How to Tame “async void” in C#

(This was originally posted on my blog as a full article)

You’re an intermediate dotnet programmer and you mostly know your way around using Tasks. You sprinkle async and await through your code, and everything is working just as expected. You’ve heard time and time again that you always want the return types of your asynchronous methods to be a Task (or Task) and that async void is essentially the root of all evil. No sweat.

One day you go to wire up an event handler using the syntax myObject.SomeEvent += SomeEventHandler, and your event handler needs to await some asynchronous code. You take all of the right steps and change your method signature to get that beautiful async Task added in, replacing void. But suddenly you get a compile error about your event handler not being compatible.

You feel trapped. You’re scared. And then you do the unspeakable…

You change your method signature for your event handler to async void and suddenly all of your compilation problems disappear right before your eyes. You hear the voices of all of the legendary dotnet programmers you look up to echoing in your mind: “What are you doing?! You cannot commit such a heinous coding crime!”. But it’s too late. You’ve succumbed to the powers of async void and all of your problems have appeared to vanish.

That is, until one day it all falls apart. And that’s when you did a search on the Internet and found this article.

Welcome, friend.

What’s Actually Wrong With async void?

Let’s start here with the basics. Here are a few of dangers of using async void in your C# code:

Exceptions thrown in an “async void” method cannot be caught in the same way as they can in an “async Task” method. When an exception is thrown in an “async void” method, it will be raised on the Synchronization Context, which can cause the application to crash.

Because “async void” methods cannot be awaited, they can lead to confusing and hard-to-debug code. For example, if an “async void” method is called multiple times, it can lead to multiple instances of the method running concurrently, which can cause race conditions and other unexpected behavior.

I’d be willing to bet you’re here because of the first point. Your application is experiencing weird crashes, and you’re having a heck of a time diagnosing the issue and getting traces. We are unable to (traditionally) wrap calls to async void methods in try/catch blocks and actually have them work as we’d expect.

I wanted to demonstrate this with a simple bit of code that you can literally try out in your browser. Let’s discuss the code below (which, by the way, is using the top level statements feature of .NET 7.0 so if you’re not used to seeing a C# program without a static void main… don’t be shocked!):

using System;
using System.Threading;
using System.Threading.Tasks;

Console.WriteLine("Start");
try
{
    // NOTE: uncomment the single line for each one of the scenarios below one at a time to try it out!

    // Scenario 1: we can await an async Task which allows us to catch exceptions
    //await AsyncTask();

    // Scenario 2: we cannot await an async void and as a result we cannot catch the exception
    //AsyncVoid();

    // Scenario 3: we purposefully wrap the async void in a Task (which we can await), but it still blows up
    //await Task.Run(AsyncVoid);
}
catch (Exception ex)
{
    Console.WriteLine("Look! We caught the exception here!");
    Console.WriteLine(ex);
}
Console.WriteLine("End");

async void AsyncVoid()
{
    Console.WriteLine("Entering async void method");
    await AsyncTask();

    // Pretend there's some super critical code right here
    // ...

    Console.WriteLine("Leaving async void method.");
}

async Task AsyncTask()
{
    Console.WriteLine("Entering async Task method");
    Console.WriteLine("About to throw...");
    throw new Exception("The expected exception");
}

As the code above shows, we have three examples scenarios to look at. And seriously, jump over to the link and try them each out by uncommenting one of those lines at a time.

Scenario 1

In scenario 1, we see our old faithful friend async Task. This task is going to throw an exception when we run the program, and since we are awaiting an async Task, the wrapping try/catch block is able to catch the exception just as we’d expect.

Scenario 2

In scenario 2, this might seem like some code that brought you here. The dreaded async void. When you run this one, you’ll notice that we print information that we’re about to throw the exception… But then we never see the indication that we’re leaving the async void method! In fact, we just see the line indicating the program has ended. How spooky is that?

Scenario 3

In scenario 3, this might look like an attempt you have tried to solve your async void woes. Surely if we wrap the async void in a Task, we can await that, and then we’ll go back to being safe… right? Right?! No. In fact, in .NET Fiddle you’ll actually see that it prints “Unhandled exception”. That’s not something we have in our code, that’s something scarier firing off AFTER our program is complete.

