Complex reverse patching

Main.cs

/* the following will print

A: val1=123, val2=456
B: val1=123
With C, we also have 456
C: val1=123
B: done
A: done
Press any key to exit
*/

using HarmonyLib;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.CompilerServices;

namespace ComplexReversePatching
{
	class Program
	{
		static void Main()
		{
			var harmony = new Harmony("test");
			harmony.PatchAll();

			var a2 = new Aclass();
			a2.A(123, 456);

			Console.Write("Press any key to exit");
			Console.ReadKey();
		}
	}

	static class Tools
	{
		internal static IEnumerable<CodeInstruction> ReplaceWithAppendedArgument(this IEnumerable<CodeInstruction> instructions, MethodInfo from, MethodInfo to, int argIndex)
		{
			var list = instructions.ToList();
			var i = list.FindIndex(code => code.Calls(from));
			if (i < 0) Console.WriteLine($"Cannot find {from}");
			list.Insert(i++, new CodeInstruction(OpCodes.Ldarg, argIndex));
			list[i].opcode = OpCodes.Call;
			list[i].operand = to;
			return list.AsEnumerable();
		}
	}

	//

	[HarmonyPatch(typeof(Aclass), "A")]
	static class APatch
	{
		public static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
		{
			var from = SymbolExtensions.GetMethodInfo(() => new Bclass().B(default));
			var to = SymbolExtensions.GetMethodInfo(() => BPatch.Bnew(default, default, default));
			return instructions.ReplaceWithAppendedArgument(from, to, 2);
		}
	}

	[HarmonyPatch]
	static class BPatch
	{
		[HarmonyReversePatch]
		[HarmonyPatch(typeof(Bclass), "B")]
		[MethodImpl(MethodImplOptions.NoInlining)]
		internal static void Bnew(Bclass instance, int val1, int val2)
		{
			IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
			{
				var from = SymbolExtensions.GetMethodInfo(() => new Cclass().C(default));
				var to = SymbolExtensions.GetMethodInfo(() => CReplacement.Cnew(default, default, default));
				return instructions.ReplaceWithAppendedArgument(from, to, 2);
			}
		}
	}

	static class CReplacement
	{
		internal static void Cnew(Cclass instance, int val1, int val2)
		{
			Console.WriteLine($"With C, we also have {val2}");
			instance.C(val1);
		}
	}

	//

	public class Aclass
	{
		public void A(int val1, int val2)
		{
			Console.WriteLine($"A: val1={val1}, val2={val2}");
			var b = new Bclass();
			b.B(val1);
			Console.WriteLine($"A: done");
		}
	}

	public class Bclass
	{
		public void B(int val1)
		{
			Console.WriteLine($"B: val1={val1}");
			var c = new Cclass();
			c.C(val1);
			Console.WriteLine($"B: done");
		}
	}

	public class Cclass
	{
		public void C(int val1)
		{
			Console.WriteLine($"C: val1={val1}");
		}
	}
}