Using the default TrimMode=link breaks FirstChanceException handler support and crashes app

[lauxjpn]

Android application type

Android for .NET (net6.0-android, etc.)

Affected platform version

.NET 6.0.100

Description

When registering an AppDomain.CurrentDomain.FirstChanceException handler, the handler works fine with non-trimmed builds.

With the default TrimMode=link option when using PublishTrimmed=true, the app crashes at runtime (presumable when the handler gets called for the first time).

I also tried the following, but the crash still happens at runtime:

<Target Name="ConfigureTrimming"
        BeforeTargets="PrepareForILLink">
    <ItemGroup>
        <ManagedAssemblyToLink Condition="'%(Filename)' == 'System.Runtime'">
            <IsTrimmable>false</IsTrimmable>
        </ManagedAssemblyToLink>
    </ItemGroup>
</Target>

---

I wanted to use a FirstChanceException handler to break into the code when an exception because of another trimmed method occurs, so that I could take a look at the call stack and the parameters of a System.Delegate.CreateDelegate(Type type, Type target, String method) call, so that I could investigate which target method got trimmed in #6612 (comment).

Unfortunately, since the TrimMode=link also trims away the FirstChanceException handler support, I can't use it for that purpose.

Steps to Reproduce

See description.

Did you find any workaround?

No response

Relevant log output

No response

[jonathanpeppers]

This example in a dotnet new android app works for me:

[Activity(Label = "@string/app_name", MainLauncher = true)]
public class MainActivity : Activity
{
    protected override void OnCreate(Bundle? savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.activity_main);

        AppDomain.CurrentDomain.FirstChanceException += (sender, e) =>
        {
            Console.WriteLine("First change exception!");
        };
    }

    public override void OnBackPressed() => Task.Factory.StartNew(() => throw new Exception("foo"));
}

However, if I do this, then FirstChanceException doesn't fire:

public override void OnBackPressed() => throw new Exception("foo");

[jonathanpeppers]

I think this behavior is correct for Debug builds ^^

The problem is Release builds (that are trimmed) crash with:

04-18 11:47:56.344 31371 31456 E companyname.fo: * Assertion at /__w/1/s/src/mono/mono/metadata/object.c:4179, condition `ctor' not met

@steveisok do you have any ideas on which type I should check to see what is linked away?

[jonathanpeppers]

@steveisok @akoeplinger here is a smaller example:

        AppDomain.CurrentDomain.FirstChanceException += (sender, e) =>
        {
            Console.WriteLine("First chance exception!");
        };

        try
        {
            throw new Exception("foo");
        }
        catch
        {
            Console.WriteLine("Exception handled!");
        }

I tested with adb shell setprop debug.mono.log default,timing,assembly,mono_log_level=debug,mono_log_mask=all

In a trimmed build (Release), this crashes without much info was to what is wrong:

04-19 15:01:49.456 24437 24437 D Mono    : Running class .cctor for System.AppDomain from 'System.Private.CoreLib.dll'
...
04-19 15:02:44.254 24437 24437 E companyname.fo: * Assertion at /__w/1/s/src/mono/mono/metadata/object.c:4179, condition `ctor' not met

adb.txt

So I was looking at:

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

What would preserve the ctor of this type?

I think ILSpy is showing this type has no ctor.

[akoeplinger]

Thanks, I opened dotnet/runtime#68235 to fix this. In the meantime you can workaround the issue by adding this to a custom linker configuration:

<assembly fullname="System.Private.CoreLib">
    <type fullname="System.Runtime.ExceptionServices.FirstChanceExceptionEventArgs">
        <method signature="System.Void .ctor(System.Exception)" />
    </type>
</assembly>