IL Trampoline

Introduction

IL Trampoline is a bit like transpilers, but instead of directly changing the IL, the IL is instead copied, and jumped to the modified IL if instance is active.

Example

Trampoline for Patching

using System;
using System.Collections.Generic;
using System.Linq;
using BeatSaberQuestPatch.Patches.Trampoline;
using CrossAccord.ILTrampoline.Attributes;
using CrossAccord.ILTrampoline.Interfaces;
using Mono.Cecil;
using Mono.Cecil.Cil;

namespace BeatSaberQuestPatch.Build;

[AccordTrampolineBuild(typeof(BeatSaberInit), "get_settingsApplicator", typeof(BeatSaberInitTrampoline))]
public class BeatSaberInitTrampolineBuild : IAccordTrampolineBuild
{
    public bool StartOffset(Instruction instructions)
    {
        return instructions.OpCode == OpCodes.Ldarg_0;
    }

    public bool EndOffset(Instruction instructions)
    {
        if (instructions.Operand is not FieldReference fieldRef)
            return false;

        return instructions.OpCode == OpCodes.Ldfld && fieldRef.Name.Contains("_standaloneSettingsApplicator");
    }

    public IEnumerable<Instruction> PatchTrampoline(IEnumerable<Instruction> instructions, TypeDefinition definition, VariableDefinition instance)
    {
        var typeDef = instance.VariableType.Resolve();
        foreach (var method in typeDef.Methods)
        {
            Console.WriteLine(method.Name);
        }
        var methodToCall = typeDef.Methods.First(it => it.Name.Contains("PatchApplicator"));
        var methodToCallRef = definition.Module.ImportReference(methodToCall);
        foreach (var instruction in instructions)
        {
            if (instruction.OpCode == OpCodes.Ldarg_0)
            {
                yield return Instruction.Create(OpCodes.Ldloc, instance);
                yield return instruction;
            }
            else if (instruction.OpCode == OpCodes.Ldfld)
            {
                yield return Instruction.Create(OpCodes.Call, methodToCallRef);
            }
        }
    }

    public int Matches => 1;
}

For runtime

using CrossAccord.Common.Interfaces;

namespace BeatSaberQuestPatch.Patches.Trampoline;

public class BeatSaberInitTrampoline : IAccordTrampolinePatch<BeatSaberInitTrampoline>
{
    public static BeatSaberInitTrampoline Instance { get; set; } = null;

    public SettingsApplicatorSO PatchApplicator(BeatSaberInit instance)
    {
        return instance._questSettingsApplicator;
    }
}