Il2CppInterop

What is Il2CppInterop

Il2CppInterop is a framework for bridging together Unity's Il2Cpp and .NET's CoreCLR runtimes.

The framework allows the use of Il2Cpp domain and objects in it from a managed .NET domain. This is extremely important for the Unity modding community and has been a key pillar of most mod loaders for several years.

Version 1 Contributions

I did not create Il2CppInterop, but I am a long-time contributor, both to Il2CppInterop and its predecessor Il2CppAssemblyUnhollower. Here are a few of my more notable contributions:

• Enable modders to use their injected classes inside asset bundles
  • Prior to this pull request, injected components had to be added with code after the asset bundle was loaded.
• Switch the assembly generator from Mono.Cecil to AsmResolver
  • This was a massive refactor, but deeply needed in order to align with the rest of the ecosystem. I wrote a tool to verify that there were no regressions from my changes.
• Improve the "unstripping" process
  • When an Il2Cpp game is compiled, unused code is stripped/trimmed from the build. Unstripping is the process of using Mono CIL instructions from public assemblies to fill method bodies in generated interop assemblies.

Version 2

Despite the unquestionable usefulness of Il2CppInterop v1, it has some friction points:

• Inheritance - Interfaces are classes; abstract classes are not abstract; virtual methods are not virtual.
• Non-blittable structs - They are generated as classes, backed by a pointer to a boxed object.
• Object type - Objects are returned from generated interop methods as the declared return type, rather than the object's actual type.

I rewrote most of Il2CppInterop to address the above issues and create a (nearly) perfect type system that faithfully models the underlying system.