monovm & Type.GetCustomAttributes() doesn't like interfaces

[jonpryor]

Context: #7190
Context: https://gist.github.com/pjcollins/a3cdf82a6e04cfafbb0ea613b769a8bb
Context: https://github.com/xamarin/java.interop/blob/bd7c60a6a5ea8abaf6f241da17c42d2d93226bf2/src/Java.Interop.Tools.TypeNameMappings/Java.Interop.Tools.TypeNameMappings/JavaNativeTypeManager.cs#L291
Context: https://github.com/xamarin/xamarin-android/tree/test-net5-drop

The attempt to run Xamarin.Android atop monovm has run into a compatibility issue around Type.GetCustomAttributes() and interfaces.

Consider the following code (philosophically similar to Issue #7190 but without external dependencies):

using System;

interface INameable {
    string Name {get;}
}

[AttributeUsage (AttributeTargets.All, AllowMultiple=true)]
class AAttribute : Attribute, INameable {
    string INameable.Name {
        get => "A";
    }
}

[AttributeUsage (AttributeTargets.All, AllowMultiple=true)]
class BAttribute : Attribute, INameable {
    string INameable.Name {
        get => "B";
    }
}

[A, B]
class Example {
}

class App {
    public static void Main ()
    {
        var attributes = (INameable[]) typeof(Example).GetCustomAttributes(typeof(INameable), inherit: false);
        Console.WriteLine ($"Found {attributes.Length} attributes");
        foreach (var attribute in attributes) {
            Console.WriteLine (attribute.Name);
        }
    }
}

The above works on desktop mono and dotnet run, but fails with the .NET 5 monovm, along the lines of this gist:

System.ArgumentException: Type passed in must be derived from System.Attribute or System.Attribute itself. INameable
   at System.Reflection.CustomAttribute.GetCustomAttributes(ICustomAttributeProvider obj, Type attributeType, Boolean inherit)
   at System.RuntimeType.GetCustomAttributes(Type attributeType, Boolean inherit)

[lambdageek]

/cc @CoffeeFlux

[CoffeeFlux]

This was caused by my commit here: 0844cb7a6152

I'd hoped to simplify the type checking, but I broke this in the process. With that said, I think this is only a problem for interfaces, so I should be able to special-case those and move along without undoing my changes? I'll also add a test to libraries so we don't regress on this again.

[marek-safar]

/cc @vitek-karas