Injected values

Prefixes, postfixes, and finalizers can receive the original's arguments, instance, and result. Declare only the parameters you need.

__instance

__instance is the original method's this. It is available for instance methods, not static methods.

__result

__result holds the returned value. Its type must match the original return type or be assignable from it. It starts with that type's default value before prefixes run. To change it, use ref, for example ref string __result.

__resultRef

__resultRef changes the reference itself for a ref return. Declare it as ref RefResult<T> __resultRef, where T is the returned element type; for ref string, use ref RefResult<string>.

__state

__state passes a per-call value from prefix to postfix or finalizer. Set it in the prefix using ref or out. It can be any type, but the patches sharing it must be in the same class.

___fields

Three underscores select a field: ___someField reads someField, including private fields. Use ref to write it, for example ref string ___name.

__args

object[] __args contains all arguments in declaration order. Editing its elements updates the corresponding arguments; ref is not needed. It has more overhead than typed argument injection.

method arguments

To read an original argument, declare a matching patch parameter; add ref to change it:

• The type of an injected argument must be assignable from the original argument (or just use object)
• Use its original name or __n, where n is its zero-based argument index. Argument annotations can map custom names.

If an original argument name conflicts with a Harmony injection name or naming convention, use ArgumentMode.Original to match its exact, case-sensitive name without interpreting it:

static void Prefix([HarmonyArgument("__result", ArgumentMode.Original)] ref bool result)

__originalMethod

MethodBase __originalMethod identifies the method being patched, useful when one patch targets several methods.

Calling it invokes the patched method, not the unmodified original. Use a reverse patch for a callable copy of the original.

__runOriginal

bool __runOriginal is read-only. In a prefix it tells you whether the original is still scheduled to run; a later prefix can still skip it. In a postfix it tells you whether the original ran.

Transpilers

In transpilers, arguments are only matched by their type so you can choose any argument name you like.

• IEnumerable<CodeInstruction> is required and receives the instructions to edit.
• ILGenerator is optional and receives the current IL generator.
• MethodBase is optional and receives the original method being patched.