bepin/BepInEx.Patcher/Program.cs

using Mono.Cecil;
using Mono.Cecil.Cil;
using System;
using System.IO;
using System.Linq;

namespace BepInEx.Patcher
{
    class Program
    {
        static void Error(string message)
        {
            Console.WriteLine($"Error: {message}");
            Console.WriteLine("Press any key to continue...");
            Console.ReadKey();
            Environment.Exit(1);
        }

        static void Main(string[] args)
        {
            string managedDir = Environment.CurrentDirectory + @"\KoikatuTrial_Data\Managed";
            if (!Directory.Exists(managedDir))
                Error($"\"{managedDir}\" not found.");

            string unityOutputDLL = Path.GetFullPath($"{managedDir}\\UnityEngine.dll");
            if (!File.Exists(unityOutputDLL))
                Error("\"UnityEngine.dll\" not found.");

            string unityOriginalDLL = Path.GetFullPath($"{managedDir}\\UnityEngine.dll.bak");
            if (!File.Exists(unityOriginalDLL))
                File.Copy(unityOutputDLL, unityOriginalDLL);

            string harmony = Path.GetFullPath($"{managedDir}\\0Harmony.dll");
            File.WriteAllBytes(harmony, Resources._0Harmony);

            string injectedDLL = Path.GetFullPath($"{managedDir}\\BepInEx.dll");
            File.WriteAllBytes(injectedDLL, Resources.BepInEx);

            var defaultResolver = new DefaultAssemblyResolver();
            defaultResolver.AddSearchDirectory(managedDir);
            var rp = new ReaderParameters
            {
                AssemblyResolver = defaultResolver
            };

            AssemblyDefinition unity = AssemblyDefinition.ReadAssembly(unityOriginalDLL, rp);
            AssemblyDefinition injected = AssemblyDefinition.ReadAssembly(injectedDLL, rp);

            InjectAssembly(unity, injected);
            
            unity.Write(unityOutputDLL);
        }

        static void InjectAssembly(AssemblyDefinition unity, AssemblyDefinition injected)
        {
            //Entry point
            var originalInjectMethod = injected.MainModule.Types.First(x => x.Name == "Chainloader")
                .Methods.First(x => x.Name == "Initialize");

            var injectMethod = unity.MainModule.ImportReference(originalInjectMethod);

            var sceneManager = unity.MainModule.Types.First(x => x.Name == "Application");

            var voidType = unity.MainModule.ImportReference(typeof(void));
            var cctor = new MethodDefinition(".cctor",
                MethodAttributes.Static
                | MethodAttributes.Private
                | MethodAttributes.HideBySig
                | MethodAttributes.SpecialName
                | MethodAttributes.RTSpecialName,
                voidType);

            var ilp = cctor.Body.GetILProcessor();
            ilp.Append(ilp.Create(OpCodes.Call, injectMethod));
            ilp.Append(ilp.Create(OpCodes.Ret));

            sceneManager.Methods.Add(cctor);
        }
    }
}