Event unsubscription and memory in C Sharp (C#) - Mini Project: Build & Apply

Event unsubscription and memory

📖 Scenario: Imagine you have a simple program where a Button can be clicked, and a Listener responds to the click event. If you don't unsubscribe the listener when it's no longer needed, it can cause memory problems.

🎯 Goal: You will create a Button class with a click event, a Listener class that subscribes to the event, then unsubscribe the listener to avoid memory leaks.

📋 What You'll Learn

Create a Button class with a Click event

Create a Listener class with a method to handle the click

Subscribe the listener to the button's click event

Unsubscribe the listener from the click event

Print messages to show when the event is handled and when unsubscribed

💡 Why This Matters

🌍 Real World

Events are used in user interfaces, games, and many applications to respond to user actions or system changes.

💼 Career

Understanding event subscription and unsubscription is important for writing efficient, memory-safe C# applications.

1

Create the Button class with a Click event

Create a class called Button with a public event Click of type EventHandler. Also add a method OnClick() that invokes the Click event if it is not null.

C Sharp (C#)

// Your code here

Need a hint?

Use public event EventHandler? Click; and invoke it with Click?.Invoke(this, EventArgs.Empty);

2

Create the Listener class and subscribe to the Click event

Create a class called Listener with a method HandleClick that matches the EventHandler signature and prints "Button clicked!". Then create a Button instance called button and a Listener instance called listener. Subscribe listener.HandleClick to button.Click.

C Sharp (C#)

using System;

public class Button
{
    public event EventHandler? Click;

    public void OnClick()
    {
        Click?.Invoke(this, EventArgs.Empty);
    }
}

// Your code here

Need a hint?

Make sure HandleClick matches EventHandler signature and subscribe with button.Click += listener.HandleClick;

3

Unsubscribe the listener from the Click event

Add a line to unsubscribe listener.HandleClick from button.Click using -=. This should be after the subscription line.

C Sharp (C#)

using System;

public class Button
{
    public event EventHandler? Click;

    public void OnClick()
    {
        Click?.Invoke(this, EventArgs.Empty);
    }
}

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

class Program
{
    static void Main()
    {
        Button button = new Button();
        Listener listener = new Listener();
        button.Click += listener.HandleClick;
        // Unsubscribe the listener here
    }
}

Need a hint?

Use button.Click -= listener.HandleClick; to unsubscribe.

4

Test the event subscription and unsubscription

Call button.OnClick() twice: once before unsubscribing and once after unsubscribing. Print "Clicked once" before the first call and "Clicked twice" before the second call to show the difference.

C Sharp (C#)

using System;

public class Button
{
    public event EventHandler? Click;

    public void OnClick()
    {
        Click?.Invoke(this, EventArgs.Empty);
    }
}

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

class Program
{
    static void Main()
    {
        Button button = new Button();
        Listener listener = new Listener();
        button.Click += listener.HandleClick;
        button.Click -= listener.HandleClick;
        // Call OnClick and print messages
    }
}

Need a hint?

Print messages before each OnClick() call to show when the event handler runs.