cs2-modding

Modding Cities: Skylines II

A mod is a .NET assembly the game loads by scanning it for a type implementing Game.Modding.IMod. What it does afterwards is add systems of its own, edit the game's data, or — rarely — rewrite a vanilla method at runtime.

The simulation is Unity ECS: city state lives in components on entities, and systems running in fixed update phases are what change it. Most of what the player sees as content is a prefab — a data-driven object the game derives entities from — so changing what the game does is often changing data rather than code. A change therefore starts as two questions: which components carry the state, and which system writes them.

Verified against game version 1.6.0f1. Every reference below states its own baseline.

Five facts to hold before writing a line

IMPORTANT: follow this skill's references on anything they own — or at the very least grep them before acting on a familiar shape, because this game diverges from the standard shapes exactly where a prior feels safest, and the guess tends to compile, run and fail silently.

Mods load late, through a two-method contract. Game.Modding.IMod declares OnLoad(UpdateSystem) and OnDispose() and nothing else, and detection requires the interface directly on a top-level class. OnLoad runs once per mod per process, after the world exists and every vanilla system is registered — on the boot path before prefabs load and before anything has ticked, though enabled mid-session it lands past all of that — so a mod can only append, and injecting ahead of the game is unavailable by construction. The mod object is allocated without running any constructor, so its instance field initialisers never run — static fields are unaffected — while a mod's systems are created normally and do get theirs. A throw during load — OnLoad, or any system's OnCreate — kills the whole mod quietly, with Modding.log as the only record; a throw from a later hook or from OnUpdate pops the game's error dialog instead, and only some hooks disable the throwing system. Mod lifecycle, loading and system ordering carries the contract; diagnostics the diagnosis order.