Async method exception crashing application

Question

I am using a third party DLL which offers an async method to perform an operation.

I run this inside a try-catch block but - with JustMyCode disabled - I get an error originating from somewhere inside the third party object which can only be handled in AppDomain.CurrentDomain.UnhandledException, which closes the app and is far to generalised as a place to handle this.

The code:

try { ResultObject result = await thirdPartyObject.MethodAsync(); }
catch { /* never get here */ }

The signature of the thirdPartyObject.MethodAsync() from reflection:

public Task<ResultObject> MethodAsync();

After hitting the internal error (which is only visible with 'Just My Code' disabled in VS settings), it goes onto a System.Threading.Tasks.TaskCanceledException: 'A task was canceled.' ...

Is there a way to intercept that task cancellation without hitting the global unhandled exception handler?

I've seen something similar with angularjs async http calls and deferred caches. You should contact the devs of the third party, maybe send a pull request. — Tamas Hegedus
– Tamas Hegedus, Commented Jan 19, 2018 at 15:45
I don't see a reason why that would not enter the catch block unless their code is bad which could be since their object is named "ResultObject". — Crowcoder
– Crowcoder, Commented Jan 19, 2018 at 15:46
@Crowcoder funnily enough the method name is renamed for this question, so don't get hung up on the method name. This is more about the exception behaviour. — James Harcourt
– James Harcourt, Commented Jan 19, 2018 at 15:47
Can you comb through their decompiled code and see if they are calling any async void methods? — Crowcoder
– Crowcoder, Commented Jan 19, 2018 at 15:50

mm8 · Accepted Answer · 2018-01-19 16:18:51Z

4

Is there a way to intercept that task cancellation without hitting the global unhandled exception handler?

Not without catching the exception in the third-party code. If the third-party method for example starts a new thread, or a new Task which it doesn't await, and an exception is thrown on this background thread, it won't get catched by your catch clause code.

Please refer to @Peter Torr's blog post for more information and some examples: https://blogs.msdn.microsoft.com/ptorr/2014/12/10/async-exceptions-in-c/

You should really ask the manufacturer of the assembly to fix their code.

answered Jan 19, 2018 at 16:18

mm8

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2 Comments

James Harcourt Over a year ago

This is correct - there is no way to solve this without fixing the 3rd party code (which has now been done, happily!)

Michal Dobrodenka Over a year ago

I'm getting exception from mono runtime in http secure handling. Would be nice, If I can prevent app crash. github.com/mono/mono/issues/8350

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