dotnet/wpf · error · InvalidOperationException

SR.Format(SR.EventTriggerTargetNameUnresolvable…

Error message

SR.Format(SR.EventTriggerTargetNameUnresolvable, eventTrigger.SourceName)

What it means

ProcessEventTrigger cannot resolve an EventTrigger's SourceName to a named child of the template. When QueryChildIndexFromChildName returns -1, the SourceName does not match any x:Name-ed element in the template, so the trigger has no target and InvalidOperationException is thrown.

Solutions

  1. Make SourceName match an x:Name on an element declared inside the same template
  2. Remove SourceName if the trigger should target the templated parent itself
  3. Search the template for the literal SourceName string to spot typos
  4. Regenerate/refresh BAML after renaming elements so compiled triggers stay in sync

Example fix

// before
<EventTrigger RoutedEvent="MouseEnter" SourceName="griipPanel">
  <BeginStoryboard>...</BeginStoryboard>
</EventTrigger>
// after (child is actually named 'gripPanel')
<EventTrigger RoutedEvent="MouseEnter" SourceName="gripPanel">
  <BeginStoryboard>...</BeginStoryboard>
</EventTrigger>
Defensive patterns

Strategy: validation

Validate before calling

bool TemplateContainsName(ControlTemplate tpl, string name)
{
    var names = new HashSet<string>();
    void Walk(object o) { /* walk XamlReader-loaded tree collecting FrameworkElement.Name */ }
    Walk(XamlReader.Parse(XamlWriter.Save(tpl)));
    return names.Contains(name);
}

Try / catch

try { control.Template = tpl; } catch (InvalidOperationException ex) when (ex.Message.Contains("SourceName") || ex.Message.Contains("name")) { Log("EventTrigger SourceName unresolved: " + ex.Message); }

Prevention

When it happens

Trigger: EventTrigger.SourceName set to a name that does not exist among the template's named children (childIndexFromChildName lookup fails); applying a shared template whose names differ; renaming a template child without updating EventTrigger.SourceName.

Common situations: Copy-pasted XAML where the storyboard targets a child name that was renamed or removed; SourceName referencing an element outside the template; MergedDictionaries templates referencing names from other resources.

Understand the failure class

Background: "Not found" and "does not exist" errors: why "Task not found", "No such folder", and "Can't find" fire when a lookup comes back empty — this error's family across 14 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/bedd3f886afe25cc. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/StyleHelper.cs:1087

            List<TriggerAction> actionsList = null;
            bool                actionsListExisted = true;
            bool                actionsListChanged = false;
            TriggerAction       action = null;

            FrameworkElementFactory childFef = null;

            // Find a ChildID for the EventTrigger.
            if( eventTrigger.SourceName == null )
            {
                eventTrigger.TriggerChildIndex = 0;
            }
            else
            {
                int childIndex = StyleHelper.QueryChildIndexFromChildName(eventTrigger.SourceName, childIndexFromChildName);

                if( childIndex == -1 )
                {
                    throw new InvalidOperationException(SR.Format(SR.EventTriggerTargetNameUnresolvable, eventTrigger.SourceName));
                }

                eventTrigger.TriggerChildIndex = childIndex;
            }

            // We have at least one EventTrigger - will need triggerActions
            // if it doesn't already exist
            if (triggerActions == null)
            {
                triggerActions = new HybridDictionary();
            }
            else
            {
                actionsList = triggerActions[eventTrigger.RoutedEvent] as List<TriggerAction>;
            }

            // Set up TriggerAction list if one doesn't already exist
            if (actionsList == null)

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