C# Events

Events are built on delegates and implement the observer (publish-subscribe) pattern — one object notifies many others when something happens.

1 - Declaring and Raising an Event

public class Button
{
    // Declare the event using EventHandler (built-in delegate)
    public event EventHandler? Clicked;

    public void Click()
    {
        Console.WriteLine("Button clicked.");
        Clicked?.Invoke(this, EventArgs.Empty); // raise the event
    }
}

var btn = new Button();

// Subscribe to the event
btn.Clicked += (sender, e) => Console.WriteLine("Handler 1: reacting to click!");
btn.Clicked += (sender, e) => Console.WriteLine("Handler 2: logging the click.");

btn.Click();
// Button clicked.
// Handler 1: reacting to click!
// Handler 2: logging the click.

2 - Custom EventArgs

public class OrderEventArgs : EventArgs
{
    public int OrderId { get; }
    public decimal Total { get; }
    public OrderEventArgs(int id, decimal total) { OrderId = id; Total = total; }
}

public class OrderService
{
    public event EventHandler<OrderEventArgs>? OrderPlaced;

    public void PlaceOrder(int id, decimal total)
    {
        // process order...
        OrderPlaced?.Invoke(this, new OrderEventArgs(id, total));
    }
}

var svc = new OrderService();
svc.OrderPlaced += (s, e) =>
    Console.WriteLine($"Order #{e.OrderId} placed — Total: {e.Total:C}");

svc.PlaceOrder(42, 99.99m);
// Order #42 placed — Total: $99.99