pardeike/Harmony · error · AmbiguousMatchException

Ambiguous match in Harmony patch for

Error message

Ambiguous match in Harmony patch for {type}:{name}

What it means

AccessTools.Method rethrows AmbiguousMatchException when Type.GetMethod finds several methods with the same name. Harmony first tries a fallback lookup via FindIncludingBaseTypes; if that also cannot resolve a unique match, it throws this message with the original exception as InnerException. It signals that the name alone is not enough to identify the patch target.

Solutions

1. Pass parameter types: AccessTools.Method(typeof(T), "Process", new[] { typeof(string) })
2. Or pass an expression: AccessTools.Method((Action<T, string>)instance.Process) to pin the exact overload
3. List candidates with AccessTools.GetDeclaredMethods(typeof(T)).Where(m => m.Name == "Process") and pick by signature

Example fix

// before
var m = AccessTools.Method(typeof(Foo), "Process");
// after
var m = AccessTools.Method(typeof(Foo), "Process", new[] { typeof(string), typeof(int) });

var candidates = typeof(T).GetMethods(BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic).Where(m => m.Name == name).ToList();
if (candidates.Count > 1 && parameters is null) throw new InvalidOperationException("Overloads found: pass parameters");

When it happens

Trigger: AccessTools.Method(typeof(T), "MethodName") where the type (including base types) has multiple methods named MethodName and no `parameters` argument was given to disambiguate; also hit by patch helpers like EnumeratorMoveNext, Finalizer lookups, and m_PrepForRemoting resolution that go through AccessTools.Method.

Common situations: Patching overloaded methods such as `Process(string)` vs `Process(object)`, interface + class implementations, or a method overloaded in a base and derived type; very common in Unity/game modding where framework types have many overloads.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 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')
• Ambiguous match for HarmonyMethod

AI-assisted analysis of pardeike/Harmony@e7872dc170 (2026-09-15). Data as JSON: /api/errors/fd49c9fc201ca914. Report an issue: GitHub.

Appendix: source

Thrown at Harmony/Tools/AccessTools.cs:707

			if (string.IsNullOrEmpty(name))
			{
				FileLog.Debug("AccessTools.Method: name is null/empty");
				return null;
			}
			MethodInfo result;
			var modifiers = new ParameterModifier[] { };
			if (parameters is null)
			{
				try
				{
					result = FindIncludingBaseTypes(type, t => t.GetMethod(name, all));
				}
				catch (AmbiguousMatchException ex)
				{
					result = FindIncludingBaseTypes(type, t => t.GetMethod(name, all, null, [], modifiers));
					if (result is null)
					{
						throw new AmbiguousMatchException($"Ambiguous match in Harmony patch for {type}:{name}", ex);
					}
				}
			}
			else
			{
				result = FindIncludingBaseTypes(type, t => t.GetMethod(name, all, null, parameters, modifiers));
			}

			if (result is null)
			{
				FileLog.Debug($"AccessTools.Method: Could not find method for type {type} and name {name} and parameters {parameters?.Description()}");
				return null;
			}

			if (generics is not null)
				result = result.MakeGenericMethod(generics);
			return result;
		}

View on GitHub (pinned to e7872dc170)