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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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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