dotnet/wpf · error · ArgumentException

SR.Format(SR.NullPropertyIllegal, "EventSetter.Event")

Error message

SR.Format(SR.NullPropertyIllegal, "EventSetter.Event")

What it means

EventSetter.Seal validates the setter before a Style/EventTrigger is finalized. It throws ArgumentException naming EventSetter.Event when the Event property (the RoutedEvent) is null, since an event setter must know which event to hook.

Solutions

Set the Event property: <EventSetter Event="Button.Click" Handler="OnButtonClick"/>
In code, assign setter.Event = ButtonBase.ClickEvent before the style is sealed
Check style resources for EventSetter entries lacking an Event attribute

Example fix

// before
var es = new EventSetter { Handler = new RoutedEventHandler(OnClick) };
style.Setters.Add(es); // throws on Seal
// after
var es = new EventSetter { Event = ButtonBase.ClickEvent, Handler = new RoutedEventHandler(OnClick) };
style.Setters.Add(es);

Defensive patterns

Strategy: validation

Validate before calling

if (eventSetter.Event == null) throw new InvalidOperationException("EventSetter.Event must be set before sealing the style");

Type guard

bool IsValidEventSetter(EventSetter s) => s?.Event != null;

Try / catch

try { style.Seal(); }
catch (ArgumentException ex) when (ex.Message.Contains("EventSetter.Event")) { fixSetter(); }

Prevention

Always set both Event and Handler on EventSetter
Validate style setters before adding to shared styles

When it happens

Trigger: Adding an EventSetter to a Style's Setters without setting Event, then sealing the style (style is used / sealed implicitly).

Common situations: XAML like <EventSetter Handler="..."/> missing the Event attribute; code-built styles that forget style.Setters.Add with ev only partially configured.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8acf7df061047ba0. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/EventSetter.cs:95

            set 
            { 
                CheckSealed();
                _handledEventsToo = value; 
            }
        }


        //
        //  Do the error checking that we can only do once all of the properties have been
        //  set, then call up to base.
        //
        
        internal override void Seal()
        {

            if (_event == null)
            {
                throw new ArgumentException(SR.Format(SR.NullPropertyIllegal, "EventSetter.Event"));
            }
            if (_handler == null)
            {
                throw new ArgumentException(SR.Format(SR.NullPropertyIllegal, "EventSetter.Handler"));
            }
            if (_handler.GetType() != _event.HandlerType)
            {
                throw new ArgumentException(SR.HandlerTypeIllegal);
            }

            base.Seal();
            
        }


        private RoutedEvent    _event;
        private Delegate         _handler;
        private bool             _handledEventsToo;

View on GitHub (pinned to 81131a70a4)