NSErrorException's Message property should contain more useful information

[johannes-schmitt]

Enhancement: From my point of view, NSErrorException's Message property should contain some information about the underlying NSError. Message is currently "Exception of type 'Foundation.NSErrorException' was thrown."

Consider a project that logs all unhandled exceptions (via AppDomain.CurrentDomain.UnhandledException) to a remote error tracking system (like Sentry) or to a log file. In almost all of the cases, these unhandled exceptions are bugs. The existing Message property does not help to understand the root cause of the bug and makes it hard to properly fix it.

Steps to Reproduce

1. in iOS (or macOS) project: throw new NSErrorException(/* ... */)
2. in platform independent project (e.g. Xamarin Forms), referencing the project from step 1: AppDomain.CurrentDomain.UnhandledException += (_, e) => { Console.WriteLine(((Exception)e.ExceptionObject).Message); } ;

Expected Behavior

Message should at least contain NSError's Code to be able to understand what the exception was about.

Actual Behavior

Message is "Exception of type 'Foundation.NSErrorException' was thrown."

[spouliot]

Make sense :)

The most useful information, coming from the native code, is the Description property, which we default for ToString () on NSObject subclasses, however NSErrorException is a subclass of Exception.

You can probably workaround this with something like
var msg = (e as NSErrorException)?.Error.Description ?? e.Message;
where e is an Exception subclass, until a release is out with the fix.

[johannes-schmitt]

I'm not sure, but I think the Description property maps to NSError's LocalizedDescription. If my assumption holds true, it'll be tough to understand what an error means for a language I don't speak. Especially, in a error tracking system it would be helpful to have some non-localized exception information to be able to understand common error across language borders.

I would suggest to override Message so that Code (which is language-neutral) is part of it. Something simple like this:
public override string Message { get { return $"{error.Description} ({error.Code})"; } }
Maybe also Domain should be included, because Code is only unique within a domain.
What do you think?

Regarding your suggested workaround: I implemented a similar approch that uses reflection. Why reflection? The type NSErrorException is not available in the platform-independent project.

[spouliot]

I'll check if it's localized after WWDC (or in between sessions)
but the output is already like Domain + Error + Description + Extra (user info)
so I don't think it can be very confusing to get the same data

[johannes-schmitt]

I've been some days away from that topic. Coming back now, I agree, NSError.ToString() is sufficient. Also other exception messages in the .NET world are actually localized.