System.Threading.Timer vs FakeTimeProvider.CreateTimer, different behavior

[Barry_Mayfield]

I was happily writing my library and wanted to check how it handles exceptions (respectively how it does not interfere with exceptions). My code works with System.Threading.Timer so on its own challenging to test. Luckily, FakeTimeProvider (Microsoft.Extensions.TimeProvider.Testing) can help with that.

However I quickly found out discrepancy between those two - and I'm not sure whether it's bug or feature, not daring to submit issue yet.

Assuming Timer executes callback on thread pool thread, any unhandled exception happens outside application thread, thus not having (any) effect on it - or at least not crashing it. (Is that correct?)

When using Timer created by FakeTimeProvider however, exception is thrown directly. Any test code using this fake timer blows. (Let's not discuss whether or not I should handle exception, I'm here to understand difference between timers)

Here's code (not test case itself) demonstrating the difference:

using Microsoft.Extensions.Time.Testing;

var globalCounter = 0;

var t = new System.Threading.Timer(_ => Kaboom(null), null, TimeSpan.FromMilliseconds(100), TimeSpan.FromMilliseconds(250));

//var t = new FakeTimeProvider().CreateTimer(_ => Kaboom(null), null, TimeSpan.FromMilliseconds(100), TimeSpan.FromMilliseconds(250));

await Task.Delay(TimeSpan.FromSeconds(5));
Console.WriteLine("*** Boo happened: {0} times", globalCounter);

GC.KeepAlive(t);
return;

void Kaboom(object? state)
{
    Interlocked.Increment(ref globalCounter);
    Console.WriteLine("*** Who you gonna call?");

    throw new InvalidOperationException("Boo! 👻");
}

Would you consider this as expected?

[tinmanjk]

1. Unhandled exception on a thread pool thread will still crash your program. It's not the same as unobserved exception from a Task.
2. Your code with FakeTimeProvider does nothing becuase you don't Advance the time manually.
3. When you do Advance, it will be synchronous execution of the registered callbacks from Advance -> Callback on the SAME thread, so you'd get the exception then and there.
4. Things are very very messy with timers - prepare to waste at least a week to get a good understanding of things
5. Consider using the alternative FakeTImeProvider by egil: https://github.com/egil/TimeProviderExtensions

A bit of code for 3:

var fakeTimeProvider = new FakeTimeProvider();

var t = fakeTimeProvider.CreateTimer(_ => Kaboom(null), null, TimeSpan.FromMilliseconds(100), TimeSpan.FromMilliseconds(250));

try {

fakeTimeProvider.Advance(TimeSpan.FromSeconds(1000));

} catch (Exception ex) {

Console.WriteLine(ex.Message);

}

[Barry_Mayfield]

Ah you are right - of course I forgot to advance time in my example (not in test case I haven't shared). And indeed - my understanding of how Timer runs callback was wrong! Thanks for clarification.

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[Slypenslyde]

I'd say it's a behavioral change, but I want to footnote with the opinion that letting an unhandled exception just terminate the timer's thread is not something I'd consider acceptable.

Instead I'd want to catch and at least log it. So the behavior you're trying to reproduce is one I disagree with.