Sequential processing of asynchronous tasks

Question

Assume the following synchronous code:

try
{
    Foo();
    Bar();
    Fubar();
    Console.WriteLine("All done");
}
catch(Exception e) // For illustration purposes only. Catch specific exceptions!
{
    Console.WriteLine(e);
}

Now assume all these methods have an Async counterpart and I have to use those for some reason, so simply wrapping the whole thing in a new task is not an option.
How would I achieve the same behavior?
What I mean with "same" is:

1. Execute a handler for the exception, if one is thrown.
2. Stop execution of the following methods, if an exception is thrown.

The only thing I was able to come up with is horrible:

var fooTask = FooAsync();
fooTask.ContinueWith(t => HandleError(t.Exception),
                     TaskContinuationOptions.OnlyOnFaulted);
fooTask.ContinueWith(
    t =>
    {
        var barTask = BarAsync();
        barTask.ContinueWith(t => HandleError(t.Exception),
                             TaskContinuationOptions.OnlyOnFaulted);
        barTask.ContinueWith(
            t =>
            {
                var fubarTask = FubarAsync();
                fubarTask.ContinueWith(t => HandleError(t.Exception),
                                       TaskContinuationOptions.OnlyOnFaulted);
                fubarTask.ContinueWith(
                    t => Console.WriteLine("All done"),
                    TaskContinuationOptions.OnlyOnRanToCompletion);
            }, 
            TaskContinuationOptions.OnlyOnRanToCompletion);
    }, 
    TaskContinuationOptions.OnlyOnRanToCompletion);

Please note:

• I need a solution that works with .NET 4, so async/await is out of the question. However, if it would work with async/await feel free to show how.
• I don't need to use the TPL. If it is impossible with the TPL another approach would be OK, maybe with Reactive Extensions?

Solution 1

Here's how it would work with async:

try
{
    await FooAsync();
    await BarAsync();
    await FubarAsync();
    Console.WriteLine("All done");
}
catch(Exception e) // For illustration purposes only. Catch specific exceptions!
{
    Console.WriteLine(e);
}

This would work on .NET 4.0 if you installed the (prerelease) Microsoft.Bcl.Async package.

---

Since you're stuck on VS2010, you can use a variant of Stephen Toub's Then:

public static Task Then(this Task first, Func<Task> next)
{
  var tcs = new TaskCompletionSource<object>();
  first.ContinueWith(_ =>
  {
    if (first.IsFaulted) tcs.TrySetException(first.Exception.InnerExceptions);
    else if (first.IsCanceled) tcs.TrySetCanceled();
    else
    {
      try
      {
        next().ContinueWith(t =>
        {
          if (t.IsFaulted) tcs.TrySetException(t.Exception.InnerExceptions);
          else if (t.IsCanceled) tcs.TrySetCanceled();
          else tcs.TrySetResult(null);
        }, TaskContinuationOptions.ExecuteSynchronously);
      }
      catch (Exception exc) { tcs.TrySetException(exc); }
    }
  }, TaskContinuationOptions.ExecuteSynchronously);
  return tcs.Task; 
}

You can use it as such:

var task = FooAsync().Then(() => BarAsync()).Then(() => FubarAsync());
task.ContinueWith(t =>
{
  if (t.IsFaulted || t.IsCanceled)
  {
    var e = t.Exception.InnerException;
    // exception handling
  }
  else
  {
    Console.WriteLine("All done");
  }
}, TaskContinuationOptions.ExcecuteSynchronously);

---

Using Rx, it would look like this (assuming you don't have the async methods already exposed as IObservable<Unit>):

FooAsync().ToObservable()
    .SelectMany(_ => BarAsync().ToObservable())
    .SelectMany(_ => FubarAsync().ToObservable())
    .Subscribe(_ => { Console.WriteLine("All done"); },
        e => { Console.WriteLine(e); });

I think. I'm not an Rx master, by any means. :)

Solution 2

Just for the sake of completeness, that's how I would implement the helper method suggested by Chris Sinclair:

public void RunSequential(Action onComplete, Action<Exception> errorHandler,
                          params Func<Task>[] actions)
{
    RunSequential(onComplete, errorHandler,
                  actions.AsEnumerable().GetEnumerator());
}

public void RunSequential(Action onComplete, Action<Exception> errorHandler,
                          IEnumerator<Func<Task>> actions)
{
    if(!actions.MoveNext())
    {
        onComplete();
        return;
    }

    var task = actions.Current();
    task.ContinueWith(t => errorHandler(t.Exception),
                      TaskContinuationOptions.OnlyOnFaulted);
    task.ContinueWith(t => RunSequential(onComplete, errorHandler, actions),
                      TaskContinuationOptions.OnlyOnRanToCompletion);
}

This ensures that each subsequent task is only requested when the previous one completed successfully.
It assumes that the Func<Task> returns an already running task.

Solution 3

What you have here is essentially a ForEachAsync. You want to run each async item, sequentially, but with some error handling support. Here is one such implementation:

public static Task ForEachAsync(IEnumerable<Func<Task>> tasks)
{
    var tcs = new TaskCompletionSource<bool>();

    Task currentTask = Task.FromResult(false);

    foreach (Func<Task> function in tasks)
    {
        currentTask.ContinueWith(t => tcs.TrySetException(t.Exception.InnerExceptions)
            , TaskContinuationOptions.OnlyOnFaulted);
        currentTask.ContinueWith(t => tcs.TrySetCanceled()
                , TaskContinuationOptions.OnlyOnCanceled);
        Task<Task> continuation = currentTask.ContinueWith(t => function()
            , TaskContinuationOptions.OnlyOnRanToCompletion);
        currentTask = continuation.Unwrap();
    }

    currentTask.ContinueWith(t => tcs.TrySetException(t.Exception.InnerExceptions)
            , TaskContinuationOptions.OnlyOnFaulted);
    currentTask.ContinueWith(t => tcs.TrySetCanceled()
            , TaskContinuationOptions.OnlyOnCanceled);
    currentTask.ContinueWith(t => tcs.TrySetResult(true)
            , TaskContinuationOptions.OnlyOnRanToCompletion);

    return tcs.Task;
}

I added in support for canceled tasks as well, just to be more general and because it took so little to do.

