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?

Solution 1:

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);
}

Solution 2:

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.

Solution 3:

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