dotnet/wpf · error · ArgumentException

Event not found: Event 'eventName' not found on type…

Error message

Event not found: Event 'eventName' not found on type 'TEventSource'.

What it means

The private WeakEventManager<TEventSource,TEventArgs> constructor resolves the event by name via Type.GetEvent on TEventSource. If no public event with that name exists, _eventInfo is null and it throws ArgumentException with SR.EventNotFound ('Event not found: Event {type} not found on type {source}.'). This is a fail-fast guard so the reflection-based manager is never built against a nonexistent event.

Solutions

Verify the exact spelling and case of the event name against typeof(TEventSource) — use nameof(SourceType.EventName) instead of a literal string.
Confirm the event is public and declared on (or inherited by) TEventSource; GetEvent does not return non-public events.
If the event was renamed, update all AddHandler/RemoveHandler call sites to the new name.
Check with typeof(TEventSource).GetEvent("YourEvent") in the debugger/immediate window to confirm the reflection lookup succeeds.

Example fix

// before
WeakEventManager<MyControl, RoutedEventArgs>.AddHandler(control, "Clic", OnClick);
// after
WeakEventManager<MyControl, RoutedEventArgs>.AddHandler(control, nameof(MyControl.Click), OnClick);

Defensive patterns

Strategy: validation

Validate before calling

var eventInfo = typeof(TEventSource).GetEvent(eventName);
if (eventInfo == null)
    throw new InvalidOperationException($"Event '{eventName}' does not exist on {typeof(TEventSource).FullName}. Available: {string.Join(", ", typeof(TEventSource).GetEvents().Select(e => e.Name))}");

WeakEventManager<TEventSource, TEventArgs>.AddHandler(source, eventName, handler);

Type guard

static bool EventExists<TSrc>(string eventName) => typeof(TSrc).GetEvent(eventName) != null;

Try / catch

try
{
    WeakEventManager<TEventSource, TEventArgs>.AddHandler(source, eventName, handler);
}
catch (ArgumentException ex) when (ex.Message.Contains("Event not found"))
{
    logger.LogError(ex, "Event '{Event}' not found on {Type}", eventName, typeof(TEventSource).Name);
    throw;
}

Prevention

Always use nameof(EventSource.EventName) instead of string literals.
Confirm the event is public and accessible on the exact source type.
When refactoring renames an event, search all WeakEventManager usages of the old string.

When it happens

Trigger: Calling WeakEventManager<TEventSource,TEventArgs>.AddHandler(source, "Click", handler) (or RemoveHandler / the constructor) where the string eventName does not match any public event on TEventSource — typo, case mismatch, event renamed, or event defined on a derived/interface type not exposed by TEventSource.

Common situations: Renaming an event during refactoring while string-based handler registrations still use the old name; using the CLR event name when the class exposes it under a different registered name; targeting events declared private/internal (GetEvent only finds public events).

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/983565b8cf5612de. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/WeakEventManagerT.cs:28

namespace System.Windows
{
    public class WeakEventManager<TEventSource, TEventArgs> : WeakEventManager
        where TEventArgs : EventArgs
    {
        #region Constructors

        //
        //  Constructors
        //

        private WeakEventManager(string eventName)
        {
            _eventName = eventName;
            _eventInfo = typeof(TEventSource).GetEvent(_eventName);

            if (_eventInfo == null)
                throw new ArgumentException(SR.Format(SR.EventNotFound, typeof(TEventSource).FullName, eventName));

            _handler = Delegate.CreateDelegate(_eventInfo.EventHandlerType, this, DeliverEventMethodInfo);
        }

        #endregion Constructors

        #region Public Methods

        //
        //  Public Methods
        //

        /// <summary>
        /// Add a handler for the given source's event.
        /// </summary>
        public static void AddHandler(TEventSource source, string eventName, EventHandler<TEventArgs> handler)
        {
            ArgumentNullException.ThrowIfNull(handler);

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