Postfix Patch

A Postfix runs after the target method executes — primarily used to read or modify the return value.

Typical Uses

• Modify return value: map, clamp, or enhance the result
• Read result: get the actual result after the method finishes (in Prefix you'd only get the default value)
• Side-effect logic: append logic after method execution completes

Basic Form

[HarmonyPatch(typeof(SomeGameClass), nameof(SomeGameClass.GetValue))]
public static class GetValue_Postfix
{
    public static void Postfix(ref float __result)
    {
        __result = Math.Max(__result, 1f);  // Prevent result from being too small
    }
}

• Use ref float __result to modify the return value
• Without ref, you can only read it

Getting Results from void Methods?

Many game methods return void, but the result is stored in a field / property. In this case, use __instance to read it:

// Target method: public void UpdateScore(), result stored in _score field
[HarmonyPatch(typeof(Scoring), nameof(Scoring.UpdateScore))]
public static class UpdateScore_Postfix
{
    public static void Postfix(Scoring __instance)
    {
        var score = __instance._score;   // Read field
        MyMod.Logger.Log($"Current score: {score}");
    }
}

To read private fields, use triple underscore: ___fieldName.

Using __state with Prefix

Sometimes you want to remember a value before the method and use it after. Use __state to pass values across patches:

[HarmonyPatch(typeof(Player), nameof(Player.Move))]
public static class Move_Patches
{
    public static void Prefix(Player __instance, ref float __state)
    {
        __state = __instance.speed;   // Remember speed before the call
    }

    public static void Postfix(Player __instance, float __state)
    {
        // Speed after the call
        var after = __instance.speed;
        // __state is the speed before the call
        if (after != __state) MyMod.Logger.Log("Speed changed");
    }
}

Rules:

• The __state type must be consistent between Prefix and Postfix
• In Prefix you must use ref (to store), in Postfix you don't use ref (to retrieve)
• If only Postfix exists, Harmony initializes __state with the default value

Modifying Return Value Type

The __result type must be compatible with the target method's return type:

// Target method returns int
public static void Postfix(ref int __result)
{
    __result = Math.Abs(__result);
}

What You Learned

• Postfix timing and use cases
• Using ref __result to modify return values
• Using __instance / ___field to read state
• Using __state to pass values across Prefix / Postfix

Next Step

Learn to handle exceptions → Finalizer Patch