Reverse Patch)¶

Quick reference for the four Harmony patch kinds, their intended use, and the implicit parameters Harmony provides. Split out from the RepairPrototype plugin template (see also ../Workflows/ModProjectSetup.md for the BepInEx scaffold). How MULTIPLE patches on one method interleave (priority sort, skip-original semantics, __state sharing) is on HarmonyPatchOrdering.

Reference¶

F0229l (Plans/RepairPrototype/plan.md:804-818):

Harmony patch types: Prefix (runs before, return false to skip, gets __instance, can modify params), Postfix (runs after, can modify __result), Transpiler (modifies IL at load), Reverse Patch (calls private/internal methods from mod code).

Prefix¶

Runs before the target method. Can:

Access and modify parameters via matching parameter names (ref T foo).
Set ref __result directly to override the return value.
Return false to block the original from running (requires the prefix be declared as bool-returning). See ./HarmonyPrefixReturnBool.md.
Access __instance for instance methods.

Use when the mod needs to intercept a call, decide whether vanilla runs, and potentially replace the result.

Postfix¶

Runs after the target method. Can:

Read or modify ref __result.
Access parameters (read-only semantics are typical, though ref parameters are still ref).
Access __instance.

Use when the mod augments vanilla's output (add a bonus, clamp a value, register side-effects). Cannot block vanilla.

Transpiler¶

Modifies the target method's IL at load time. Receives an IEnumerable<CodeInstruction> and returns a modified enumerable.

Use only when Prefix/Postfix can't express the change: inserting code mid-method, replacing a specific opcode, rewriting a loop body. Transpilers are brittle against game-version updates; prefer Prefix/Postfix when they suffice.

Reverse Patch¶

Provides a mod-owned delegate that calls a private/internal game method without reflection at call-time. The Reverse Patch class's static method has the same signature as the target; Harmony redirects the call into the target at load time.

Use when the mod needs to call a private/internal vanilla method hot-path frequently; Reverse Patch is faster than MethodInfo.Invoke reflection. See ./AccessToolsRecipes.md for the reflection alternative.

Implicit parameter names¶

__instance: the target object for instance methods.
__result: the method's return value (Prefix / Postfix may modify).
__state: Prefix-to-Postfix state handoff (Prefix's out becomes Postfix's in).
____<fieldName> (four underscores + field name): private field of the target class. Harmony binds the parameter to the field automatically.

Verification history¶

2026-07-06: added a cross-link to the new HarmonyPatchOrdering page (multi-patch priority sort, skip semantics, __state sharing). Navigational only; no factual claim on this page changed.
2026-04-20: page created from the Research migration; single source (F0229l).

Open questions¶

None at creation.