wiki/se-launcher/development/patching.md

title, description, published, date, tags, editor, dateCreated

title	description	published	date	tags	editor	dateCreated
Patching	This article explains how to use versatile Harmony library in launcher environment	true	2025-07-07T15:32:19.754Z		markdown	2025-07-07T15:32:17.950Z

Patching Game Code

The SE Launcher uses the Harmony library to patch and extend the behavior of the game at runtime. Plugins are also encouraged to use Harmony, as it allows for safe and compatible method patching even when multiple plugins modify the same methods.

Harmony is widely used in the modding ecosystem and offers powerful features for injecting code before, after, or instead of existing method logic.

This article does not cover the full scope of Harmony’s capabilities. Developers should refer to the official documentation for detailed information:
https://harmony.pardeike.net/articles/intro.html

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Simple Hello World Patch

Below is an example plugin patch that adds a simple message to the game log each time the world is autosaved.

Patch Class

using HarmonyLib;
using NLog;
using Sandbox.Game.World;

namespace TestPlugin;

[HarmonyPatch(typeof(MySession), nameof(MySession.SaveEnded))]
public static class SavePatch
{
    private static readonly Logger Log = LogManager.GetCurrentClassLogger();

    public static void Prefix()
    {
        Log.Info("Hello world!");
    }
}

What it does:

• The patch targets MySession.SaveEnded, which is called by the game when an autosave completes.
• A Prefix() method is defined, which means it will run before the original method.
• Log.Info("Hello world!") will be printed to the log each time the game autosaves.

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Applying the Patch in Your Plugin

To apply the patch, you must initialize Harmony in your plugin’s Init() method:

using HarmonyLib;

public void Init(object gameInstance)
{
    new Harmony("TestPlugin").PatchAll(typeof(Plugin).Assembly);
}

This line scans your plugin assembly for [HarmonyPatch] attributes and applies all defined patches.

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Notes

• The Harmony ID ("TestPlugin" in this case) should be unique per plugin to avoid conflicts.
• Harmony patches are designed to coexist across multiple plugins, so long as they do not directly conflict by trying to replace the same method behavior entirely.
• If needed, you can control patching more granularly with Patch() and Unpatch() methods instead of PatchAll().

For more complex patching scenarios, refer to the Harmony documentation: https://harmony.pardeike.net/articles/intro.html