Mono & .NET 5 & The Debugger & Exceptions, oh my!

[jonpryor]

Related: dotnet/android#4877
Related: dotnet/android#4864

Related: https://xamarinhq.slack.com/archives/C03CEGRUW/p1593198033319900
Related: https://files.slack.com/files-pri/T03C6CW9H-F0164ULFMEE/image.png

Xamarin.Android v10.x uses the mono/mono/2020-02 branch, and uses the System.Diagnostics.Debugger.Mono_UnhandledException() internal call when a debugger is attached, in order to simulate the first chance notification within the debugger.

Debugger.Mono_UnhandledException() does not exist in .NET 5 mono. Consequently, when an exception is thrown in a .NET 5 Xamarin.Android app, the debugger "first chance notification" behavior differs.

As such, when an exception is thrown and not caught, there is no "first chance" notification, nor is there a "last chance" notification. Instead, execution continues, the entire call stack is unwound, and then the exception is visible within the debugger.

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This might be a Glorious Opportunity to rework how exceptions work within Xamarin.Android apps when a debugger is attached. dotnet/android#4877 describes the current state of affairs, and it's "not good." In particular, current behavior is that when a managed exception is caught while the debugger is attached, when execution continues mono will unwind stack frames, without allowing execution to return to Java code.

This basically means that if you have a Managed > Java > Managed callstack transition, the process is effectively unusable if the rightmost frame throws an exception while the process is being debugged.

(Not using a debugger results in "sane" exception handling, but means you don't have a debugger…)

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[lambdageek]

I'm curious what the debugger situation is like on macos/ios - I recall we convert managed exceptions into objective-C exceptions and back at Managed->Native->Managed transitions - and the unwinding in native frames is handled by objC. But I don't know what the debugger behavior is like.

It would be good if Android and ios didn't diverge too much here...

[thaystg]

I'm planning to take a look at this next week, to understand how iOS and Android works to understand which would be the best approach to follow on wasm and on Android using .NET 5 mono.
I will do some tests and I can write here everything that I understand. :)

[jonathanpeppers]

There is also another call related to exceptions:

https://github.com/xamarin/xamarin-android/blob/4cae5f5e40896c69b7448cb78cf613cf6327c97c/src/Mono.Android/Android.Runtime/JNIEnv.cs#L251-L262

This allows the AppDomain.UnhandledException event to work, but AppDomain.DoUnhandledException does not exist in .NET 5.