[Bug] Null reference exception inside ICommand instantly terminates app

[Tommigun1980]

Hi.

If a null reference exception happens inside an executed Command, the app terminates instantly, nothing is printed to the log, and no error propagation takes place. This is quite troublesome as it makes it hard to track down what happened and also impossible to log these errors for diagnostic purposes. You can't even catch this exception -- it literally just terminates on a null reference error, even when a debugger is connected, and shows absolutely no information about what went wrong. AppDomain.CurrentDomain.UnhandledException is also not invoked when this happens.

I have isolated this in the iOS simulator only, so don't know if it happens on a real device or on Android.

So please put this command in a view model and run it, and the app will terminate instantly on the null reference error, without any error handling being possible:

            this.TestCommand = new Command(() =>
            {
                try
                {
                    int[] ha = null;
                    var he = ha[0];
                }
                catch (Exception e)
                {
                    Console.WriteLine($"NEVER GETS HERE. Exception: '{e}'");
                    throw e;
                }
            });

The app only terminates if this code is inside an ICommand's action; null reference exceptions thrown from other places work normally. Also, the app gets instantly terminated only on a real null reference error, manually thrown errors propagate properly (including a manually thrown NullReferenceException) from Command actions.

Latest nightly XF version, iOS simulator.

[StephaneDelcroix]

Console.WriteLine calls are buffered, so if the app is crashing (because of the re-throw), you might miss the message. Try Debug.WriteLine.

Now it is possible that Command are executed async, and exception being thrown on async thread are not known to be marshalled correctly back to the main thread. We should check with the Xamarin.iOS guys about it

[StephaneDelcroix]

(for the record, @rachelkang was able to reproduce the behaviour)

[rachelkang]

@Tommigun1980 I was able to repro the crash you described, but I want to note that your code did reach the catch clause for me and the exception was thrown!

Repro: Issue12774.zip

[Tommigun1980]

@StephaneDelcroix You are right, but when it happens for me it won't run any code in the catch clause (nor hit breakpoints), it just stops. I added the output to the repro for clarity.

@rachelkang That is peculiar. I will make a separate repro project to help isolate this.

[StephaneDelcroix]

well, @Tommigun1980, you know the drill: provide a repro project

[Tommigun1980]

well, @Tommigun1980, you know the drill: provide a repro project

@StephaneDelcroix I would love to but I don't know how. It happens with a 100% repro rate in my project. I don't know what to look for in order to isolate the issue.

I have been testing random things to try to isolate the condition but this is extremely time consuming. Isn't there a way to upload the project so you could use it for reproducing the issue? I don't even know what I am looking for -- the command issue happens in a real project but not in an isolated test case. I don't know the inner functionality of Xamarin well enough to guess what could be affecting this. Maybe you can give me some pointers? Thank you.

[Tommigun1980]

@StephaneDelcroix I bit the bullet and spent my Friday evening on this.

1. Please download the modified sample project: Issue12774.zip
2. Please edit iOS's Info.plist: Set the FacebookAppID property to a valid FB app id.
3. Do the above before opening the solution. If the solution has been opened even once, you have to delete all obj and bin folders, restart VS and restore packages after editing Info.plist, or the issue will not appear.
4. Run the project in the iOS simulator.

My additions to the project:

1. I added the Plugin.FacebookClient NuGet.
2. I reference something inside it, in this case App.xaml.cs has the following: Plugin.FacebookClient.FacebookResponse<string> thisMustBeReferenced = null;
3. I added the FB properties to Info.plist.

But this is not yet enough, one has to set a valid FacebookAppID in Info.plist, and clear obj and bin folders (in case the solution has been opened even once). This was very painful to isolate.

By following these steps, null reference exceptions inside ICommand actions will now hard crash the app and can no longer be caught.
Thanks.

PS: Alternatively I can email my FB app's id to someone over there which you can use for testing purposes, but I don't want to put it in the repro project directly.

[Tommigun1980]

When the crash happens with the above repro project the following is printed to the VS log:

When firing up simulator:
INFO [2020-11-23 23:29:59Z]: XAML Hot Reload initializing...
WARNING [2020-11-23 23:29:59Z]: (MyApp.iOS) Unknown Breakpoint Hit: UIKit.UIApplication.Main(string[] args, principal, delegate) <--- it says 'Unknown Breakpoint Hit' even when no breakpoints are active
INFO [2020-11-23 23:30:02Z]: (MyApp.iOS) (HotReloadAgent) HotReload: Initialized Agent.
INFO [2020-11-23 23:30:02Z]: XAML Hot Reload Connected and Ready.
...
When exception is thrown inside ICommand's action (that instantly crashes everything and can't be caught):
INFO [2020-11-23 23:30:09Z]: (MyApp.iOS) Stopped Hot Reload session due to ExplicitlyEnded ...
INFO [2020-11-23 23:30:09Z]: Stopped Hot Reload session.

[Tommigun1980]

Hi again.

@StephaneDelcroix Any chance this crash bug could get fixed? As mentioned it makes debugging very hard as the app just hard crashes on exceptions, and it prevents diagnostics logging on actual devices. It is hurting me pretty badly.

I also spent a lot of time on isolating the exact issue and making the repro project, so could at least a generic timeline be given?

Thanks so much!

[Tommigun1980]

This crash bug is hurting me pretty badly. Is there any way crash bugs could be prioritised?

Friendly reminder that repro is at #12774 (comment).

Thank you.