Unified global Unhandled Exception event for .NET MAUI

LICENSE

MIT License

Copyright (c) 2022 Matt Johnson-Pint

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MauiExceptions.cs

public static class MauiExceptions
{
#if WINDOWS
    private static Exception _lastFirstChanceException;
#endif

    // We'll route all unhandled exceptions through this one event.
    public static event UnhandledExceptionEventHandler UnhandledException;

    static MauiExceptions()
    {
        // This is the normal event expected, and should still be used.
        // It will fire for exceptions from iOS and Mac Catalyst,
        // and for exceptions on background threads from WinUI 3.
        
        AppDomain.CurrentDomain.UnhandledException += (sender, args) => 
        {
            UnhandledException?.Invoke(sender, args);
        };

#if IOS || MACCATALYST

        // For iOS and Mac Catalyst
        // Exceptions will flow through AppDomain.CurrentDomain.UnhandledException,
        // but we need to set UnwindNativeCode to get it to work correctly. 
        // 
        // See: https://github.com/xamarin/xamarin-macios/issues/15252
        
        ObjCRuntime.Runtime.MarshalManagedException += (_, args) =>
        {
            args.ExceptionMode = ObjCRuntime.MarshalManagedExceptionMode.UnwindNativeCode;
        };

#elif ANDROID

        // For Android:
        // All exceptions will flow through Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser,
        // and NOT through AppDomain.CurrentDomain.UnhandledException

        Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser += (sender, args) =>
        {
            UnhandledException?.Invoke(sender, new UnhandledExceptionEventArgs(args.Exception, true));
        };

#elif WINDOWS

        // For WinUI 3:
        //
        // * Exceptions on background threads are caught by AppDomain.CurrentDomain.UnhandledException,
        //   not by Microsoft.UI.Xaml.Application.Current.UnhandledException
        //   See: https://github.com/microsoft/microsoft-ui-xaml/issues/5221
        //
        // * Exceptions caught by Microsoft.UI.Xaml.Application.Current.UnhandledException have details removed,
        //   but that can be worked around by saved by trapping first chance exceptions
        //   See: https://github.com/microsoft/microsoft-ui-xaml/issues/7160
        //

        AppDomain.CurrentDomain.FirstChanceException += (_, args) =>
        {
            _lastFirstChanceException = args.Exception;
        };

        Microsoft.UI.Xaml.Application.Current.UnhandledException += (sender, args) =>
        {
            var exception = args.Exception;

            if (exception.StackTrace is null)
            {
                exception = _lastFirstChanceException;
            }

            UnhandledException?.Invoke(sender, new UnhandledExceptionEventArgs(exception, true));
        };
#endif
    }
}

@Danielku15, @rumbledot: you shouldn't need to set Thread.DefaultUncaughtExceptionHandler, as .NET for Android already sets that property and forwards exceptions to the AppDomain.UnhandledException event:

• https://github.com/dotnet/android/blob/fc88da7f8bbbed08ca449e3e1c37d99904151ed8/src/java-runtime/java/mono/android/Runtime.java#L14
• https://github.com/dotnet/android/blob/fc88da7f8bbbed08ca449e3e1c37d99904151ed8/src/java-runtime/java/mono/android/Runtime.java#L50
• https://github.com/dotnet/android/blob/fc88da7f8bbbed08ca449e3e1c37d99904151ed8/src/Mono.Android/Android.Runtime/JNIEnv.cs#L140
• https://github.com/dotnet/runtime/blob/731a96b0018cda0c67bb3fab7a4d06538fe2ead5/src/mono/mono/metadata/object.c#L4447-L4462

@jonpryor Interesting, in my case none of the other exception handlers were invoked in scenarios like ClassNotFoundException (related to broken bindings) so I had to register Thread.DefaultUncaughtExceptionHandler where I got the callback and details I needed.

I have followed what everybody has mentioned here. Though the customer exception handler code is getting executed which i can verify from the console during debug run on android emulator as well as on real device, app still closes down. I can't get to handle the error and keep the app open. Is that the expected behaviour? Is there a way i can keep the app open and show a error alert?

this is my code

` public static class GlobalExceptions
{
#if WINDOWS
private static Exception _lastFirstChanceException;
#endif

	// We'll route all unhandled exceptions through this one event.
	public static event UnhandledExceptionEventHandler UnhandledException;

	static GlobalExceptions()
	{
		// This is the normal event expected, and should still be used.
		// It will fire for exceptions from iOS and Mac Catalyst,
		// and for exceptions on background threads from WinUI 3.
		AppDomain.CurrentDomain.UnhandledException += (sender, args) =>
		{
			UnhandledException?.Invoke(sender, args);
		};

		// Events fired by the TaskScheduler. That is calls like Task.Run(...)     
		TaskScheduler.UnobservedTaskException += (sender, args) =>
		{
			UnhandledException?.Invoke(sender, new UnhandledExceptionEventArgs(args.Exception, false));
		};

#if IOS
// For iOS and Mac Catalyst
// Exceptions will flow through AppDomain.CurrentDomain.UnhandledException,
// but we need to set UnwindNativeCode to get it to work correctly.
//
// See: dotnet/macios#15252
ObjCRuntime.Runtime.MarshalManagedException += (_, args) =>
{
args.ExceptionMode = ObjCRuntime.MarshalManagedExceptionMode.UnwindNativeCode;
};

#elif ANDROID

    // For Android:
    // All exceptions will flow through Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser,
    // and NOT through AppDomain.CurrentDomain.UnhandledException

    Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser += (sender, args) =>
    {
		args.Handled = true;
        UnhandledException?.Invoke(sender, new UnhandledExceptionEventArgs(args.Exception, true));
    };

	Java.Lang.Thread.DefaultUncaughtExceptionHandler = new CustomUncaughtExceptionHandler(e =>
        UnhandledException?.Invoke(null, new UnhandledExceptionEventArgs(e, false)));

#elif WINDOWS

    // For WinUI 3:
    //
    // * Exceptions on background threads are caught by AppDomain.CurrentDomain.UnhandledException,
    //   not by Microsoft.UI.Xaml.Application.Current.UnhandledException
    //   See: https://github.com/microsoft/microsoft-ui-xaml/issues/5221
    //
    // * Exceptions caught by Microsoft.UI.Xaml.Application.Current.UnhandledException have details removed,
    //   but that can be worked around by saved by trapping first chance exceptions
    //   See: https://github.com/microsoft/microsoft-ui-xaml/issues/7160
    //

    AppDomain.CurrentDomain.FirstChanceException += (_, args) =>
    {
        args.Handled = true;
        _lastFirstChanceException = args.Exception;
    };

    Microsoft.UI.Xaml.Application.Current.UnhandledException += (sender, args) =>
    {
        var exception = args.Exception;

        if (exception.StackTrace is null)
        {
            exception = _lastFirstChanceException;
        }
        args.Handled = true;
        UnhandledException?.Invoke(sender, new UnhandledExceptionEventArgs(exception, true));
    };

#endif
}
}

#if ANDROID
public class CustomUncaughtExceptionHandler(Action<Java.Lang.Throwable> callback)
: Java.Lang.Object, Java.Lang.Thread.IUncaughtExceptionHandler
{
public void UncaughtException(Java.Lang.Thread t, Java.Lang.Throwable e)
{
callback(e);
}
}
#endif

}`