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Harmony Guide

Intro to Prefixes and Postfixes

Harmony Patches are defined as Prefixes and Postfixes. Extensive Harmony patch documentation is here.

Prefixes run before the original method. It can be used to:

  • Access and edit the arguments of the original method.
  • Set the result of the original method.
  • Skip the original method.
  • One key difference to keep in mind is that while in true Harmony, patched prefixes can cancel out other prefixes, this is NOT THE CASE with HarmonyX, the fork of Harmony that BepInEx uses. With HarmonyX, you cannot cancel out other prefixes when returning false and attempting to skip the original method.

Postfixes run after the original method. They can be used to:

  • Read or change the result of the original method.
  • Access the arguments of the original method.
  • Run custom code after the logic of the original method has executed.

They are usually defined as attributes above a patch class, which contains static methods named Prefix and/or Postfix. You can access the current instance by adding a __instance parameter to the method (with the type of the class you are patching).

Understanding how to Patch Classes

An example patch is below. In particular, we call which class we want to detect, for instance, we will use the PlayerControl class. After that, we would like to define what method we want to detect in that class. In this scenario, we will use the FixedUpdate method which runs every time the engine updates physics.

In order for us to patch this class and method, we can use the following line:

// The type in the typeof() is the class being patched.
// The method in the nameof() is the method of the class that is being patched
[HarmonyPatch(typeof(PlayerControl), nameof(PlayerControl.FixedUpdate))]

Make sure that this is above your class

After that, we can create a Postfix method since it is not necessary to use a Prefix It is easy practice to make sure your method is public, static, and that your variable is called __instance You then can access the PlayerControl from the variable __instance

using HarmonyLib;

namespace ExampleMod
[HarmonyPatch(typeof(PlayerControl), nameof(PlayerControl.FixedUpdate))]
public static class PlayerControlPatch
// It is nice to know that this method runs for all players so all player's names are changed to "Apollo was here"
public static void Postfix(PlayerControl __instance)
// This changes all player's name into "Apollo was here"
__instance.nameText.Text = "Apollo was here";