// Example event.
public class ProductRegisteredEvent
{
    public int ProductId { get; }
    public string ProductName { get; }
    public ProductRegisteredEvent(int productId, string productName)
    {
        ProductId = productId;
        ProductName = productName;
    }
}
// Example event handlers.
public class ProductRegisteredEventHandlers : IEventHandler<ProductRegisteredEvent>
{
    // Sync event handler.
    [EventHandler]
    public void Handle(ProductRegisteredEvent @event)
    {
        System.Console.WriteLine($"ProductRegisteredEventHandler handled {@event.GetType()}.");
    }
    
    // Async event handler.
    [EventHandler]
    public Task SendEmailNotificationAsync(ProductRegisteredEvent @event, CancellationToken ct)
    {
        System.Console.WriteLine($"Sending email notification...");
        return Task.CompletedTask;
    }
}

// Event Handler Registration.
public EventDelegator RegisterEventHandlers()
{
    // MultiMessageHandlerRegistration allows registration of a multiple message handlers per message type.
    var registration = new MultiMessageHandlerRegistration();
    // Register all methods with [EventHandler] attribute.
    registration.RegisterEventHandlerAttributes(() => new ProductRegisteredEventHandlers(new ProductRepository());
    // Build the event delegator.
    IMessageHandlerResolver resolver = registration.BuildMessageHandlerResolver();
    return new EventDelegator(resolver);
}