LuckyPennySoftware/MediatR · error · ArgumentException

notification does not implement $INotification

Error message

notification does not implement $INotification

What it means

The non-generic Publish(object) overload pattern-matches the payload: nulls throw ArgumentNullException, INotification instances are forwarded, and anything else throws ArgumentException. Note the message text itself contains a literal bug — the source reads `$"{nameof(notification)} does not implement ${nameof(INotification)}"`, so the rendered text is `notification does not implement $INotification` (a stray `$` before the interface name).

Source

Thrown at src/MediatR/Mediator.cs:137

    }

    public Task Publish<TNotification>(TNotification notification, CancellationToken cancellationToken = default)
        where TNotification : INotification
    {
        if (notification == null)
        {
            throw new ArgumentNullException(nameof(notification));
        }

        return PublishNotification(notification, cancellationToken);
    }

    public Task Publish(object notification, CancellationToken cancellationToken = default) =>
        notification switch
        {
            null => throw new ArgumentNullException(nameof(notification)),
            INotification instance => PublishNotification(instance, cancellationToken),
            _ => throw new ArgumentException($"{nameof(notification)} does not implement ${nameof(INotification)}")
        };

    /// <summary>
    /// Override in a derived class to control how the tasks are awaited. By default the implementation calls the <see cref="INotificationPublisher"/>.
    /// </summary>
    /// <param name="handlerExecutors">Enumerable of tasks representing invoking each notification handler</param>
    /// <param name="notification">The notification being published</param>
    /// <param name="cancellationToken">The cancellation token</param>
    /// <returns>A task representing invoking all handlers</returns>
    protected virtual Task PublishCore(IEnumerable<NotificationHandlerExecutor> handlerExecutors, INotification notification, CancellationToken cancellationToken) 
        => _publisher.Publish(handlerExecutors, notification, cancellationToken);

    private Task PublishNotification(INotification notification, CancellationToken cancellationToken = default)
    {
        var handler = _notificationHandlers.GetOrAdd(notification.GetType(), static notificationType =>
        {
            var wrapperType = typeof(NotificationHandlerWrapperImpl<>).MakeGenericType(notificationType);
            var wrapper = Activator.CreateInstance(wrapperType) ?? throw new InvalidOperationException($"Could not create wrapper for type {notificationType}");

View on GitHub (pinned to 916ef1b3d6)

Solutions

1. Make the type implement INotification: `public record FooHappened : INotification`.
2. If the payload is actually a request, call Send instead of Publish.
3. Use the generic Publish<TNotification> overload where TNotification : INotification to catch mismatches at compile time.

Example fix

// before
public class OrderShipped { public Guid Id; }
mediator.Publish(orderShipped); // ArgumentException
// after
public record OrderShipped(Guid Id) : INotification;
mediator.Publish(orderShipped);

if (notification is not INotification)
    throw new ArgumentException($"{notification?.GetType().Name ?? "null"} is not an INotification.", nameof(notification));

static bool IsNotification(object o) => o is INotification;

When it happens

Trigger: Calling mediator.Publish(someObject) where someObject does not implement INotification (e.g. a plain DTO, an event from another library, or a request mistakenly sent as a notification).

Common situations: Cross-system dispatch that boxes payloads as object and passes the wrong type; reusing a request type as a notification without implementing INotification; legacy payloads not yet migrated to the notification contract.

Related errors

• {requestType.Name} does not implement IRequest
• Could not create wrapper for type {notificationType}
• {requestType.Name} does not implement IStreamRequest<TRespon
• The type "{openBehaviorType.Name}" must implement IPipelineB
• Could not create wrapper type for {requestType}

AI-assisted analysis of LuckyPennySoftware/MediatR@916ef1b3d6 (2026-08-13). Data as JSON: /api/errors/7a369a91d7839b8a. Report an issue: GitHub.