C# – beware of async void in your code

In general, when you see async void in your code it’s bad news, because:

  1. you can’t wait for its completion (as mentioned in this post already)
  2. any unhandled exceptions will terminate your process (ouch!)

 

Suppose you have a timer event that fires every once in a while and you want to do some asynchronous processing inside the handler, so you proceed to write something along the lines of:

The fact that your timer event excepted might hurt, that it took your whole process with it is likely to hurt an awful lot more!

Well, what are your options? If you change the handler to return Task instead the code won’t compile because ElapsedEventHandler delegate type specifies a void return type. Annoyed

Two options springs to mind here.

First, you can always just execute the whole block of code synchronously instead.

This is not ideal because it’s going to block the thread whilst it’s waiting for the asynchronous operation to come back, preventing the thread from being used to do other useful work in the mean time.

If you wish to persist with using async-await and just wish to swallow any exceptions then you can consider option two:

In this case, we’ve simply used a continuation (which fires regarded whether the preceding task had faulted) to swallow any exception that had been thrown by the asynchronous operations.

 

Hope this helps, and remember, whenever you see async void in your code it should be triggering off your spider senses!

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  • l1x

    “any unhan­dled excep­tions will ter­mi­nate your process (ouch!)” I see this as an excellent design decision. Many programmers thinks it is ok to have unhandled exception, and it is absolutely not, only if you are working in Erlang and you supervising the process. In C#/Java land please deal with your exceptions and log a meaningful error instead of letting a stacktrace end up in the logs.

  • Yan Cui

    That’s a fair point. I think the important difference from Erlang here is the lack of memory isolation and an unhandled exception in a background process could have left your application potentially in all kinds of invalid state that you shouldn’t be allowed to continue. Where as a crashed Erlang process couldn’t have corrupted any other process’s memory so the failure is contained and the application as a whole can be allowed to continue unless the superviser’s restart strategy decides otherwise.

    The point I was really trying to make (and failing to do so by the sound of it) is that with ‘async void’ you don’t have a way to catch these exceptions at all. With ‘async Task’ the caller is able to catch any unhandled exceptions from the method and reason about with them, and choose to let the application continue where appropriate, whereas with ‘async void’ that option is off the table.