pardeike/Harmony · error · Exception

Method returned an unexpected result

Error message

Method {method.FullDescription()} returned an unexpected result: {error}

What it means

A HarmonyTargetMethods auxiliary method must return a non-empty IEnumerable<MethodBase> with no null elements. When it returns null or an empty/null-containing sequence, Harmony wraps the failure in an Exception identifying the method via FullDescription(), since there is otherwise nothing to patch.

Solutions

Filter nulls in TargetMethods(): use 'var m = AccessTools.Method(...); if (m != null) yield return m;'
Never return null — return an empty sequence only when intentionally patching nothing, and guard the caller
Log/skip missing targets explicitly so you control the failure instead of Harmony

Example fix

// before
static IEnumerable<MethodBase> TargetMethods() =>
    new[] { AccessTools.Method(typeof(Foo), "Bar"), AccessTools.Method(typeof(Foo), "Baz") };
// after
static IEnumerable<MethodBase> TargetMethods()
{
    foreach (var name in new[] { "Bar", "Baz" })
    {
        var m = AccessTools.Method(typeof(Foo), name);
        if (m != null) yield return m;
        else FileLog.Log($"Foo.{name} not found, skipping");
    }
}

Defensive patterns

Strategy: validation

Validate before calling

var methods = TargetMethods();
if (methods == null || methods.Any(m => m == null)) throw new InvalidOperationException("TargetMethods must return non-null MethodBase elements");

Type guard

static bool IsValidTargets(IEnumerable<MethodBase> ms) => ms != null && ms.All(m => m != null);

Try / catch

try { harmony.PatchAll(); } catch (Exception e) when (e.Message.Contains("returned an unexpected result")) { LogNullTargets(e); }

Prevention

Never return null from TargetMethods(); filter null AccessTools results before yielding
Log skipped targets instead of yielding nulls

When it happens

Trigger: A TargetMethods() method returning null; returning an empty IEnumerable; yielding null elements (e.g. AccessTools.Method returning null for a missing overload collected into a list).

Common situations: Collecting targets with 'yield return AccessTools.Method(...)' where some lookups return null after a version change; conditionally returning no methods when a feature is absent.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

AI-assisted analysis of pardeike/Harmony@e7872dc170 (2026-09-15). Data as JSON: /api/errors/1d45e2bb71494967. Report an issue: GitHub.

Appendix: source

Thrown at Harmony/Public/PatchClassProcessor.cs:276

				list.AddRange(props.Select(prop => prop.GetSetMethod(true)).Where(method => method is not null).Cast<MethodBase>());
				return list;
			}

			var result = new List<MethodBase>();

			var targetMethods = RunMethod<HarmonyTargetMethods, IEnumerable<MethodBase>>(null, null);
			if (targetMethods is object)
			{
				string error = null;
				result = [.. targetMethods];
				if (result is null)
					error = "null";
				else if (result.Any(m => m is null))
					error = "some element was null";
				if (error != null)
				{
					if (auxilaryMethods.TryGetValue(typeof(HarmonyTargetMethods), out var method))
						throw new Exception($"Method {method.FullDescription()} returned an unexpected result: {error}");
					else
						throw new Exception($"Some method returned an unexpected result: {error}");
				}
				return result;
			}

			var targetMethod = RunMethod<HarmonyTargetMethod, MethodBase>(null, null, method => method is null ? "null" : null);
			if (targetMethod is not null)
				result.Add(targetMethod);

			return result;
		}

		void ReportException(Exception exception, MethodBase original)
		{
			if (exception is null)
				return;
			if ((containerAttributes.debug ?? false) || Harmony.DEBUG)

View on GitHub (pinned to e7872dc170)