Event-driven design pattern in C Sharp (C#)

Introduction

Event-driven design helps programs respond to actions like clicks or messages. It makes programs wait for events and react when they happen.

When building a user interface that reacts to button clicks or keyboard input.

When you want parts of your program to communicate without being tightly connected.

When handling messages or signals from sensors or external devices.

When you want to run code only after something specific happens, like a file download finishing.

Syntax

C Sharp (C#)

public class Publisher
{
    public event EventHandler? OnChange;

    public void RaiseEvent()
    {
        OnChange?.Invoke(this, EventArgs.Empty);
    }
}

public class Subscriber
{
    public void Subscribe(Publisher pub)
    {
        pub.OnChange += HandleEvent;
    }

    private void HandleEvent(object? sender, EventArgs e)
    {
        Console.WriteLine("Event received!");
    }
}

Events use delegates to connect publishers and subscribers.

The question mark (?) after the event name checks if there are subscribers before calling.

Examples

This line subscribes a method to an event so it runs when the event happens.

C Sharp (C#)

publisher.OnChange += SubscriberMethod;

This line unsubscribes a method so it no longer runs on the event.

C Sharp (C#)

publisher.OnChange -= SubscriberMethod;

This triggers the event safely, only if there are subscribers.

C Sharp (C#)

OnChange?.Invoke(this, EventArgs.Empty);

Sample Program

This program shows a publisher that raises an event and a subscriber that listens and reacts by printing a message.

C Sharp (C#)

using System;

public class Publisher
{
    public event EventHandler? OnChange;

    public void RaiseEvent()
    {
        Console.WriteLine("Publisher: Raising event...");
        OnChange?.Invoke(this, EventArgs.Empty);
    }
}

public class Subscriber
{
    public void Subscribe(Publisher pub)
    {
        pub.OnChange += HandleEvent;
    }

    private void HandleEvent(object? sender, EventArgs e)
    {
        Console.WriteLine("Subscriber: Event received!");
    }
}

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

        subscriber.Subscribe(publisher);

        publisher.RaiseEvent();
    }
}

Important Notes

Always unsubscribe from events when no longer needed to avoid memory leaks.

Events help keep code organized by separating who sends and who reacts.

Summary

Event-driven design lets programs react to actions or changes.

Publishers send events; subscribers listen and respond.

This pattern helps build interactive and flexible programs.

Practice

1. What is the main role of an event subscriber in the event-driven design pattern?

easy

A. To manage the program's user interface layout

B. To listen for events and respond when they occur

C. To store data for the program's use

D. To create and send events to other parts of the program

Solution

1. Step 1: Understand the roles in event-driven design

   In event-driven design, publishers send events and subscribers listen for these events.

2. Step 2: Identify subscriber's responsibility

   The subscriber's job is to react or respond when an event happens, meaning it listens and acts accordingly.

3. Final Answer:

   To listen for events and respond when they occur -> Option B

4. Quick Check:

   Subscriber listens and reacts = D [OK]

Hint: Subscribers always listen and react to events [OK]

Common Mistakes:

• Confusing subscriber with publisher
• Thinking subscriber creates events
• Assuming subscriber manages UI layout

2. Which of the following is the correct syntax to declare an event in C#?

easy

A. public void event MyEvent();

B. event public MyEvent EventHandler;

C. public event EventHandler MyEvent;

D. EventHandler public event MyEvent;

Solution

1. Step 1: Recall event declaration syntax in C#

   Events are declared with the keyword 'event' followed by a delegate type, then the event name.

2. Step 2: Check each option

   public event EventHandler MyEvent; matches the correct syntax: 'public event EventHandler MyEvent;'. Others have wrong order or keywords.

3. Final Answer:

   public event EventHandler MyEvent; -> Option C

4. Quick Check:

   Correct event declaration = B [OK]

Hint: Event declaration: 'public event DelegateType EventName;' [OK]

Common Mistakes:

• Mixing method and event syntax
• Wrong keyword order
• Missing 'event' keyword

3. What will be the output of the following C# code?

using System;
class Program {
  public static event Action OnNotify;
  static void Main() {
    OnNotify += () => Console.WriteLine("Event triggered");
    OnNotify?.Invoke();
  }
}

medium

A. Event triggered

B. No output

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand event subscription and invocation

   The event OnNotify is subscribed with a lambda that prints "Event triggered". The event is then invoked safely with ?.Invoke().

2. Step 2: Predict output

   Since OnNotify has a subscriber, invoking it calls the lambda, printing "Event triggered" to the console.

3. Final Answer:

   Event triggered -> Option A

4. Quick Check:

   Subscribed event invoked prints message = A [OK]

Hint: Subscribed event invocation prints output [OK]

Common Mistakes:

• Ignoring null-conditional operator ?.Invoke()
• Expecting no output without subscription
• Confusing event with method call syntax

4. Identify the error in this C# event-driven code snippet:

public class Publisher {
  public event EventHandler OnChange;
  public void Raise() {
    OnChange();
  }
}

medium

A. OnChange event is invoked without checking for null

B. EventHandler delegate is not declared

C. Raise method should be static

D. Event should be private

Solution

1. Step 1: Review event invocation rules

   Events can be null if no subscribers exist. Invoking without null check causes runtime error.

2. Step 2: Identify the error in code

   OnChange() is called directly without checking if OnChange is null, risking NullReferenceException.

3. Final Answer:

   OnChange event is invoked without checking for null -> Option A

4. Quick Check:

   Invoke event safely with null check = C [OK]

Hint: Always check event for null before invoking [OK]

Common Mistakes:

• Calling event without null check
• Assuming event is never null
• Misunderstanding event access modifiers

5. You want to design a system where multiple subscribers react differently to a button click event. Which approach best uses the event-driven pattern in C# to achieve this?

hard

A. Make subscribers poll the button state continuously in a loop

B. Create a list of methods and call them manually without events

C. Use a static method to handle all button clicks centrally

D. Declare a public event, let subscribers add their handlers, and invoke event on button click

Solution

1. Step 1: Understand event-driven design for multiple subscribers

   Events allow multiple subscribers to register handlers that get called automatically when the event fires.

2. Step 2: Evaluate options for best practice

   Declare a public event, let subscribers add their handlers, and invoke event on button click uses a public event and lets subscribers add handlers, which is the standard event-driven approach. Others are less efficient or not event-driven.

3. Final Answer:

   Declare a public event, let subscribers add their handlers, and invoke event on button click -> Option D

4. Quick Check:

   Use events for multiple subscriber reactions = A [OK]

Hint: Use events to notify multiple subscribers automatically [OK]

Common Mistakes:

• Manually calling subscriber methods instead of using events
• Using polling instead of event notifications
• Centralizing all logic in static methods