Fix BepInEx 6 for Unity 4.x games and below

BepInEx.Core/BepInEx.Core.csproj

@@ -22,8 +22,8 @@
     <ItemGroup>
         <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All"/>
         <PackageReference Include="HarmonyX" Version="2.10.0"/>
-        <PackageReference Include="MonoMod.Utils" Version="22.3.23.4"/>
         <PackageReference Include="SemanticVersioning" Version="2.0.2"/>
+        <PackageReference Include="MonoMod.Utils" Version="22.5.1.1" />
     </ItemGroup>
     <ItemGroup>
         <Compile Remove="Contract\IPlugin.cs"/>

BepInEx.Preloader.Core/BepInEx.Preloader.Core.csproj

@@ -22,7 +22,7 @@
     <ItemGroup>
         <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All"/>
         <PackageReference Include="HarmonyX" Version="2.10.0"/>
-        <PackageReference Include="MonoMod.RuntimeDetour" Version="22.3.23.4"/>
-        <PackageReference Include="MonoMod.Utils" Version="22.3.23.4"/>
+        <PackageReference Include="MonoMod.RuntimeDetour" Version="22.5.1.1" />
+        <PackageReference Include="MonoMod.Utils" Version="22.5.1.1" />
     </ItemGroup>
 </Project>

BepInEx.Preloader.Unity/EntrypointPatcher.cs

@@ -110,8 +110,7 @@ internal class EntrypointPatcher : BasePatcher
                 var ins = il.Body.Instructions.First();
 
                 il.InsertBefore(ins,
-                                il.Create(OpCodes
-                                              .Ldnull)); // gameExePath (always null, we initialize the Paths class in Entrypoint)
+                                il.Create(OpCodes.Ldnull)); // gameExePath (always null, we initialize the Paths class in Entrypoint)
 
                 il.InsertBefore(ins,
                                 il.Create(OpCodes.Call,

BepInEx.Unity/BepInEx.Unity.csproj

@@ -7,7 +7,7 @@
         <LangVersion>10</LangVersion>
     </PropertyGroup>
     <PropertyGroup Condition="'$(TargetFramework)' == 'net35'">
-        <!-- Output only net35 version to distribution; netstadnard is only used for generating NuGet package -->
+        <!-- Output only net35 version to distribution; netstandard is only used for generating NuGet package -->
         <OutputPath>..\bin\Unity\</OutputPath>
         <DocumentationFile>$(OutputPath)\BepInEx.Unity.xml</DocumentationFile>
         <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>

BepInEx.Unity/Bootstrap/UnityChainloader.cs

@@ -1,4 +1,5 @@
-using System.Diagnostics;
+using System;
+using System.Diagnostics;
 using System.IO;
 using System.Reflection;
 using System.Runtime.CompilerServices;
@@ -10,6 +11,10 @@ using BepInEx.Unity.Logging;
 using MonoMod.Utils;
 using UnityEngine;
 using Logger = BepInEx.Logging.Logger;
+using Object = UnityEngine.Object;
+
+[assembly: InternalsVisibleTo("UnityEngine")]
+[assembly: InternalsVisibleTo("UnityEngine.Core")]
 
 namespace BepInEx.Unity.Bootstrap;
 
@@ -53,31 +58,81 @@ public class UnityChainloader : BaseChainloader<BaseUnityPlugin>
         get => Application.unityVersion;
     }
 
-    private static void StaticStart(string gameExePath = null)
+    private static Version UnityVersionAsVersion => new Version(UnityVersion.Substring(0, UnityVersion.LastIndexOf('.') + 2));
+
+    private static bool staticStartHasBeenCalled = false;
+
+    /// <summary>
+    /// This method is public so that BepInEx can correctly initialize on some versions of Unity that do not respect InternalsVisibleToAttribute. Do not call this yourself
+    /// </summary>
+    [Obsolete("This method is public so that BepInEx can correctly initialize on some versions of Unity that do not respect InternalsVisibleToAttribute. DO NOT CALL", true)]
+    public static void StaticStart(string gameExePath = null)
     {
-        var instance = new UnityChainloader();
-        instance.Initialize(gameExePath);
-        instance.Execute();
+        try
+        {
+            if (staticStartHasBeenCalled)
+                throw new InvalidOperationException("Cannot call StaticStart again");
+
+            Logger.Log(LogLevel.Debug, "Entering chainloader StaticStart");
+
+            var instance = new UnityChainloader();
+            instance.Initialize(gameExePath);
+            instance.Execute();
+
+            staticStartHasBeenCalled = true;
+
+            Logger.Log(LogLevel.Debug, "Exiting chainloader StaticStart");
+        }
+        catch (Exception ex)
+        {
+            Logger.Log(LogLevel.Fatal, $"Unable to complete chainloader StaticStart: {ex.Message}");
+            Logger.Log(LogLevel.Fatal, ex.StackTrace);
+        }
     }
 
 
     public override void Initialize(string gameExePath = null)
     {
-        Instance = this;
+        try
+        {
+            Logger.Log(LogLevel.Debug, "Entering chainloader initialize");
 
-        UnityTomlTypeConverters.AddUnityEngineConverters();
+            Instance = this;
 
-        ThreadingHelper.Initialize();
+            UnityTomlTypeConverters.AddUnityEngineConverters();
 
-        ManagerObject = new GameObject("BepInEx_Manager") { hideFlags = HideFlags.HideAndDontSave };
-        Object.DontDestroyOnLoad(ManagerObject);
+            Logger.Log(LogLevel.Debug, "Creating Manager object");
 
-        var productNameProp =
-            typeof(Application).GetProperty("productName", BindingFlags.Public | BindingFlags.Static);
-        _consoleTitle =
-            $"{CurrentAssemblyName} {CurrentAssemblyVersion} - {productNameProp?.GetValue(null, null) ?? Path.GetFileNameWithoutExtension(Process.GetCurrentProcess().ProcessName)}";
+            ManagerObject = new GameObject("BepInEx_Manager") { hideFlags = HideFlags.HideAndDontSave };
+            Object.DontDestroyOnLoad(ManagerObject);
 
-        base.Initialize(gameExePath);
+            Logger.Log(LogLevel.Debug, "Getting game product name");
+
+            var productNameProp = typeof(Application).GetProperty("productName", BindingFlags.Public | BindingFlags.Static);
+
+            _consoleTitle = $"{CurrentAssemblyName} {CurrentAssemblyVersion} - {productNameProp?.GetValue(null, null) ?? Path.GetFileNameWithoutExtension(Process.GetCurrentProcess().ProcessName)}";
+
+            if (UnityVersionAsVersion.Major >= 5)
+            {
+                Logger.Log(LogLevel.Debug, "Initializing ThreadingHelper");
+                ThreadingHelper.Initialize();
+            }
+            else
+            {
+                Logger.Log(LogLevel.Debug, "Skipping ThreadingHelper init due to unity version (<= 4)");
+            }
+
+            Logger.Log(LogLevel.Debug, "Falling back to BaseChainloader initializer");
+
+            base.Initialize(gameExePath);
+
+            Logger.Log(LogLevel.Debug, "Exiting chainloader initialize");
+        }
+        catch (Exception ex)
+        {
+            Logger.Log(LogLevel.Fatal, $"Unable to complete chainloader init: {ex.Message}");
+            Logger.Log(LogLevel.Fatal, ex.StackTrace);
+        }
     }
 
     protected override void InitializeLoggers()