Patching

Harmony patch methods

Inside the class Harmony searches for methods with the specific names TargetMethod(), Prepare(), Prefix() or Postfix(). Instead of relying on those names, you can also use the method annoations [HarmonyTargetMethod
], [HarmonyPrepare], [HarmonyPrefix] or [HarmonyPostfix].

TargetMethod (Optional)

Most of the times, you will use a combination of HarmonyPatch() annotations on the class to define the method you want to patch. Sometimes though, it is necessary to calculate the method with code. For this, Harmony searches for a method called

static MethodInfo TargetMethod()
static MethodInfo TargetMethod(Harmony instance)
// or
[HarmonyTargetMethod]
static MethodInfo CalculateMethod(...)

That method, if it exists, is expected to return a MethodInfo of the method to be patched. You can optionally receive the harmony instance if you want to run other Harmony methods inside your code.

Prepare (Optional)

Before the patching, Harmony gives you a chance to prepare your state. For this, Harmony searches for a method called

static bool Prepare()
static bool Prepare(Harmony instance)
// or
[HarmonyPrepare]
static bool MyInitializer(...)

That method, if it exists, is expected to return a boolean that controls if patching will happen. You can optionally receive the harmony instance if you want to run other Harmony methods inside your code.

Prefix (Optional)

static bool Prefix(...)
// or
[HarmonyPrefix]
static bool MyPrefix(...)

This method defines the code that is executed before the original method. It follows the guidelines defined in [Patching]

Postfix (Optional)

static bool Postfix(...)
// or
[HarmonyPostfix]
static bool MyPostfix(...)

This method defines the code that is executed after the original method. It follows the guidelines defined in Patching.

Patch method parameters

Each prefix and postfix gets all parameters of the original method as well as the instance (if original method is not static) and the return value. In order to patch a method your patches need to follow the following principles when defining them:

• Prefix and postfix are static methods
• A prefix must return a boolean
• If original method is not static, prefix and postfix get the instance as the first parameter
• A prefix gets a reference to the return value
• Prefix and postfix get all original parameters as references
• Prefix will not get any out parameters

Example:

// original method in class Customer
private List<string> getNames(int count, out Error error)

// prefix
static bool Prefix(Customer instance, ref List<string> result, ref int count)

// postfix
static void Postfix(Customer customer, ref List<string> result, ref int count, ref Error error)