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I use crashlytics and my app is using 6.4.3 and Mono 3.2.0 in the App Store (production). I'm getting crashlytics errors for "GC_start_routine" in the DidEnterBackground event. I am running a background task so I'm not sure if that's contributing to this but it shouldn't, but it appears maybe the GC is trying to do a collect in the DidEnterBackground and it's crashing?
Please attach the crash log or (better) a test case that can be used to reproduce the issue.
See if this can help you see what's going on:
GC_start_routine is just the entry point for a thread mono has started (so it'll show up on most of the threads).
The important thread is #5 - exit is called here. Xamarin.iOS has not been thoroughly tested when the process is exiting (because it is very rare that an app actually exits), so we've historically had a few crashes at exit time (which you would never see, because the app was exiting anyway).
Now the frame above the call to 'exit' is 'mono_internal_thread_unhandled_exception' - and that indicates that an exception occurred, and that no catch handler was found for that exception, in which case we'll abort the process. The crash report doesn't contain any information about managed exceptions, so it's impossible to say what happened in this case.
However you might be able to listen on the AppDomain.UnhandledException event and log more information about this when it occurs, so the next time you'll have more information.