dotnet/wpf · error · InvalidOperationException

SR.EventTriggerDoesNotEnterExit

Error message

SR.EventTriggerDoesNotEnterExit

What it means

EventTrigger.Seal() throws if EnterActions or ExitActions are set. These collections are only meaningful on property-based triggers (Trigger/MultiTrigger), where they fire when a condition becomes true/false. On an EventTrigger they have no semantics, so the library rejects them.

Solutions

  1. Move the actions into the EventTrigger's Actions collection (executed when the event fires).
  2. If enter/exit semantics are needed, use a property Trigger with EnterActions/ExitActions instead.
  3. Use event-handler code-behind or a Storyboard in Actions to express the on/off transition manually.

Example fix

<!-- before -->
<EventTrigger RoutedEvent="Mouse.MouseEnter">
  <EventTrigger.EnterActions>
    <BeginStoryboard>...</BeginStoryboard>
  </EventTrigger.EnterActions>
</EventTrigger>

<!-- after -->
<EventTrigger RoutedEvent="Mouse.MouseEnter">
  <BeginStoryboard>...</BeginStoryboard>
</EventTrigger>
Defensive patterns

Strategy: validation

Validate before calling

if (eventTrigger.HasEnterActions || eventTrigger.HasExitActions)
    throw new InvalidOperationException("EventTrigger does not support EnterActions/ExitActions.");

Try / catch

try { trigger.Seal(); }
catch (InvalidOperationException) { /* move actions into Actions collection or a property Trigger */ }

Prevention

When it happens

Trigger: Assigning EventTrigger.EnterActions or EventTrigger.ExitActions (in XAML or code) and then sealing the trigger during style/template application.

Common situations: XAML written for a property Trigger pasted into an EventTrigger; assuming EventTrigger supports enter/exit semantics; code generators or converters emitting EnterActions for all trigger types.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/62dc20d9f3199f49. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/EventTrigger.cs:219

        public bool ShouldSerializeActions()
        {
            return ( _actions != null && _actions.Count > 0 );
        }

        ///////////////////////////////////////////////////////////////////////
        // Internal members
        
        internal sealed override void Seal()
        {
            if( PropertyValues.Count > 0 )
            {
                throw new InvalidOperationException(SR.EventTriggerDoNotSetProperties);
            }

            // EnterActions/ExitActions aren't meaningful on event triggers.
            if( HasEnterActions || HasExitActions )
            {
                throw new InvalidOperationException(SR.EventTriggerDoesNotEnterExit);
            }

            if (_routedEvent != null && _actions != null && _actions.Count > 0)
            {
                _actions.Seal(this);  // TriggerActions need a link back to me to fetch the childId corresponding the sourceId string.
            }

            base.Seal(); // Should be almost a no-op given lack of PropertyValues
        }

        ///////////////////////////////////////////////////////////////////////
        // Private members

        // Event that will fire this trigger
        private RoutedEvent _routedEvent = null;

        // Name of the Style.VisualTree node whose event to listen to.
        //  May remain the default value of null, which  means the object being

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