It adds each task as a continuation of the previous task, and all along the line it ensures that any exceptions result in the final task's exception being set.

Here is an example usage:

public static Task FooAsync()
{
    Console.WriteLine("Started Foo");
    return Task.Delay(1000)
        .ContinueWith(t => Console.WriteLine("Finished Foo"));
}

public static Task BarAsync()
{
    return Task.Factory.StartNew(() => { throw new Exception(); });
}

private static void Main(string[] args)
{
    List<Func<Task>> list = new List<Func<Task>>();

    list.Add(() => FooAsync());
    list.Add(() => FooAsync());
    list.Add(() => FooAsync());
    list.Add(() => FooAsync());
    list.Add(() => BarAsync());

    Task task = ForEachAsync(list);

    task.ContinueWith(t => Console.WriteLine(t.Exception.ToString())
        , TaskContinuationOptions.OnlyOnFaulted);
    task.ContinueWith(t => Console.WriteLine("Done!")
        , TaskContinuationOptions.OnlyOnRanToCompletion);
}

Solution 4

You should be able to create a method to combine two tasks, and only start the second if the first succeeds.

public static Task Then(this Task parent, Task next)
{
    TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
    parent.ContinueWith(pt =>
    {
        if (pt.IsFaulted)
        {
            tcs.SetException(pt.Exception.InnerException);
        }
        else
        {
            next.ContinueWith(nt =>
            {
                if (nt.IsFaulted)
                {
                    tcs.SetException(nt.Exception.InnerException);
                }
                else { tcs.SetResult(null); }
            });
            next.Start();
        }
    });
    return tcs.Task;
}

you can then chain tasks together:

Task outer = FooAsync()
    .Then(BarAsync())
    .Then(FubarAsync());

outer.ContinueWith(t => {
    if(t.IsFaulted) {
        //handle exception
    }
});

If your tasks are started immediately you can just wrap them in a Func:

public static Task Then(this Task parent, Func<Task> nextFunc)
{
    TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
    parent.ContinueWith(pt =>
    {
        if (pt.IsFaulted)
        {
            tcs.SetException(pt.Exception.InnerException);
        }
        else
        {
            Task next = nextFunc();
            next.ContinueWith(nt =>
            {
                if (nt.IsFaulted)
                {
                    tcs.SetException(nt.Exception.InnerException);
                }
                else { tcs.SetResult(null); }
            });
        }
    });
    return tcs.Task;
}

Solution 5

Now, I haven't really used the TPL much, so this is just a stab in the dark. And given what @Servy mentioned, perhaps this won't run completely asynchronously. But I figured I'd post it and if it's way off the mark, you can downvote me to oblivion or I can have it deleted (or we can just fix what needs fixing)

public void RunAsync(Action onComplete, Action<Exception> errorHandler, params Action[] actions)
{
    if (actions.Length == 0)
    {
        //what to do when no actions/tasks provided?
        onComplete();
        return;
    }

    List<Task> tasks = new List<Task>(actions.Length);
    foreach(var action in actions)
    {
        Task task = new Task(action);
        task.ContinueWith(t => errorHandler(t.Exception), TaskContinuationOptions.OnlyOnFaulted);
        tasks.Add(task);
    }

    //last task calls onComplete
    tasks[actions.Length - 1].ContinueWith(t => onComplete(), TaskContinuationOptions.OnlyOnRanToCompletion);

    //wire all tasks to execute the next one, except of course, the last task
    for (int i = 0; i <= actions.Length - 2; i++)
    {
        var nextTask = tasks[i + 1];
        tasks[i].ContinueWith(t => nextTask.Start(), TaskContinuationOptions.OnlyOnRanToCompletion);
    }

    tasks[0].Start();
}

And it would have usage like:

RunAsync(() => Console.WriteLine("All done"),
            ex => Console.WriteLine(ex),
            Foo,
            Bar,
            Fubar);

Thoughts? Downvotes? :)

(I definitely prefer async/await though)

EDIT: Based on your comments to take Func<Task>, would this be a proper implementation?

public void RunAsync(Action onComplete, Action<Exception> errorHandler, params Func<Task>[] actions)
{
    if (actions.Length == 0)
    {
        //what to do when no actions/tasks provided?
        onComplete();
        return;
    }

    List<Task> tasks = new List<Task>(actions.Length);
    foreach (var action in actions)
    {
        Func<Task> nextActionFunc = action;
        Task task = new Task(() =>
        {
            var nextTask = nextActionFunc();
            nextTask.ContinueWith(t => errorHandler(t.Exception), TaskContinuationOptions.OnlyOnFaulted);
            nextTask.Start();
        });
        tasks.Add(task);
    }

    //last task calls onComplete
    tasks[actions.Length - 1].ContinueWith(t => onComplete(), TaskContinuationOptions.OnlyOnRanToCompletion);

    //wire all tasks to execute the next one, except of course, the last task
    for (int i = 0; i <= actions.Length - 2; i++)
    {
        var nextTask = tasks[i + 1];
        tasks[i].ContinueWith(t => nextTask.Start(), TaskContinuationOptions.OnlyOnRanToCompletion);
    }

    tasks[0].Start();
}

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