{"record":{"id":"55ff2598ff2e8e6d","repo":"aspnetboilerplate/aspnetboilerplate","slug":"registered-event-handler-for-event-type-eventtype","errorCode":null,"errorMessage":"Registered event handler for event type {eventType.Name} is null!","messagePattern":"Registered event handler for event type (.+?) is null!","errorType":"exception","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"src/Abp/Events/Bus/EventBus.cs","lineNumber":370,"sourceCode":"            if (exceptions.Any())\n            {\n                if (exceptions.Count == 1)\n                {\n                    exceptions[0].ReThrow();\n                }\n\n                throw new AggregateException(\"More than one error has occurred while triggering the event: \" + eventType, exceptions);\n            }\n        }\n\n        private void TriggerHandlingException(IEventHandlerFactory handlerFactory, Type eventType, IEventData eventData, List<Exception> exceptions)\n        {\n            var eventHandler = handlerFactory.GetHandler();\n            try\n            {\n                if (eventHandler == null)\n                {\n                    throw new ArgumentNullException($\"Registered event handler for event type {eventType.Name} is null!\");\n                }\n\n                var handlerType = typeof(IEventHandler<>).MakeGenericType(eventType);\n\n                var method = handlerType.GetMethod(\n                    \"HandleEvent\",\n                    new[] { eventType }\n                );\n\n                method.Invoke(eventHandler, new object[] { eventData });\n            }\n            catch (TargetInvocationException ex)\n            {\n                exceptions.Add(ex.InnerException);\n            }\n            catch (Exception ex)\n            {\n                exceptions.Add(ex);","sourceCodeStart":352,"sourceCodeEnd":388,"githubUrl":"https://github.com/aspnetboilerplate/aspnetboilerplate/blob/2323c13a152aa0ebfb37af3830f687d7f5b53795/src/Abp/Events/Bus/EventBus.cs#L352-L388","documentation":"EventBus.TriggerHandlingException resolves a handler instance from its factory before invoking HandleEvent via reflection; when the factory returns null it throws ArgumentNullException with this message. It means a registered handler factory produced no usable handler instance for the event type — an invalid registration rather than a handler bug.","triggerScenarios":"Publishing an event whose registered IEventHandlerFactory.GetHandler() returns null — e.g. a handler registered as transient with a broken IoC registration, an unresolvable dependency, or a custom factory that can't create the handler.","commonSituations":"Custom IEventHandlerFactory implementations returning null on failure; handler registered with a type whose constructor dependencies fail silently; removal of a service the handler depends on during refactoring; faulty UntypedHandlerFactory usage.","solutions":["Inspect the handler registration for that event type and fix the factory/IoC registration so GetHandler returns a valid instance.","Ensure the handler class's constructor dependencies are all registered in the DI container.","If using a custom factory, make GetHandler throw a descriptive exception instead of returning null.","Test handler resolution in isolation (resolve the handler type from IocManager) to find the null source."],"exampleFix":"// before\nConfiguration.EventBus.Register<MyEvent>(() => null); // or custom factory returning null\n// after\nConfiguration.EventBus.Register<MyEvent>(() => new MyEventHandler(new Dep()));","handlingStrategy":"validation","validationCode":"var handler = iocManager.IsRegistered<MyEventHandler>() ? iocManager.Resolve<MyEventHandler>() : null;\nif (handler == null)\n    throw new InvalidOperationException(\"MyEventHandler cannot be resolved — fix registration before publishing\");","typeGuard":null,"tryCatchPattern":"try\n{\n    _eventBus.Trigger(myEvent);\n}\ncatch (ArgumentNullException ex) when (ex.Message.Contains(\"event handler\") && ex.Message.Contains(\"is null\"))\n{\n    Logger.Error(\"Broken event handler registration\", ex);\n}","preventionTips":["Never let custom handler factories return null; throw with context instead","Resolve-ability-test all handlers at startup","Keep registrations and handler implementations in sync (delete both together)","Ensure handler constructor dependencies are registered in DI"],"tags":["abp","eventbus","null","registration"],"backgroundTag":"null-argument","analyzedSha":"2323c13a152aa0ebfb37af3830f687d7f5b53795","analyzedAt":"2026-09-08T08:00:50.638Z","contentChangedAt":"2026-09-08T08:00:50.638Z","schemaVersion":2},"datasetVersion":"2026-09-17T15:17:12.973Z"}