How can I allow Task exceptions to propagate back to the UI thread?

Question

In TPL, if an exception is thrown by a Task, that exception is captured and stored in Task.Exception, and then follows all the rules on observed exceptions. If it's never observed, it's eventually rethrown on the finalizer thread and crashes the process.

Is there a way to prevent the Task from catching that exception, and just letting it propagate instead?

The Task I'm interested in would already be running on the UI thread (courtesy of TaskScheduler.FromCurrentSynchronizationContext), and I want the exception to escape so it can be handled by my existing Application.ThreadException handler.

I basically want unhandled exceptions in the Task to behave like unhandled exceptions in a button-click handler: immediately propagate on the UI thread, and be handled by ThreadException.

Answer 1

I found a solution that works adequately some of the time.

Single task

var synchronizationContext = SynchronizationContext.Current;
var task = Task.Factory.StartNew(...);

task.ContinueWith(task =>
    synchronizationContext.Post(state => {
        if (!task.IsCanceled)
            task.Wait();
    }, null));

This schedules a call to task.Wait() on the UI thread. Since I don't do the Wait until I know the task is already done, it won't actually block; it will just check to see if there was an exception, and if so, it will throw. Since the SynchronizationContext.Post callback is executed straight from the message loop (outside the context of a Task), the TPL won't stop the exception, and it can propagate normally -- just as if it was an unhandled exception in a button-click handler.

One extra wrinkle is that I don't want to call WaitAll if the task was canceled. If you wait on a canceled task, TPL throws a TaskCanceledException, which it makes no sense to re-throw.

Multiple tasks

In my actual code, I have multiple tasks -- an initial task and multiple continuations. If any of those (potentially more than one) get an exception, I want to propagate an AggregateException back to the UI thread. Here's how to handle that:

var synchronizationContext = SynchronizationContext.Current;
var firstTask = Task.Factory.StartNew(...);
var secondTask = firstTask.ContinueWith(...);
var thirdTask = secondTask.ContinueWith(...);

Task.Factory.ContinueWhenAll(
    new[] { firstTask, secondTask, thirdTask },
    tasks => synchronizationContext.Post(state =>
        Task.WaitAll(tasks.Where(task => !task.IsCanceled).ToArray()), null));

Same story: once all the tasks have completed, call WaitAll outside the context of a Task. It won't block, since the tasks are already completed; it's just an easy way to throw an AggregateException if any of the tasks faulted.

At first I worried that, if one of the continuation tasks used something like TaskContinuationOptions.OnlyOnRanToCompletion, and the first task faulted, then the WaitAll call might hang (since the continuation task would never run, and I worried that WaitAll would block waiting for it to run). But it turns out the TPL designers were cleverer than that -- if the continuation task won't be run because of OnlyOn or NotOn flags, that continuation task transitions to the Canceled state, so it won't block the WaitAll.

Edit

When I use the multiple-tasks version, the WaitAll call throws an AggregateException, but that AggregateException doesn't make it through to the ThreadException handler: instead only one of its inner exceptions gets passed to ThreadException. So if multiple tasks threw exceptions, only one of them reaches the thread-exception handler. I'm not clear on why this is, but I'm trying to figure it out.

Answer 2

Ok Joe... as promised, here's how you can generically solve this problem with a custom TaskScheduler subclass. I've tested this implementation and it works like a charm. Don't forget you can't have the debugger attached if you want to see Application.ThreadException to actually fire!!!

The Custom TaskScheduler

This custom TaskScheduler implementation gets tied to a specific SynchronizationContext at "birth" and will take each incoming Task that it needs to execute, chain a Continuation on to it that will only fire if the logical Task faults and, when that fires, it Posts back to the SynchronizationContext where it will throw the exception from the Task that faulted.

public sealed class SynchronizationContextFaultPropagatingTaskScheduler : TaskScheduler
{
    #region Fields

    private SynchronizationContext synchronizationContext;
    private ConcurrentQueue<Task> taskQueue = new ConcurrentQueue<Task>();

    #endregion

    #region Constructors

    public SynchronizationContextFaultPropagatingTaskScheduler() : this(SynchronizationContext.Current)
    {
    }

    public SynchronizationContextFaultPropagatingTaskScheduler(SynchronizationContext synchronizationContext)
    {
        this.synchronizationContext = synchronizationContext;
    }

    #endregion

    #region Base class overrides

    protected override void QueueTask(Task task)
    {
        // Add a continuation to the task that will only execute if faulted and then post the exception back to the synchronization context
        task.ContinueWith(antecedent =>
            {
                this.synchronizationContext.Post(sendState =>
                {
                    throw (Exception)sendState;
                },
                antecedent.Exception);
            },
            TaskContinuationOptions.OnlyOnFaulted | TaskContinuationOptions.ExecuteSynchronously);

        // Enqueue this task
        this.taskQueue.Enqueue(task);

        // Make sure we're processing all queued tasks
        this.EnsureTasksAreBeingExecuted();
    }

    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
    {
        // Excercise for the reader
        return false;
    }

    protected override IEnumerable<Task> GetScheduledTasks()
    {
        return this.taskQueue.ToArray();
    }

    #endregion

    #region Helper methods

    private void EnsureTasksAreBeingExecuted()
    {
        // Check if there's actually any tasks left at this point as it may have already been picked up by a previously executing thread pool thread (avoids queueing something up to the thread pool that will do nothing)
        if(this.taskQueue.Count > 0)
        {
            ThreadPool.UnsafeQueueUserWorkItem(_ =>
            {
                Task nextTask;

                // This thread pool thread will be used to drain the queue for as long as there are tasks in it
                while(this.taskQueue.TryDequeue(out nextTask))
                {
                    base.TryExecuteTask(nextTask);
                }
            },
            null);
        }
    }

    #endregion
}

Some notes/disclaimers on this implementation:

• If you use the parameterless constructor, it will pick up the current SynchronizationContext... so if you just construct this on a WinForms thread (main form constructor, whatever) and it will work automatically. Bonus, I also have a constructor where you can explicitly pass in the SynchronizationContext that you got from somewhere else.
• I have not provided an implementation of TryExecuteTaskInline so this implementation will just always just queue the Task to be worked on. I leave this as an excercise for the reader. It's not hard, just... not necessary to demonstrate the functionality you're asking for.
• I'm using a simple/primitive approach to scheduling/executing the Tasks that leverages the ThreadPool. There are definitely richer implementations to be had, but again the focus of this implementation is simply about marshaling exceptions back to the "Application" thread

Ok, now you have a couple options for using this TaskScheduler:

Pre-configure TaskFactory Instance

This approach allows you to setup a TaskFactory once and then any task you start with that factory instance will use the custom TaskScheduler. That would basically look something like this:

At application startup

private static readonly TaskFactory MyTaskFactory = new TaskFactory(new SynchronizationContextFaultPropagatingTaskScheduler());

Throughout code

MyTaskFactory.StartNew(_ =>
{
    // ... task impl here ...
});

Explicit TaskScheduler Per-Call

Another approach is to just create an instance of the custom TaskScheduler and then pass that into StartNew on the default TaskFactory every time you start a task.

At application startup

private static readonly SynchronizationContextFaultPropagatingTaskScheduler MyFaultPropagatingTaskScheduler = new SynchronizationContextFaultPropagatingTaskScheduler();

Throughout code

Task.Factory.StartNew(_ =>
{
    // ... task impl here ...
},
CancellationToken.None // your specific cancellationtoken here (if any)
TaskCreationOptions.None, // your proper options here
MyFaultPropagatingTaskScheduler);