stride3d/stride · error · ArgumentNullException

Value cannot be null. (Parameter 'routedEvent')

Error message

Value cannot be null. (Parameter 'routedEvent')

What it means

Argument-null validation in UIElement.AddHandler: the routedEvent parameter is null. AddHandler stores the handler keyed by the RoutedEvent; without a valid event identifier the handler could never be matched during routing, so the call is rejected with ArgumentNullException named 'routedEvent'.

Solutions

1. Pass the static RoutedEvent field (e.g. UIElement.SomeEventEvent) to AddHandler instead of a null reference.
2. Ensure the RoutedEvent was registered via RoutedEvent.Register before any AddHandler call runs; check static-initialization order in partial/event definition classes.
3. Guard call sites where the routed event reference is resolved dynamically and may be null.

When it happens

Trigger: Thrown at sources/engine/Stride.UI/UIElement.Events.cs:199 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/7242e836ac1df4bf. Report an issue: GitHub.

Appendix: source

Thrown at sources/engine/Stride.UI/UIElement.Events.cs:199

            e.EndEventRouting();

            if (sourceWasNull) // reset the source if it was not explicitly set (event might be reused again for other sources)
                e.Source = null;
        }

        /// <summary>
        /// Adds a routed event handler for a specified routed event, adding the handler to the handler collection on the current element. 
        /// Specify handledEventsToo as true to have the provided handler be invoked for routed event that had already been marked as handled by another element along the event route.
        /// </summary>
        /// <param name="routedEvent">An identifier for the routed event to be handled.</param>
        /// <param name="handler">A reference to the handler implementation.</param>
        /// <param name="handledEventsToo">true to register the handler such that it is invoked even when the routed event is marked handled in its event data; 
        /// false to register the handler with the default condition that it will not be invoked if the routed event is already marked handled.</param>
        /// <exception cref="ArgumentNullException">Provided handler or routed event is null.</exception>
        public void AddHandler<T>(RoutedEvent<T> routedEvent, EventHandler<T> handler, bool handledEventsToo = false) where T : RoutedEventArgs
        {
            if (routedEvent == null) throw new ArgumentNullException(nameof(routedEvent));
            if (handler == null) throw new ArgumentNullException(nameof(handler));

            if (!eventsToHandlers.TryGetValue(routedEvent, out List<RoutedEventHandlerInfo> handlerInfos))
                eventsToHandlers[routedEvent] = handlerInfos = new List<RoutedEventHandlerInfo>();

            handlerInfos.Add(new RoutedEventHandlerInfo<T>(handler, handledEventsToo));
        }

        /// <summary>
        /// Removes the specified routed event handler from this element.
        /// </summary>
        /// <param name="routedEvent">The identifier of the routed event for which the handler is attached.</param>
        /// <param name="handler">The specific handler implementation to remove from the event handler collection on this element.</param>
        /// <exception cref="ArgumentNullException">Provided handler or routed event is null.</exception>
        public void RemoveHandler<T>(RoutedEvent<T> routedEvent, EventHandler<T> handler) where T : RoutedEventArgs
        {
            if (routedEvent == null) throw new ArgumentNullException(nameof(routedEvent));
            if (handler == null) throw new ArgumentNullException(nameof(handler));

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