Fixes the Duplicate type '<>O' crash caused by MelonLoader 0.7.2 on fresh installs.
This affects all Unity games using MelonLoader with Il2Cpp — not just one specific game. If MelonLoader crashes on first launch with a duplicate type error, this tool fixes it.
- Close all affected games
- Download
FixCoreModule-v*.zipfrom the latest release and extract it - Run
FixCoreModule.exe - Launch your game
The tool automatically scans your Steam libraries and finds all affected games. It asks for confirmation before fixing each one. If auto-detection fails, a folder picker opens so you can browse to your game folder.
A backup of each original DLL is saved as .bak. If a game updates and the error returns, run it again.
MelonLoader's Il2CppInterop unstripper sometimes writes duplicate <>O type definitions into UnityEngine.CoreModule.dll during first-time assembly generation. This causes a crash before any mod can load.
FixCoreModule uses Mono.Cecil to read the DLL, remove duplicate type definitions, and rewrite it cleanly. No network calls are made — it only touches files in your local game folders.
The tool searches for affected games automatically:
- Checks the current directory (if you placed it in a game folder)
- Scans all Steam library locations (reads
libraryfolders.vdfand the registry) - Lists all games with a corrupted
UnityEngine.CoreModule.dll - Asks
Fix this game? [Y/n]before touching each one
If auto-detection finds nothing, a Windows folder picker dialog opens so you can browse to your game folder. If that's cancelled, you can type the path manually.
Requires .NET 8 SDK.
dotnet build -c Release
For a self-contained exe (no .NET runtime required on target machine):
dotnet publish -c Release -r win-x64 --self-contained -p:PublishSingleFile=true
Developed with AI assistance (Claude)
MIT License. See LICENSE.
| Library | License | Author |
|---|---|---|
| Mono.Cecil | MIT | Jb Evain |
See THIRD-PARTY-LICENSES.txt for full license texts.