In C#, events are a powerful way to implement the publish-subscribe pattern. They allow one object to notify another when something happens.

For example, a button click, a file download completed, or a value change can be raised as events.

What is an Event?

An event is a notification sent by an object when an action occurs. Other objects can subscribe to these events and respond accordingly.

Real-world Analogy:

When the doorbell rings (event), the person inside (subscriber) gets notified and reacts.

Events Involve Two Main Components:

• Publisher – The class that raises the event.
• Subscriber – The class that responds to the event.

Syntax of Events

Events in C# are usually based on delegates.

public delegate void MyEventHandler(string message);

public class Publisher
{
    public event MyEventHandler OnPublish;

    public void TriggerEvent()
    {
        OnPublish?.Invoke("Hello from Publisher!");
    }
}

Example: Basic Event Usage

using System;

class Publisher
{
    public event Action OnDataPublished;

    public void PublishData()
    {
        Console.WriteLine("Publishing data...");
        OnDataPublished?.Invoke(); // Raise the event
    }
}

class Subscriber
{
    public void HandleData()
    {
        Console.WriteLine("Subscriber received the data.");
    }
}

class Program
{
    static void Main()
    {
        Publisher publisher = new Publisher();
        Subscriber subscriber = new Subscriber();

        // Subscribe to the event
        publisher.OnDataPublished += subscriber.HandleData;

        publisher.PublishData();
    }
}

Output: 

Publishing data...
Subscriber received the data.

Unsubscribing from Events

You can unsubscribe from events using -= operator to avoid memory leaks:

publisher.OnDataPublished -= subscriber.HandleData;

Event with Parameters (Using EventHandler)

You can pass data with events using EventHandler<T>.

public class DataEventArgs : EventArgs
{
    public string Data { get; set; }
}

public class Publisher
{
    public event EventHandler<DataEventArgs> DataPublished;

    public void Publish(string data)
    {
        DataPublished?.Invoke(this, new DataEventArgs { Data = data });
    }
}

class Subscriber
{
    public void OnDataReceived(object sender, DataEventArgs e)
    {
        Console.WriteLine("Data Received: " + e.Data);
    }
}

Why Use Events Instead of Direct Method Calls?

Summary

Events are crucial in building interactive, responsive, and decoupled systems — especially in GUI apps, services, and libraries.

Want to explore custom events, async event handlers, or how events work in WinForms/WPF?

Read More -

• Overview
• How to subscribe to and unsubscribe from events
• How to raise base class events in derived classes
• How to implement interface events
• How to implement custom event accessors