By avo


2013-10-16 23:21:03 8 Comments

I know it is considered generally a bad idea to use fire-and-forget async void methods to start tasks, because there is no track of the pending task and it is tricky to handle exceptions which might be thrown inside such a method.

Should I generally avoid async void event handlers, as well? For example,

private async void Form_Load(object sender, System.EventArgs e)
{
        await Task.Delay(2000); // do async work
        // ...
} 

I can rewrite it like this:

Task onFormLoadTask = null; // track the task, can implement cancellation

private void Form_Load(object sender, System.EventArgs e)
{
        this.onFormLoadTask = OnFormLoadTaskAsync(sender, e);
} 

private async Task OnFormLoadTaskAsync(object sender, System.EventArgs e)
{
        await Task.Delay(2000); // do async work
        // ...
} 

What are the underwater rocks for async event handlers, besides possible re-entrancy?

4 comments

@Alexander Zwitbaum 2016-01-31 00:34:10

If you use ReSharper, a free ReCommended Extension could be helpful for you. It analyzes the "async void" methods and highlights when used inappropriately. The extension can distinguish different usages of async void and provide appropriated quick fixes described here: ReCommended-Extension wiki.

@DotNet Dreamer 2014-09-07 06:34:31

Yes, generally async void of event handlers is the only case. If you want to know more about it you can check out a great video here at channel 9

The only case where this kind of fire-and-forget is appropriate is in top-level event-handlers. Every other async method in your code should return "async Task".

here is the link

@Portikus 2017-11-06 10:27:17

The 'top-level event-handlers' is a important hint. When using async void event handler on lower level event handler it can cause huge problems with not caught exceptions.

@lsp 2018-11-29 11:12:57

Thanks for the video link, very useful

@Stephen Cleary 2013-10-16 23:25:38

The guideline is to avoid async void except when used in an event handler, so using async void in an event handler is OK.

That said, for unit testing reasons I often like to factor out the logic of all async void methods. E.g.,

public async Task OnFormLoadAsync(object sender, EventArgs e)
{
  await Task.Delay(2000);
  ...
}

private async void Form_Load(object sender, EventArgs e)
{
  await OnFormLoadAsync(sender, e);
}

@InteXX 2015-02-18 07:31:32

I'm curious... is there a reason why you don't just change Form_Load's access to public? It seems like the code would be less verbose that way.

@InteXX 2015-02-18 07:33:33

Oops, never mind... VBer trying to read C# here... I just noticed the return type of OnFormLoadAsync. I see now that this makes for a handy trick. Thanks.

@InteXX 2015-02-18 07:37:04

All that said, could you have a look and offer an opinion here. Thanks!

@Alex Hope O'Connor 2018-11-05 06:10:00

In WPF is it fine to mark routed event handlers with async? I am not sure how the Handled flag on the event args would be checked in such a case...

@Stephen Cleary 2018-11-05 13:00:42

@AlexHopeO'Connor: The Handled flag must be set synchronously; it's not possible to use async to make a decision on whether the event is handled or not.

@Alex Hope O'Connor 2018-11-06 02:30:31

@StephenCleary so would it be alright to fire off an async void from the top level event handlers for command bindings in WPF? Been reading this: msdn.microsoft.com/en-us/magazine/dn630647.aspx But I have been using CommandBinding and RoutedUICommand in my application and was wondering how to best execute my commands asynchronously and provide things like user cancellation etc

@Stephen Cleary 2018-11-06 17:35:21

@AlexHopeO'Connor: It's been a while since I've worked with a WPF app, but I've used solutions similar to that in the past. I.e., make the ICommand.Execute method async void; I consider this acceptable since ICommand.Execute is logically an event handler.

@Eric Lippert 2013-10-16 23:25:33

Should I generally avoid async void event handlers, as well?

Generally event handlers are the one case where a void async method is not a potential code smell.

Now, if you do need to track the task for some reason then the technique you describe is perfectly reasonable.

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