What’s Actually Actually The Problem?

Fundamentally, no matter how you try massage code around, if you have an event handler the return type of it must be void to hook up to a C# event using the + operator.

Period.

This means that even if you try to refactor all of your code to avoid this, you’re only “fixing” the issue by running away from it. And yes, while I agree if possible you should try and write code that doesn’t trap you into crappy patterns, sometimes curiosity wins out.

Is there a way that we could still have our event handlers as async void (so that we can await things inside of them)? Yes, of course! And the added bonus of the method that we’re going to dive into is that it’s not restricted to just event handlers. You can leverage this pattern on any old async void method you have laying around.

Do I recommend it? Absolutely not. I often find myself designing objects with events on them and I try to restrict my usage to this exact scenario. You know, the one that all of the C# gurus say “Fine, if you MUST use async void then… Make sure it’s for an event handler on a button or something”.

So let’s look at how we can make that a less painful experience going forward.

Waking Up From The Nightmare

You’ve been patient enough, so let’s dive right into it. If you want to follow along with a working demo right in your browser, check out this link.

Here’s the demo code, which we’ll cover right after:

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.ExceptionServices;

var someEventRaisingObject = new SomeEventRaisingObject();

// notice the async void. BEHOLD!!
someEventRaisingObject.TheEvent += async (s, e) =>
{
    Console.WriteLine("Entering the event handler...");
    await TaskThatIsReallyImportantAsync();
    Console.WriteLine("Exiting the event handler...");
};

try
{
    // forcefully fire our event!
    await someEventRaisingObject.RaiseEventAsync();
}
catch (Exception ex)
{
    Console.WriteLine($"Caught an exception in our handler: {ex.Message}");
}

// just block so we can wait for async operations to complete
Console.WriteLine("Press Enter to exit");
Console.ReadLine();

async Task TaskThatIsReallyImportantAsync()
{
    // switch between these two lines (comment one or the other out) to play with the behavior
    throw new Exception("This is an expected exception");
    //await Task.Run(() => Console.WriteLine("Look at us writing to the console from a task!"));
}

class SomeEventRaisingObject
{
    // you could in theory have your own event args here
    public event EventHandler<EventArgs> TheEvent;

    public Task RaiseEventAsync()
    {
        // the old way (if you toggle this way with the exception throwing, it will not hit our handler!)
        //TheEvent?.Invoke(this, EventArgs.Empty);
        //return Task.CompletedTask;

        // the new way (if you toggle this way with the exception throwing, it WILL hit our handler!)
        return InvokeAsync(TheEvent, true, true, this, EventArgs.Empty);
    }

    internal static async Task InvokeAsync(
            MulticastDelegate multicastDelegate,
            bool forceOrdering,
            bool stopOnFirstError,
            params object[] args)
        {
            if (multicastDelegate is null)
            {
                return;
            }

            var tcs = new TaskCompletionSource<bool>();
            var delegates = multicastDelegate.GetInvocationList();
            var count = delegates.Length;

            // keep track of exceptions along the way and a separate collection
            // for exceptions we have assigned to the TCS
            var assignedExceptions = new List<Exception>();
            var trackedExceptions = new ConcurrentQueue<Exception>();

            foreach (var @delegate in multicastDelegate.GetInvocationList())
            {
                var async = @delegate.Method
                    .GetCustomAttributes(typeof(AsyncStateMachineAttribute), false)
                    .Any();

                bool waitFlag = false;
                var completed = new Action(() =>
                {
                    if (Interlocked.Decrement(ref count) == 0)
                    {
                        lock (tcs)
                        {
                            assignedExceptions.AddRange(trackedExceptions);
                            if (!trackedExceptions.Any())
                            {
                                tcs.SetResult(true);
                            }
                            else if (trackedExceptions.Count == 1)
                            {
                                tcs.SetException(assignedExceptions[0]);
                            }
                            else
                            {
                                tcs.SetException(new AggregateException(assignedExceptions));
                            }
                        }
                    }

