Why does catch block never execute on the device?

Question

Consider this code:

Task.Factory.StartNew (() => {
    try {
        Console.WriteLine ("Throwing");
        throw new Exception ();
    } catch {
        Console.WriteLine ("Gotcha!");
    }
});

On iOS Simulator, it prints Throwing and Gotcha! as expected.
However, when compiled for the device, it only prints Throwing and never prints Gotcha!.

Why does this happen?
I'm using MonoTouch 6.0.8.
I can also reproduce it on 6.0.6.

I put up a sample project here, you can try to run it on the device.
(never mind the name—I was investigating a different crash when I found this).

Try looking at this previous stackoverflow post-http://stackoverflow.com/questions/363495/difference-between-catch-block-and-throw-new-exception-in-method — MethodMan, Dec 21 '12 at 23:24
@DJ: Not sure how it is relevant. I'm throwing an exception, and `catch` doesn't catch it. — Dan Abramov, Dec 21 '12 at 23:26
@DJ, this is obviously a trivial example to illustrate the issue. — Kirk Woll, Dec 21 '12 at 23:31
Here's a similar bug report from Xamarin that claims it's been fixed in relase 5.3.4: https://bugzilla.xamarin.com/show_bug.cgi?id=5423 . Have you tried updating your Monotouch version? — Diego, Dec 21 '12 at 23:55
@Diego: Mine is the newest. :-) But this looks very similar indeed. — Dan Abramov, Dec 21 '12 at 23:56
Dear downvoter: you're just like that `catch` block, so silent and mysterious. — Dan Abramov, Dec 25 '12 at 01:02

Dan Abramov · Answer 1 · 2012-12-21T23:57:20.813

2

I just found that it only happens with LLVM enabled.
Perhaps it's a bug so I'll report it to Xamarin.

Interestingly, adding

BeginInvokeOnMainThread (() => {
    new UIAlertView ("Gotta catch 'em all!", "", null, "OK", null).Show ();
});

after Console.WriteLine call helps—I see both alert and log entry.

There's my sample project.

edited Dec 21 '12 at 23:57

answered Dec 21 '12 at 23:28

Dan Abramov

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It sure sounds like a bug! – poupou Dec 21 '12 at 23:35
Strangely, I can't reproduce it in another project. Hum hum. – Dan Abramov Dec 21 '12 at 23:35
For what it's worth, I could not repro it either in my project. Hope you can track down the particulars. :) – Kirk Woll Dec 21 '12 at 23:36
@Kirk: Do all `Console.WriteLine` messages appear in Device Console? – Dan Abramov Dec 21 '12 at 23:37
Yes, it appears on the device's console as well. (with LLVM enabled) – Kirk Woll Dec 21 '12 at 23:39
@Kirk: Can you please try [my project](http://rghost.net/42418971) (never mind the name; the code is in the controller's ViewDidLoad)? – Dan Abramov Dec 21 '12 at 23:52
@poupou: I attached a sample project if you feel like giving it a spin. I can reproduce it both on iPad 2 and iPad Mini with iOS 6.0.1. But I suppose it might be MonoTouch 6.0.8-only. – Dan Abramov Dec 21 '12 at 23:58
1

@DanAbramov please fill such issues on http://bugzilla.xamarin.com so the right developers can fix them :-) – poupou Dec 22 '12 at 17:56

score 1 · Answer 2 · answered Dec 21 '12 at 23:29

1

Maybe a silly question but have the conditions been met to require the catch command?

If so perhaps the application breaks before returning the "Gotcha" and iOS commands take over.

I believe iOS listens to see if and when your command/application breaks because default functions kick in to handle this. Similar to the dealloc commands.

answered Dec 21 '12 at 23:29

AJames

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But the curious thing is the app doesn't crash. – Dan Abramov Dec 21 '12 at 23:45

Why does catch block never execute on the device?

2 Answers2