Workaround FirstChanceExceptionEventArgs being trimmed

[akoeplinger]

Fixes: #6626

[jonathanpeppers]

This looks like the right names: https://source.dot.net/#System.Private.CoreLib/FirstChanceExceptionEventArgs.cs,9

[jonpryor]

@akoeplinger: why doesn't the linker already preserve this method?

At a glance, it's because the only managed code that creates an instance of FirstChanceExceptionEventArgs is in NativeAOT: https://github.com/dotnet/runtime/blob/2389815a1bc810b5b3c39586b570cbaae2f0f3a5/src/coreclr/nativeaot/System.Private.CoreLib/src/System/AppContext.CoreRT.cs#L14

Oddly, the only native code I see which uses FirstChanceExceptionEventArgs is CoreCLR, e.g.

• https://github.com/dotnet/runtime/blob/2389815a1bc810b5b3c39586b570cbaae2f0f3a5/src/coreclr/vm/corelib.h#L101
• https://github.com/dotnet/runtime/blob/2389815a1bc810b5b3c39586b570cbaae2f0f3a5/src/coreclr/vm/excep.cpp#L11069

I can't easily find any Mono dotnet/runtime code which mentions FirstChanceExceptionEventArgs.

Thus it looks like we need the PreserveList because it's native code which is creating an instance of the type, but it would be nice to know which mono code is creating this instance. Or does Mono also use excep.cpp?

[jonathanpeppers]

I think it's:

https://github.com/dotnet/runtime/blob/6c7ce85e149eaaf4cc452a7008754d5c799db938/src/mono/mono/metadata/object.c#L94

[akoeplinger]

Yes it's that object.c code from mono, nothing in the BCL references the ctor.

[jonpryor]

Given that dotnet/runtime#68235 has been merged and backported, do we really need this fix?

[akoeplinger]

@jonpryor the backport won't be merged in time for the 6.0 release that you're shipping with, so this is to fix the issue until that backport reaches you.

[jonpryor]

[Microsoft.Android.Sdk.ILLink] Preserve FirstChanceExceptionEventArgs

Fixes: https://github.com/xamarin/xamarin-android/issues/6626

Context: https://github.com/dotnet/runtime/pull/68235

If you create a .NET 6 app whichs ubscribes to the
[`AppDomain.FirstChanceException`][0] event:

	partial class MainActivity {
	    protected override void OnCreate (Bundle? savedInstanceState)
	    {
	        AppDomain.CurrentDomain.FirstChanceException += (o, e) => {
	            Console.WriteLine ("First chance exception!");
	       };
	    }
	}

and has a way to cause an exception to be thrown:

	partial class MainActivity {
	    public override void OnBackPressed() => throw new Exception ("wee?");
	}

and is built in Release configuration with Trimming enabled,

then the app will crash when the exception is thrown (in the above
case, by launching the app and then pressing the Back button):

	E companyname.fo: * Assertion at /__w/1/s/src/mono/mono/metadata/object.c:4179, condition `ctor' not met

The cause of the crash is that the linker removes the
[`FirstChanceExceptionEventArgs(Exception)`][1] constructor.

This was fixed in dotnet/runtime#68235 by using appropriate
`[DynamicDependency]` attributes.  However, this runtime fix is
unlikely to be released before our next RC.

Add a workaround by updating
`src/Microsoft.Android.Sdk.ILLink/PreserveLists/System.Private.CoreLib.xml`
so that the linker won't remove the
`FirstChanceExceptionEventArgs(Exception)` constructor.

[0]: https://docs.microsoft.com/en-us/dotnet/api/system.appdomain.firstchanceexception?view=net-6.0
[1]: https://docs.microsoft.com/en-us/dotnet/api/system.runtime.exceptionservices.firstchanceexceptioneventargs.-ctor?view=net-6.0