                    waitFlag = true;
                });
                var failed = new Action<Exception>(e =>
                {
                    trackedExceptions.Enqueue(e);
                });

                if (async)
                {
                    var context = new EventHandlerSynchronizationContext(completed, failed);
                    SynchronizationContext.SetSynchronizationContext(context);
                }

                try
                {
                    @delegate.DynamicInvoke(args);
                }
                catch (TargetParameterCountException e)
                {
                    throw;
                }
                catch (TargetInvocationException e) when (e.InnerException != null)
                {
                    // When exception occured inside Delegate.Invoke method all exceptions are wrapped in
                    // TargetInvocationException.
                    failed(e.InnerException);
                }
                catch (Exception e)
                {
                    failed(e);
                }

                if (!async)
                {
                    completed();
                }

                while (forceOrdering && !waitFlag)
                {
                    await Task.Yield();
                }

                if (stopOnFirstError && trackedExceptions.Any())
                {
                    lock (tcs)
                    {
                        if (!assignedExceptions.Any())
                        {
                            assignedExceptions.AddRange(trackedExceptions);
                            if (trackedExceptions.Count == 1)
                            {
                                tcs.SetException(assignedExceptions[0]);
                            }
                            else
                            {
                                tcs.SetException(new AggregateException(assignedExceptions));
                            }
                        }
                    }

                    break;
                }
            }

            await tcs.Task;
        }

        private class EventHandlerSynchronizationContext : SynchronizationContext
        {
            private readonly Action _completed;
            private readonly Action<Exception> _failed;

            public EventHandlerSynchronizationContext(
                Action completed,
                Action<Exception> failed)
            {
                _completed = completed;
                _failed = failed;
            }

            public override SynchronizationContext CreateCopy()
            {
                return new EventHandlerSynchronizationContext(
                    _completed,
                    _failed);
            }

            public override void Post(SendOrPostCallback d, object state)
            {
                if (state is ExceptionDispatchInfo edi)
                {
                    _failed(edi.SourceException);
                }
                else
                {
                    base.Post(d, state);
                }
            }

            public override void Send(SendOrPostCallback d, object state)
            {
                if (state is ExceptionDispatchInfo edi)
                {
                    _failed(edi.SourceException);
                }
                else
                {
                    base.Send(d, state);
                }
            }

            public override void OperationCompleted() => _completed();
        }
}

Understanding The Scenarios

There are two classes of things that we can play with in this code snippet:

• Toggle between what TaskThatIsReallyImportantAsync does. You can either safely print to the console, or have it throw an exception so you can experiment with the happy vs bad path. This alone isn’t the focus of the article, but it allows you to try different situations.
• Toggle the behavior of RaiseEventAsync, which will show you the not-so-great behavior of async void compared to our solution! Mix this with the previous option to see how we can improve our ability to catch exceptions.

How It Works

This solution is something I heavily borrowed from Oleg Karasik. In fact, his original article does such a fantastic job explaining how he built out the algorithm feature at a time. So I give full credit to him because without his post, I never would have put together what I am showing here.

The main take aways regarding how this works are that:

• We can use a custom synchronization context, specifically for handling events, that take a custom completed/failed callback.
• We can use reflection to check for AsyncStateMachineAttribute to see if our handler is truly marked as async or not.
• GetInvocationList allows us to get the entire chain of handled registered to an event.
• DynamicInvoke allows us to invoke the delegate without the compiler screaming about types.

When you combine the above, we can also layer in a couple of other features:

• forceOrdering: This boolean flag can force each handler to run in the order that they were registered or if it’s disabled, the handlers can run asynchronously with each other.
• stopOnFirstError: This boolean flag can prevent the code from firing off the subsequent event handlers if one of them raises an exception.

In the end, we are able to set information on a TaskCompletionSource instance that indicates completion or an exception being tracked.

What’s Next?

If you’d like to see how I expanded upon some of this code for my own projects, please continue reading the full original article. I hope this article serves as a reminder to folks that when we hear “It can’t be done” or “Never do X” or “Always do Y”, we shouldn’t always stop in our tracks without trying to understand further. When we have constraints, this is when we come up with the most creative solutions!