Custom logic-value injection on a vanilla device

Pattern for adding a new readable logic value to a vanilla device (e.g. exposing a private field as an IC10-readable LogicType). Uses a Prefix on CanLogicRead (sets __result = true, returns false to skip original) and a Prefix on GetLogicValue that reads the private field via the Harmony parameter-naming convention. Originating code is from the Re-Volt mod's TransformerLogicPatch.cs.

Problem

F0229i (Plans/RepairPrototype/plan.md:727-755):

Code pattern from Re-Volt TransformerLogicPatch.cs showing [HarmonyPatch(typeof(Transformer))] class with [HarmonyPrefix] on CanLogicRead (sets __result = true and returns false to skip original) and GetLogicValue (accesses private field ____powerProvided via convention). Demonstrates adding custom logic values to an existing device via Harmony prefixes.

Solution / recipe

Skeleton:

[HarmonyPatch(typeof(Transformer))]
internal static class TransformerLogicPatch
{
    [HarmonyPrefix]
    [HarmonyPatch(nameof(Transformer.CanLogicRead))]
    public static bool CanLogicReadPrefix(LogicType logicType, ref bool __result)
    {
        if (logicType == MyCustomLogicType)
        {
            __result = true;
            return false;  // skip original
        }
        return true;  // fall through to vanilla
    }

    [HarmonyPrefix]
    [HarmonyPatch(nameof(Transformer.GetLogicValue))]
    public static bool GetLogicValuePrefix(LogicType logicType, ref double __result, float ____powerProvided)
    {
        if (logicType == MyCustomLogicType)
        {
            __result = ____powerProvided;
            return false;
        }
        return true;
    }
}

Key elements:

• ref parameters __result let the prefix set the return value.
• The prefix returns false to block vanilla (see ./HarmonyPrefixReturnBool.md).
• Private-field access via ____<fieldName> (four underscores + field name) is Harmony's parameter-naming convention; the parameter is bound by name to the field. See ./AccessToolsRecipes.md.

Inheritance considerations

If the target device's CanLogicRead / GetLogicValue are inherited from a base class (e.g. WirelessPower), target the declaring class, not the subclass. See ./HarmonyInheritedMethods.md.

Registering the LogicType itself

Adding a LogicType value the game recognizes is a separate piece of work involving the three-registries mechanism. See ../GameSystems/LogicType.md.

Verification history

• 2026-04-20: page created from the Research migration; single source (F0229i) generalized from Re-Volt's code.

Open questions

None at creation.