Cannot patch default interface methods

[rhoot]

When a method has been implemented on an interface directly as opposed to in a class implementing that interface, there appears to be no way to patch it. Instead there is an exception raised that says "Owner can't be an array or an interface."

Repro:

interface IFoo
{
    public void FooImpl();
    public void Foo() { FooImpl(); }
}

class Foo : IFoo
{
    public void FooImpl() { Console.WriteLine("Foo"); }
}

[HarmonyPatch(typeof(IFoo), nameof(IFoo.Foo))]
static class FooPatch
{
    static void Prefix(IFoo __instance) { Console.WriteLine("Bar"); }
}

[ManlyMarco]

I didn't even realize this was possible since it needs at least .NET Core 3 so it's basically not supported on Unity (Unity 6 supports it but I rarely see it used). What game is this required for? I'd like to see how the IL looks for that case, if it's secretly using a compiler generated class then you can just patch that directly.

[rhoot]

Constance is using it. In the Constance.Entities.dll assembly there is an IConLootBag type that has a few of these methods. There are likely other types too but that's the one I bumped into.

[rhoot]

Maybe it doesn't require .NET Core 3? I was able to compile a binary that was using it with <TargetFramework>netstandard2.1</TargetFramework>.

[ManlyMarco]

Everything I've seen states it needs .NET Core 3 and I can't compile it unless I set the runtime to this or newer. (unless it's another "only in Unity" thing)

[rhoot]

I guess another option is that as long as the compiler is new enough it can be used even when targeting older frameworks? Constance has all its scripts compiled externally rather than letting Unity do it, and I'm also compiling using a very recent version.

[xADDBx]

.NET Standard 2.1 is supported from .NET Core 3.0+ and .NET 5+
.NET Standard therefore supports default interface implementations

Unity supports .NET Standard 2.1 starting with 2021.2

For this specific issue I think it's a relatively minor issue directly in MonoMod as I've mentioned on the Harmony Discord.

[xADDBx]

MonoMod/MonoMod#300

If I correctly assume that this is the same underlying issue then the next MonoMod release (whenever that happens) might fix it.