Unhandled exception in the target app shows a .NET host error

[andyld97]

Version

ver 2.0.3

Details

Hi!

First of all I am not really sure if this should be a bug report or a feature request!

In my app I am using this exception handler event: AppDomain.CurrentDomain.UnhandledException to show a MessageBox and try to create a backup in case of an unhandled exception.
But with DotnetRuntimeBootstrapper this event doesn't get called, instead it's showing this MessageBox:

AppHost_Error_20220125131642.txt

I've tested Dispatcher.UnhandledException instead and it works fine though, but it only catches Exceptions from the UI-Thread. So is there a way to make this other exception handler working?

The thing is that the text from this default MessageBox isn't really helpful, because it hides the exception that actually occurs.
So is there a way to disable this MessageBox and this logfile somehow (or at least for unhandled exceptions inside the application)?

I think it could be useful if the user can implement his own MessageBox instead (showing two MessageBoxes is not really useful)

Steps to reproduce

• throw an exception and don't catch it inside a project where DotnetRuntimeBootstrapper is used (WPF)

[Tyrrrz]

That does look like a bug, but I wonder why it happens. The host error you're seeing should only appear if the host itself fails to initialize:

DotnetRuntimeBootstrapper/DotnetRuntimeBootstrapper.AppHost/Dotnet/DotnetHost.cs

Lines 51 to 79 in 5a1706c

	public int Run(string targetFilePath, string[] args)
	{
	// Route errors to a buffer
	var errorBuffer = new StringBuilder();
	GetHostfxrSetErrorWriterFn()(s => errorBuffer.AppendLine(s));
	// Initialize the host as if we're running the app from command line
	var argsCombined = args.Prepend(targetFilePath).ToArray();
	if (GetInitializeForCommandLineFn()(argsCombined.Length, argsCombined, IntPtr.Zero, out var handle) != 0)
	{
	throw new ApplicationException(
	$"Failed to initialize .NET host for '{targetFilePath}' with arguments [{string.Join(", ", args)}]. " +
	(errorBuffer.Length > 0
	? "Error:" + Environment.NewLine + errorBuffer
	: "Host resolver did not report any errors.")
	);
	}
	// Run the app
	try
	{
	return GetRunAppFn()(handle);
	}
	finally
	{
	// Ignore errors when tearing down the host
	GetCloseFn()(handle);
	}
	}

Given that the application did run, that doesn't make sense, so maybe something triggered the bootstrapper to reinitialize the host?

There could be a flaw in this sequence, we should really only catch exceptions related to .NET host initialization because as it stands, something potentially unrelated can cause the host to re-initialize:

DotnetRuntimeBootstrapper/DotnetRuntimeBootstrapper.AppHost/Program.cs

Lines 71 to 90 in 5a1706c

	private int Run(string[] args)
	{
	try
	{
	// Attempt to run the target first without any checks (hot path)
	return _targetAssembly.Run(args);
	}
	catch
	{
	if (!InstallMissingPrerequisites())
	{
	// User canceled or reboot is required
	return 0xB007;
	}
	// Attempt to run the target again
	return _targetAssembly.Run(args);
	}
	}
	}

The fix would be to change the exception thrown in DotnetHost.cs to something more specific, and change the catch to only match on that exception. PR is welcome.

That would explain the error message box though. I don't know why UnhandledException doesn't trigger. Are you sure that's the case? Can you try to put some File.WriteAllText(...) in your exception handler to determine that it definitely doesn't run?

[andyld97]

I tested it and CurrentDomain_UnhandledException definitely doesn't trigger!
I only got the first file on my desktop: Dispatcher_UnhandledException.

Then I did another test, I deleted this file and run this again but I uninstalled your nuget package and then I got both files.

[Tyrrrz]

I fixed an issue that resulted in the original error. The cause turned out to be not what I expected.
As for CurrentDomain_UnhandledException, I still don't know why that doesn't trigger. There doesn't seem to be anything in .NET host that would correspond to that. It could be that the default .NET apphost does something more to facilitate that, but I don't know what.

[andyld97]

Hmm, this is really strange. I just updated the nuget package and now if an unhandeld exception occurs, the application crashes sliently (without any MessageBox)

[Tyrrrz]

Was there a message box before (apart from the faulty "host already initialized" one)? Does it work the same way if you run your app from the console (dotnet myapp.dll)?

The fix was to add this block here:

DotnetRuntimeBootstrapper/DotnetRuntimeBootstrapper.AppHost/Dotnet/DotnetHost.cs

Lines 75 to 82 in b6d6f3a

	catch (SEHException ex)
	{
	// This is thrown when the app crashes with an unhandled exception.
	// Unfortunately, there is no way to get that exception or its message,
	// so the best we can do is to return the associated exit code.
	// https://github.com/Tyrrrz/DotnetRuntimeBootstrapper/issues/23
	return ex.ErrorCode;
	}

All it does is fail fast without trying to run the app again (that's where the original error came from). There is no way to get more information about the underlying error.

I did test it in my demo app and if the exception happened prior to Application.Run(...) (WinForms) then the app would terminate without any message. However, if it happened later, then it would show a message box with the error. So I assume it has to be handled at the application boundary.

I also tried that with dotnet CLI and the result was the same, except it also printed the error message to the console.

[andyld97]

Was there a message box before (apart from the faulty "host already initialized" one)? Does it work the same way if you run your app from the console (dotnet myapp.dll)?

No. The only MessageBox I got was "host already initalized".
If I try to run it from console (dotnet myapp.dll) it works as excepted: my MessageBox was shown.

The fix was to add this block here:

Yeah, I've read your commit and I think I understood what you've been done.

I did test it in my demo app and if the exception happened prior to Application.Run(...) (WinForms) then the app would terminate without any message. However, if it happened later, then it would show a message box with the error. So I assume it has to be handled at the application boundary.

In my case the app also crashes if the exception occurs later on when the app is already started. I've renamed an assembly which is required for a special task, so that it cannot be loaded and cause the unhandeld exception. I don't know if this matters, but my app is using WPF instead of WinForms.

[Tyrrrz]

If I try to run it from console (dotnet myapp.dll) it works as excepted: my MessageBox was shown.

Hmm interesting. Could you make a simple repro in a zip file and create a new issue for it?
I'm assuming the fact that the event handler doesn't trigger is related to the missing message box.

To be entirely honest, I still have no idea what could cause this. I also tried to see how .NET's standard apphost handles it, but this is the only part I was able to find:

https://github.com/dotnet/runtime/blob/3610ac1e08c478b3852d0a5f23ff1595b4d15c5e/src/native/corehost/apphost/apphost.windows.cpp

This doesn't cover the actual host initialization logic and I'm not sure where the rest of it is or if it's even open source.