CS2M - Cities: Skylines 2 Multiplayer

Quick Links

• Discord Server
• Developer Resources

Introduction

CS2M is an in-development multiplayer / co-op mod for Cities: Skylines 2.

Build & Deploy

Prerequisites

• .NET SDK (net472, tested with 9.x)
• Node.js 18+ and npm
• Game installed at path in Toolchain/Mod.props (edit if different)

One-shot build

dotnet build CS2M.sln -c Debug

ILRepack merges CS2M.API.dll, CS2M.BaseGame.dll, LiteNetLib, 0Harmony, MessagePack.* and System.* deps into a single CS2M.dll. Merged intermediates are auto-deleted from the build output.

The UI bundle (CS2M.mjs, CS2M.css, mod.json) is built into dist/Mods/CS2M/ via the UI's webpack build (triggered by the C# project).

Deploy & run

./deploy_and_run.ps1

Copies CS2M.dll + UI files + lang/ to the game's Mods folder and launches the game in -developerMode. Do not ship the satellite DLLs — orphan assemblies with no IMod cause NullReferenceException in the mod manager.

Project layout

Directory	Purpose
CS2M/	C# mod entry point (Mod.cs). Minimal OnLoad: registers UISystem, skips BaseGame sync systems & Harmony patches
CS2M.API/	Shared serialisable command / network types
CS2M.BaseGame/	Base-game integration: commands, harmony patches (off by default)
CS2M.UI/	Webpack + TypeScript UI bundle
Toolchain/	MSBuild props/targets (game path, ILRepack)

UI architecture

The mod mounts its main-menu multiplayer launcher via moduleRegistry.append("GameTopLeft", MainMenuButton) rather than moduleRegistry.extend(...). This avoids a cohtml/AMD-driver hang (driver 32.0.21033.2001) triggered by extending transition-group-coordinator.tsx. SVG icons are inlined as data URLs for the same reason.

Known issues

• I18NEverywhere (PDX #75426) must be disabled in the active playset — it was never updated for game 1.5.7f1 and throws TypeLoadException at mod init.
• AMD driver 32.0.21033.2001 (Sept 2025) hangs ~30s after entering the main menu when cohtml loads SVGs or extends transition-group-coordinator. Fixed in this repo by using append + inline icons.
• The BaseGame sync systems (TimeSystem, MoneySyncSystem, etc.) and Harmony patches are skipped in OnLoad to avoid ECS deadlocks on AMD; they can be re-enabled once a multiplayer server is running.

License

MIT