Introduction

Events enable a class or object to notify other classes or objects when something of interest occurs. The class that sends (or raises) the event is called the publisher and the classes that receive (or handle) the event are called subscribers.

Delegate

All events are based on the EventHandler delegate, which is defined as follows:

public delegate void EventHandler(object sender, EventArgs e);

Steps in setting up an event

Publish the the event
Add a method for event invocation
Raise the event
Subscribe to the event
Add a method to handle the event

Publishing an event

Consider the following Car class that publishes and event

public class Car
    {
        public int Speed { get; set; } = 0;
        
        // 1. Publish an event
        public event EventHandler SpeedLimitReached;

        public Car(int speed)
        {
            Speed = speed;
        }

        public void Accelerate(int amount)
        {
            Speed = Speed + amount;

            // 3. Raise the event
            if(Speed > 50)
            {
                OnLimitReached(new EventArgs());
            }
        }

        // 2. Method to invoke the event
        public virtual void OnLimitReached(EventArgs e)
        {
            EventHandler handler = SpeedLimitReached;
            handler?.Invoke(this, e);
        }
    }

The EventHandler delegate is used to publish an event named ‘SpeedLimitReached’
A method named OnLimitReached is used for the event invocation. It must meet the signature requirements for the EventHandler delegate, ie returns a void and take an EventArgs argument
When the speed of the car reaches above 50, the event is raised

Subscribing to an event

Consider the following console app :

public class Program
    {
        static void Main(string[] args)
        {
            Car car = new Car(30);

            // 4. Subscribe to the event
            car.SpeedLimitReached += HandleSpeedLimitReached;

            Console.WriteLine("Car is accelerating ..");

            car.Accelerate(30);

        }

        // 5. Method to handle the event
        public static void HandleSpeedLimitReached(object sender, EventArgs e)
        {
            Console.WriteLine("Speed limit reached, time to slow down!");
        }
    }

Car is accelerating ..
Speed limit reached, time to slow down!

Subscribe to the event by specifying a handler
The method HandleSpeedLimitReached is used to handle the event. It must satisfy the signature for the EventHandler delegate. When the speed limit is reached, it raises a message in the console window