CODEWITHSUNDEEP
c#.neteventsasynchronousasync-await
avo
avo

Reputation: 10701

Should I avoid 'async void' event handlers?

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?

Upvotes: 170

Views: 64383

Answers (4)

Idrees Khan
Idrees Khan

Reputation: 7752

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

Upvotes: 7

Alexander Zwitbaum
Alexander Zwitbaum

Reputation: 4846

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.

Upvotes: 8

Stephen Cleary
Stephen Cleary

Reputation: 456397

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);
}

Upvotes: 202

Eric Lippert
Eric Lippert

Reputation: 659984

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.

Upvotes: 62

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