unoplatform/uno · error · XamlObjectWriterException

Referenced value method {0} in type {1} indicated by event {

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.

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);

View on GitHub (pinned to 0418340488)

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.