Is there a simple way to return a task with an exception?

Question

My understanding is that return Task.FromResult(foo) is a simple shorthand for:

var tcs = new TaskCompletionSource<TFoo>();
tcs.SetResult(foo);
return tcs.Task;

Is there some equivalent for a task that returns an exception state?

var tcs = new TaskCompletionSource<TFoo>();
tcs.SetException(new NotSupportedException()); // or whatever is appropriate
return tcs.Task;

I don't see anything like Task.FromException. Or would it be more appropriate to just throw the exception without returning a task?

Looking at your code makes me want to select it and use "Extract Method" in ReSharper. — John Saunders, Mar 25 '14 at 00:50
It appears that such a method was intentionally not exposed: it exists, but is `internal`. — dlev, Mar 25 '14 at 00:50
The trouble with that approach is that the exception you return wouldn't have a stack trace, since it is never actually thrown. Perhaps that's why the FromException is not exposed. — Thomas Levesque, Mar 25 '14 at 01:14
@JohnSaunders - Sure, I can put it in a method, but I'm more interested if this is the right approach or not. — Matt Johnson-Pint, Mar 25 '14 at 02:53
@ThomasLevesque - Hmmm.. so should I throw it, catch it and then return it in the Task? I don't think it should just be thrown. Or should it? — Matt Johnson-Pint, Mar 25 '14 at 02:54
@MattJohnson, well, throwing it just to catch it doesn't seem a very good idea... I guess Noseratio's answer is a good solution — Thomas Levesque, Mar 25 '14 at 08:38

score 21 · Accepted Answer · answered Jun 03 '15 at 12:10

21

Starting with .NET 4.6 there is a Task.FromException method in the BCL.

There's also Task.FromCanceled.

answered Jun 03 '15 at 12:10

Drew Noakes

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noseratio · Answer 2 · 2014-04-03T06:07:58.633

My understanding is that return Task.FromResult(foo) is a simple shorthand for... [TaskCompletionSource.SetResult].

Actually, Task.FromResult doesn't use TaskCompletionSource, its implementation is much simpler than that.

Is there some equivalent for a task that returns an exception state?

I reckon TaskCompletionSource would be the best option for this. You could also do something like this:

static Task FromExAsync(Exception ex) 
{
    var task = new Task(() => { throw ex; });
    task.RunSynchronously();
    return task;
}

The exception will not be propagated outside the returned task, until observed via await task or task.Wait(), which should be a desired behavior.

Note that if the exception passed to FromExAsync is an active exception (i.e. has been already thrown and caught elsewhere), re-throwing it like this would loose the current stack trace and Watson buckets information stored inside the exception. There are two ways of dealing with it:

Wrap the exception with AggregateException. This will make the original exception available as AggregateException.InnerException:

static Task FromExAsync(Exception ex) 
{
    var task = new Task(() => { throw new AggregateException(ex); });
    task.RunSynchronously();
    return task;
}

Use ExceptionDispatchInfo.Capture to flow the active exception's state:

static Task FromExAsync(Exception ex) 
{
    var ei = System.Runtime.ExceptionServices.ExceptionDispatchInfo.Capture(ex);
    var task = new Task(() => { ei.Throw(); });
    task.RunSynchronously();
    return task;
}

Finally, perhaps the simplest yet the worst option (due to the overhead of the state machine and the compiler warning) is to throw from an async method:

static async Task FromExAsync(Exception ex)
{
    throw ex;
}

Given this talk of overhead, perhaps one could consider using a custom Awaiter? — Aron, Mar 25 '14 at 03:35
@Aron, I don't see how a custom awaiter would eliminate `async` and the task machine, as @MattJohnson needs a `Task` object for the return value. For some extra details, check [this](http://stackoverflow.com/q/22268569/1768303). — noseratio, Mar 25 '14 at 03:39
Thanks. Async programming is very powerful, but sometimes even the smallest things get confusing. — Matt Johnson-Pint, Mar 25 '14 at 04:26
Perhaps worth noting is that `Task` has an internal `FromException` function, and that it internally uses the exact same `Task.TrySetException` function that `TaskCompletionSource.TrySetException` also uses. That's a strong reason for me to use `TaskCompletionSource.TrySetException`: you get the exact result that MS thinks is a good idea for `Task.FromException`. — , Mar 12 '15 at 08:53
@hvd, there's new `Task.FromException` API in .NET 4.6: http://blogs.msdn.com/b/pfxteam/archive/2015/02/02/new-task-apis-in-net-4-6.aspx — noseratio, Mar 12 '15 at 11:58
@Noseratio Heh, nice, thanks. I was looking at .NET 4.5, which does already have `FromException`, it just isn't public yet. — , Mar 12 '15 at 12:04

Is there a simple way to return a task with an exception?

2 Answers2