Event-driven design pattern in C Sharp (C#) - Mini Project: Build & Apply

Event-driven design pattern

📖 Scenario: You are building a simple notification system where a Button can be clicked, and other parts of the program can react to this click event.This is like a real-world button that when pressed, triggers some action like turning on a light or playing a sound.

🎯 Goal: Create a basic event-driven program in C# where a Button class raises a Clicked event, and a Notifier class listens and responds by printing a message.

📋 What You'll Learn

Create a Button class with a Clicked event

Create a Notifier class with a method to handle the Clicked event

Raise the Clicked event when the button is pressed

Subscribe the Notifier to the Clicked event

Print a message when the event is handled

💡 Why This Matters

🌍 Real World

Event-driven design is used in user interfaces, games, and any program that reacts to user actions or other events.

💼 Career

Understanding events is essential for software developers working with GUI applications, web development, and real-time systems.

1

Create the Button class with a Clicked event

Create a class called Button with a public event Clicked of type EventHandler. Inside the class, add a method Press() that will later raise this event.

C Sharp (C#)

public class Button
{
    // Your code here
}

Need a hint?

Use public event EventHandler? Clicked; to declare the event.

The Press method is empty for now.

2

Create the Notifier class with an event handler method

Create a class called Notifier with a public method OnButtonClicked that takes object? sender and EventArgs e as parameters. Inside this method, write code to print "Button was clicked!".

C Sharp (C#)

using System;

public class Button
{
    public event EventHandler? Clicked;

    public void Press()
    {
        // Event will be raised later
    }
}

public class Notifier
{
    // Your code here
}

Need a hint?

The event handler method must match the EventHandler delegate signature.

3

Raise the Clicked event inside the Press method and subscribe the Notifier

Inside the Press() method of Button, raise the Clicked event safely using the null-conditional operator. Then, create a Button object and a Notifier object. Subscribe the Notifier's OnButtonClicked method to the Clicked event of the Button.

C Sharp (C#)

using System;

public class Button
{
    public event EventHandler? Clicked;

    public void Press()
    {
        // Raise the Clicked event here
    }
}

public class Notifier
{
    public void OnButtonClicked(object? sender, EventArgs e)
    {
        Console.WriteLine("Button was clicked!");
    }
}

// Your code here to create objects and subscribe

Need a hint?

Use Clicked?.Invoke(this, EventArgs.Empty); to raise the event safely.

Subscribe with button.Clicked += notifier.OnButtonClicked;.

4

Press the button to trigger the event and print the message

In the Main method, after subscribing, call the Press() method on the button object to trigger the Clicked event and print "Button was clicked!".

C Sharp (C#)

using System;

public class Button
{
    public event EventHandler? Clicked;

    public void Press()
    {
        Clicked?.Invoke(this, EventArgs.Empty);
    }
}

public class Notifier
{
    public void OnButtonClicked(object? sender, EventArgs e)
    {
        Console.WriteLine("Button was clicked!");
    }
}

class Program
{
    static void Main()
    {
        Button button = new Button();
        Notifier notifier = new Notifier();
        button.Clicked += notifier.OnButtonClicked;
        // Your code here to press the button
    }
}

Need a hint?

Call button.Press(); to trigger the event and see the message.