unoplatform/uno · error · XamlObjectWriterException

Referenced value method

Error message

Referenced value method {0} in type {1} indicated by event {2} was not found

What it means

Thrown in SetEvent after resolving the owner type and splitting the handler method name: the writer looks for an instance method (public or non-public) on the root-state value's runtime type whose parameter types match the event handler delegate's Invoke signature. If no such method exists, it throws.

Solutions

Add an instance method with the signature matching the event's delegate: `void Handler(object sender, TEventArgs e)`.
Ensure the method is an instance method (the binding flags request Instance), not static.
Confirm the handler lives on the type that is the writer's root-state value (the target).

Example fix

// before
private static void OnClick() { }
// after
private void OnClick(object sender, RoutedEventArgs e) { }

Defensive patterns

Strategy: validation

Validate before calling

// Confirm an instance method with the delegate's signature exists on the target.
var invokeParams = ev.EventHandlerType.GetMethod("Invoke").GetParameters()
    .Select(p => p.ParameterType).ToArray();
var handler = target.GetType().GetMethod(handlerName,
    BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic,
    null, invokeParams, null);
if (handler == null)
    throw new InvalidOperationException($"No matching handler '{handlerName}'.");

Try / catch

try { /* write event */ }
catch (XamlObjectWriterException ex) when (ex.Message.Contains("value method") && ex.Message.Contains("was not found")) {
    // add/rename the handler method with the delegate's exact signature.
}

Prevention

Match the handler signature to the event delegate exactly (object sender, TEventArgs e).
Keep handlers as instance methods on the root-state value type.
Rename handler call sites when renaming methods.

When it happens

Trigger: Event handler value 'ClickHandler' where ClickHandler does not exist on the root type, or exists but with the wrong signature (e.g. `void ClickHandler()` instead of `void ClickHandler(object, RoutedEventArgs)`), or is static rather than instance.

Common situations: Handler method renamed; signature changed; method is static; method lives on a different type than the root state value; typo in handler name.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/10713caa91335f99. Report an issue: GitHub.

Appendix: source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlObjectWriter.cs:391

			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)
		{
			if (member == XamlLanguage.FactoryMethod)
				object_states.Peek ().FactoryMethod = (string) value;
			else if (member.IsDirective)
				return;
			else
				SetValue (member, object_states.Peek ().Value, value);
		}
		
		void SetValue (XamlMember member, object target, object value)
		{
			if (!source.OnSetValue (target, member, value))
				member.Invoker.SetValue (target, value);

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