pardeike/Harmony · error · ArgumentNullException

Value cannot be null. (Parameter 'config.original')

Error message

Value cannot be null. (Parameter 'config.original')

What it means

MethodCreator's constructor requires a MethodCreatorConfig whose 'original' (the method being replaced/wrapped) is non-null. This is an internal guard hit when the wrapper-generation pipeline is invoked without a resolved original method — almost always downstream of a null patch target that earlier code let through. It surfaces as ArgumentNullException with parameter name 'config.original'.

Solutions

1. Validate the patch target is non-null before calling harmony.Patch/PatchProcessor/ReversePatch
2. Fix the reflection/AccessTools lookup so it resolves the intended method
3. Log the exact target type+name in your patch bootstrap so null resolution fails loudly at startup

Example fix

// before
var original = AccessTools.Method(typeof(Hud), "Render");
harmony.Patch(original, postfix: postfix); // original may be null
// after
var original = AccessTools.Method(typeof(Hud), "Render") ?? throw new InvalidOperationException("Hud.Render not found");
harmony.Patch(original, postfix: postfix);

if (original is null)
    throw new InvalidOperationException($"Refusing to patch: original method not found ({type}.{name})");

static MethodBase EnsureTarget(MethodBase? m, string name) =>
    m ?? throw new InvalidOperationException($"Patch target '{name}' not found");

try { harmony.Patch(original, prefix: prefix); }
catch (ArgumentNullException ex) when (ex.ParamName == "config.original")
{ Log.Error("Patch target was null — check your reflection lookup"); }

When it happens

Trigger: Internal wrapper/patch creation where config.original was never set because the MethodBase resolution returned null; calling Harmony patching APIs with a null original MethodInfo that bypassed earlier null guards.

Common situations: Mods patching methods found by reflection that no longer exist after a game update; reverse-patch attempts against a null source method; stale cached method references across hot reloads.

Related errors

• Value cannot be null. (Parameter 'fromMethod')
• Value cannot be null. (Parameter 'method')
• The type must declare an empty constructor (the constructor…
• Value cannot be null. (Parameter 'config.methodbase')
• Value cannot be null. (Parameter 'generator')

AI-assisted analysis of pardeike/Harmony@e7872dc170 (2026-09-15). Data as JSON: /api/errors/4fd1b44474e6dd62. Report an issue: GitHub.

Appendix: source

Thrown at Harmony/Internal/MethodCreator.cs:18

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Reflection.Emit;

using static HarmonyLib.Code;

namespace HarmonyLib
{
	internal class MethodCreator
	{
		internal MethodCreatorConfig config;

		internal MethodCreator(MethodCreatorConfig config)
		{
			if (config.original is null)
				throw new ArgumentNullException("config.original");
			this.config = config;
			if (config.debug)
			{
				FileLog.LogBuffered($"### Patch: {config.original.FullDescription()}");
				FileLog.FlushBuffer();
			}
			if (config.Prepare() == false)
				throw new Exception("Could not create replacement method");
		}

		internal (MethodInfo, Dictionary<int, CodeInstruction>) CreateReplacement()
		{
			config.originalVariables = this.DeclareOriginalLocalVariables(config.MethodBase);
			config.localVariables = new VariableState();

			if (config.Fixes.Any() && config.returnType != typeof(void))
			{
				config.resultVariable = config.DeclareLocal(config.returnType);

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