Modding Monster Train - Harmony Reverse Patches

July 14, 2021

Modding with Harmony

Monster Train

Title: Harmony ParDeike - Reverse Patch
By: Harmony
Harmony - Documentation
Description: Harmony documentation on patching with the Reverse Patch.

Overview

I was investigating ways to use Harmony to access private methods and fields from classes within the Unity target game itself, Monster Train in this case. One of the first direct options I came across was using reverse patches.

Basics of the Reverse Patch

"A reverse patch is a stub method in your own code that 'becomes' the original or part of the original method so you can use that yourself."

    Typical use cases:
  • easily call private methods
  • no reflection, no delegate, native performance
  • you can cherry-pick part of the original method by using a transpiler
  • can give you the unmodified original
  • will freeze the implementation to the time you reverse patch

Implementation

After seeing its first use case being calling private methods, I thought this would be a perfect tool to use. As far as I have found, it appears to need a reference to an instance of the object that the private method belongs to. This has proved rather troublesome. While the setting up of the reverse patch has appeared to work and run, it is running into other issues that require further research.

The class I am trying to access is a Unity Scriptable Object, and has no easy way to directly access the existing instance (that I have found so far). I tried circumventing this by creating my own instance to work from, but that has its own issues. The new instance, if it is even being generated properly, is not intialized in any way and is missing integral references, such as to the AllGameData, which are also difficult to inject into the instance. This is again because it is a Unity Scriptable Object, so most fields are private but serialized, so they are very protected and only exposed in the Unity Inspector to make the necessary references I am missing in a newly generated instance.

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