitoring. I think the TPL will manage this well, but I would like to confirm.

Does this make sense? Is there a better way to do this?

private Task TimeoutSyncMethod( Action<CancellationToken> syncAction, TimeSpan timeout )
{
  var cts = new CancellationTokenSource();

  var outer = Task.Run( () =>
  {
     try
     {
        //Start the synchronous method - passing it a cancellation token
        var inner = Task.Run( () => syncAction( cts.Token ), cts.Token );

        if( !inner.Wait( timeout ) )
        {
            //Try give the sync method a chance to abort grecefully
            cts.Cancel();
            //There was a timeout regardless of what the sync method does - so throw
            throw new TimeoutException( "Timeout waiting for method after " + timeout );
        }
     }
     finally
     {
        cts.Dispose();
     }
  }, cts.Token );

  return outer;
}

---

Edit:

Using @Timothy's answer I'm now using this. While not significantly less code it is a lot clearer. Thanks!

  private Task TimeoutSyncMethod( Action<CancellationToken> syncAction, TimeSpan timeout )
  {
    var cts = new CancellationTokenSource();

    var inner = Task.Run( () => syncAction( cts.Token ), cts.Token );
    var delay = Task.Delay( timeout, cts.Token );

    var timeoutTask = Task.WhenAny( inner, delay ).ContinueWith( t => 
      {
        try
        {
          if( !inner.IsCompleted )
          {
            cts.Cancel();
            throw new TimeoutException( "Timeout waiting for method after " + timeout );
          }
        }
        finally
        {
          cts.Dispose();
        }
      }, cts.Token );

    return timeoutTask;
  }

Answer 1

I have re-written this solution for .NET 4.0 where some methods are not available e.g.Delay. This version is monitoring a method which returns object. How to implement Delay in .NET 4.0 comes from here: How to put a task to sleep (or delay) in C# 4.0?

public class OperationWithTimeout
{
    public Task<object> Execute(Func<CancellationToken, object> operation, TimeSpan timeout)
    {
        var cancellationToken = new CancellationTokenSource();

        // Two tasks are created. 
        // One which starts the requested operation and second which starts Timer. 
        // Timer is set to AutoReset = false so it runs only once after given 'delayTime'. 
        // When this 'delayTime' has elapsed then TaskCompletionSource.TrySetResult() method is executed. 
        // This method attempts to transition the 'delayTask' into the RanToCompletion state.
        Task<object> operationTask = Task<object>.Factory.StartNew(() => operation(cancellationToken.Token), cancellationToken.Token);
        Task delayTask = Delay(timeout.TotalMilliseconds);

        // Then WaitAny() waits for any of the provided task objects to complete execution.
        Task[] tasks = new Task[]{operationTask, delayTask};
        Task.WaitAny(tasks);

        try
        {
            if (!operationTask.IsCompleted)
            {
                // If operation task didn't finish within given timeout call Cancel() on token and throw 'TimeoutException' exception.
                // If Cancel() was called then in the operation itself the property 'IsCancellationRequested' will be equal to 'true'.
                cancellationToken.Cancel();
                throw new TimeoutException("Timeout waiting for method after " + timeout + ". Method was to slow :-)");
            }
        }
        finally
        {
            cancellationToken.Dispose();
        }

        return operationTask;
    }

    public static Task Delay(double delayTime)
    {
        var completionSource = new TaskCompletionSource<bool>();
        Timer timer = new Timer();
        timer.Elapsed += (obj, args) => completionSource.TrySetResult(true);
        timer.Interval = delayTime;
        timer.AutoReset = false;
        timer.Start();
        return completionSource.Task;
    }
}

How to use it then in Console app.

    public static void Main(string[] args)
    {
        var operationWithTimeout = new OperationWithTimeout();
        TimeSpan timeout = TimeSpan.FromMilliseconds(10000);

        Func<CancellationToken, object> operation = token =>
        {
            Thread.Sleep(9000); // 12000

            if (token.IsCancellationRequested)
            {
                Console.Write("Operation was cancelled.");
                return null;
            }

            return 123456;
        };

        try
        {
            var t = operationWithTimeout.Execute(operation, timeout);
            var result = t.Result;
            Console.WriteLine("Operation returned '" + result + "'");
        }
        catch (TimeoutException tex)
        {
            Console.WriteLine(tex.Message);
        }

        Console.WriteLine("Press enter to exit");
        Console.ReadLine();
    }

Answer 2

If you have a Task called task, you can do this:

var delay = Task.Delay(TimeSpan.FromSeconds(3));
var timeoutTask = Task.WhenAny(task, delay);

If timeoutTask.Result ends up being task, then it didn't timeout. Otherwise, it's delay and it did timeout.

I don't know if this is going to behave identically to what you have implemented, but it's the built-in way to do this.

Answer 3

Jasper's answer got me most of the way, but I specifically wanted a void function to call a non-task synchronous method with a timeout. Here's what I ended up with:

public static void RunWithTimeout(Action action, TimeSpan timeout)
{
    var task = Task.Run(action);
    try
    {
        var success = task.Wait(timeout);
        if (!success)
        {
            throw new TimeoutException();
        }
    }
    catch (AggregateException ex)
    {
        throw ex.InnerException;
    }
}

Call it like:

RunWithTimeout(() => File.Copy(..), TimeSpan.FromSeconds(3));

Answer 4

To elabolate on Timothy Shields clean solution:

        if (task == await Task.WhenAny(task, Task.Delay(TimeSpan.FromSeconds(3))))
        {
            return await task;
        }
        else
            throw new TimeoutException();

This solution, I found, will also handle the case where the Task has a return value - i.e:

async Task<T>

More to be found here: MSDN: Crafting a Task.TimeoutAfter Method