stride3d/stride · error · ArgumentNullException

Value cannot be null. (Parameter 'routedEvent')

Error message

Value cannot be null. (Parameter 'routedEvent')

What it means

Stride's UI EventManager refuses to register a class handler when the routedEvent parameter is null. Class handlers are stored in a dictionary keyed by RoutedEvent, so a null event has no meaning and would corrupt the routing tables. This is a fail-fast guard in RegisterClassHandler<T>.

Solutions

1. Ensure the RoutedEvent<T> instance is created via EventManager.RegisterRoutedEvent in a static field/ctor that runs before RegisterClassHandler is called
2. If obtaining the event via GetRoutedEvent, check the result for null before passing it and fix the name/ownerType if null
3. Pass the strongly-typed static field (e.g. UIElement.MouseMoveEvent) instead of a nullable local

Example fix

// before
EventManager.RegisterClassHandler<UIElement>(typeof(UIElement), null, OnMouseMove);
// after
var evt = EventManager.GetRoutedEvent<MouseButtonEventArgs>(typeof(UIElement), "MouseDown");
if (evt != null)
    EventManager.RegisterClassHandler<UIElement>(typeof(UIElement), evt, OnMouseMove);

if (routedEvent == null) throw new InvalidOperationException("RegisterClassHandler requires a non-null RoutedEvent");

static bool IsValidRoutedEvent<T>(RoutedEvent<T> evt) where T : RoutedEventArgs => evt != null;

try { EventManager.RegisterClassHandler<T>(classType, routedEvent, handler); }
catch (ArgumentNullException ex) { logger.LogError(ex, "Class handler registration failed: {Param}", ex.ParamName); }

When it happens

Trigger: Calling EventManager.RegisterClassHandler<T>(classType, routedEvent, handler) with a null routedEvent argument, typically because a static RoutedEvent field was not yet initialized or a GetRoutedEvent lookup returned null.

Common situations: Static initialization order problems where the handler registration runs before the RoutedEvent<T> static field is assigned; renaming an event so a lookup like EventManager.GetRoutedEvent returns null; copy-pasted registration code with a placeholder null.

Related errors

• Value cannot be null. (Parameter 'handler')
• Value cannot be null. (Parameter 'name')
• Value cannot be null. (Parameter 'ownerType')
• A routed event named
• The routed event cannot be changed while the event is being…

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/06d9e4d34a301eec. Report an issue: GitHub.

Appendix: source

Thrown at sources/engine/Stride.UI/Events/EventManager.cs:78

                currentType = currentType.GetTypeInfo().BaseType;
            }

            return types.Where(t => OwnerToEvents.ContainsKey(t)).SelectMany(t => OwnerToEvents[t].Values).ToArray();
        }

        /// <summary>
        /// Registers a class handler for a particular routed event, with the option to handle events where event data is already marked handled.
        /// </summary>
        /// <param name="classType">The type of the class that is declaring class handling.</param>
        /// <param name="routedEvent">The routed event identifier of the event to handle.</param>
        /// <param name="handler">A reference to the class handler implementation.</param>
        /// <param name="handledEventsToo">true to invoke this class handler even if arguments of the routed event have been marked as handled; 
        /// false to retain the default behavior of not invoking the handler on any marked-handled event.</param>
        /// <exception cref="ArgumentNullException"><paramref name="classType"/>, <paramref name="routedEvent"/>, or <paramref name="handler"/> is null.</exception>
        public static void RegisterClassHandler<T>(Type classType, RoutedEvent<T> routedEvent, EventHandler<T> handler, bool handledEventsToo = false) where T : RoutedEventArgs
        {
            if (classType == null) throw new ArgumentNullException(nameof(classType));
            if (routedEvent == null) throw new ArgumentNullException(nameof(routedEvent));
            if (handler == null) throw new ArgumentNullException(nameof(handler));

            lock(SyncRoot)
            {
                if (!ClassesToClassHandlers.ContainsKey(classType))
                    ClassesToClassHandlers[classType] = new Dictionary<RoutedEvent, RoutedEventHandlerInfo>();

                ClassesToClassHandlers[classType][routedEvent] = new RoutedEventHandlerInfo<T>(handler, handledEventsToo); 
            }
        }

        /// <summary>
        /// Get the class handler for the class <paramref name="classType"/> and routed event <paramref name="routedEvent"/>.
        /// </summary>
        /// <param name="classType">The type of the class that is handling the event.</param>
        /// <param name="routedEvent">The routed event to handle</param>
        /// <returns>The class handler</returns>
        /// <exception cref="ArgumentNullException"><paramref name="classType"/>, or <paramref name="routedEvent"/> is null.</exception>

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