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Lesson 3515 min read

Events

Learn how C# events let objects notify subscribers when something happens, using the event keyword.

What is an Event?

An event is a special kind of delegate that implements the publish-subscribe pattern: one object announces "something happened," and any number of other objects can subscribe to react to it.

Declaring and Raising an Event

class Order
{
    public event Action<string> OrderShipped;


    public void Ship()
    {
        Console.WriteLine("Shipping order...");
        OrderShipped?.Invoke("Order #1234"); // raise the event
    }
}

Subscribing to an Event

var order = new Order();
order.OrderShipped += (id) => Console.WriteLine($"Email: {id} has shipped!");
order.OrderShipped += (id) => Console.WriteLine($"SMS: {id} is on its way.");


order.Ship();
Output

Click Run to see what this code prints.

The ?. Null-Conditional Operator

OrderShipped?.Invoke(...) safely does nothing if no one has subscribed — without the `?.`, invoking a null event would throw a NullReferenceException.

FAQs

An event is a delegate with restricted access — outside code can subscribe (+=) and unsubscribe (-=), but only the declaring class can raise (invoke) it.

Summary

  • Events implement the publish-subscribe pattern on top of delegates.
  • Multiple subscribers can react to a single event.
  • Always use `?.Invoke()` to raise an event safely.
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Lambda Expressions