unoplatform/uno · error · XamlObjectWriterException

Referenced type {0} in event {1} was not found

Error message

Referenced type {0} in event {1} was not found

What it means

Thrown in SetEvent while resolving the owner type of a handler value. The handler string (e.g. 'MyType.MyHandler') is split on the last '.'; the left portion is resolved as a XamlType via the name resolver. If resolution returns null the referenced type cannot be located, and the event cannot be wired.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlObjectWriter.cs:379

				name_scope.RegisterName (name, state.Value);
			} else {
				if (xm.IsEvent)
					SetEvent (xm, (string) CurrentMemberState.Value);
				else if (!xm.IsReadOnly) // exclude read-only object such as collection item.
					SetValue (xm, CurrentMemberState.Value);
			}
		}

		[UnconditionalSuppressMessage("Trimming", "IL2075", Justification = "Types manipulated here have been marked earlier")]
		void SetEvent (XamlMember member, string value)
		{
			if (member.UnderlyingMember == null)
				throw new XamlObjectWriterException (String.Format (CultureInfo.InvariantCulture, "Event {0} has no underlying member to attach event", member));

			int idx = value.LastIndexOf ('.');
			var xt = idx < 0 ? root_state.Type : ResolveTypeFromName (value.Substring (0, idx));
			if (xt == null)
				throw new XamlObjectWriterException (String.Format (CultureInfo.InvariantCulture, "Referenced type {0} in event {1} was not found", value, member));
			if (xt.UnderlyingType == null)
				throw new XamlObjectWriterException (String.Format (CultureInfo.InvariantCulture, "Referenced type {0} in event {1} has no underlying type", value, member));
			string mn = idx < 0 ? value : value.Substring (idx + 1);
			var ev = (EventInfo) member.UnderlyingMember;
			// get an appropriate MethodInfo overload whose signature matches the event's handler type.
			// FIXME: this may need more strict match. RuntimeBinder may be useful here.
			var eventMethodParams = ev.EventHandlerType.GetMethod ("Invoke").GetParameters ();
			
			var target = root_state.Value;
			var mi = target.GetType().GetMethod (mn, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public, null, (from pi in eventMethodParams select pi.ParameterType).ToArray (), null);
			if (mi == null)
				throw new XamlObjectWriterException (String.Format (CultureInfo.InvariantCulture, "Referenced value method {0} in type {1} indicated by event {2} was not found", mn, value, member));
			var obj = object_states.Peek ().Value;
			ev.AddEventHandler (obj, Delegate.CreateDelegate (ev.EventHandlerType, target, mi));
		}

		void SetValue (XamlMember member, object value)
		{

View on GitHub (pinned to 0418340488)

Solutions

  1. Verify the type portion of the handler value is resolvable (correct xmlns prefix and assembly reference).
  2. Add the missing xmlns prefix mapping for the handler's namespace.
  3. For local handlers with no '.', the root state type is used — ensure the root type is correct.

Example fix

<!-- before -->
<Button Click="wrongns:MyType.ClickHandler" />
<!-- after -->
<Button Click="local:MyType.ClickHandler" xmlns:local="clr-namespace:MyApp" />
Defensive patterns

Strategy: validation

Validate before calling

// Verify the handler owner type is resolvable before writing the event.
var ownerType = schemaContext.GetXamlType(ownerNamespace, ownerName);
if (ownerType == null)
    throw new InvalidOperationException($"Cannot resolve handler owner type '{ownerName}'.");

Try / catch

try { /* write event with handler value */ }
catch (XamlObjectWriterException ex) when (ex.Message.Contains("in event") && ex.Message.Contains("was not found")) {
    // fix the type prefix / xmlns for the handler owner.
}

Prevention

When it happens

Trigger: An event handler value like 'Foo.ClickHandler' where 'Foo' cannot be resolved in the current type/name context — wrong xmlns prefix, type not in referenced assemblies, or a typo.

Common situations: Namespace prefix omitted or wrong; assembly containing the handler type not referenced; type renamed; using a fully-qualified name where a prefix is expected.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/ced2e2dc4d899b04. Report an issue: GitHub.