pardeike/Harmony · error · ArgumentException

Undefined target method for patch method

Error message

Undefined target method for patch method {patchMethod.info.method.FullDescription()}

What it means

PatchWithAttributes resolves each annotated patch method's target via PatchMethodinfo.GetOriginalMethod(). When the target cannot be found (wrong type/name/argumentTypes, or the method was renamed/removed), Harmony throws this ArgumentException naming the offending patch method.

Solutions

1. Verify the target type and member name exist at runtime with AccessTools.TypeByName/AccessTools.Method before patching
2. Correct the class/type, method name, and argumentTypes in the [HarmonyPatch] attribute to match the current assembly
3. For version-dependent targets, resolve with TargetMethod() and search candidates dynamically instead of hard-coded attributes

Example fix

// before
[HarmonyPatch(typeof(Player), "MoveTo", typeof(Vector3))]
// after
static MethodBase TargetMethod() =>
    AccessTools.Method(typeof(Player), "MoveTo", new[] { typeof(Vector3) })
    ?? AccessTools.Method(typeof(Player), "MoveTo");

var target = patchMethod.info.GetOriginalMethod();
if (target is null) throw new ArgumentException($"{patchMethod.info.method} does not resolve to a target");

try { harmony.PatchAll(); } catch (ArgumentException e) when (e.Message.StartsWith("Undefined target method")) { LogMissingTarget(e); }

When it happens

Trigger: A [HarmonyPatch] attribute on a patch method references a type or member that does not exist at runtime (renamed, stripped by trimming/IL2CPP, wrong assembly version, wrong argumentTypes for overloads).

Common situations: Game/framework updates that renamed internal methods; targeting methods in optional dependencies; AOT/IL2CPP builds where reflection metadata was stripped; typos in target class or method names.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• The type must declare an empty constructor (the constructor…
• Value cannot be null. (Parameter 'fromMethod')
• Value cannot be null. (Parameter 'method')
• Value cannot be null. (Parameter 'config.original')
• Method has wrong return type (should be assignable to )

AI-assisted analysis of pardeike/Harmony@e7872dc170 (2026-09-15). Data as JSON: /api/errors/f12a515547462372. Report an issue: GitHub.

Appendix: source

Thrown at Harmony/Public/PatchClassProcessor.cs:175

			foreach (var job in jobs.GetJobs())
			{
				lastOriginal = job.original;
				if (unpatch)
					ProcessUnpatchJob(job);
				else
					ProcessPatchJob(job);
			}
			return jobs.GetReplacements();
		}

		List<MethodInfo> PatchWithAttributes(ref MethodBase lastOriginal, bool unpatch)
		{
			var jobs = new PatchJobs<MethodInfo>();
			foreach (var patchMethod in patchMethods)
			{
				lastOriginal = patchMethod.info.GetOriginalMethod();
				if (lastOriginal is null)
					throw new ArgumentException($"Undefined target method for patch method {patchMethod.info.method.FullDescription()}");

				var job = jobs.GetJob(lastOriginal);
				job.AddPatch(patchMethod);
			}
			foreach (var job in jobs.GetJobs())
			{
				lastOriginal = job.original;
				if (unpatch)
					ProcessUnpatchJob(job);
				else
					ProcessPatchJob(job);
			}
			return jobs.GetReplacements();
		}

		void ProcessPatchJob(PatchJobs<MethodInfo>.Job job)
		{
			MethodInfo replacement = default;

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