Publisher-subscriber execution model in C Sharp (C#)

Introduction

The publisher-subscriber model helps different parts of a program talk to each other without being tightly connected. It makes programs easier to change and grow.

When you want one part of your program to send messages to many other parts without knowing who they are.

When you want to react to events like button clicks, data updates, or messages from other systems.

When you want to keep your code organized by separating who sends information and who listens.

When building apps that need to update many things at once after an action happens.

When you want to add new features without changing the existing code that sends messages.

Syntax

C Sharp (C#)

using System;

// Define a delegate for the event
public delegate void MessageReceivedHandler(string message);

// Publisher class
public class Publisher
{
    // Event based on the delegate
    public event MessageReceivedHandler MessageReceived;

    // Method to send message to subscribers
    public void SendMessage(string message)
    {
        MessageReceived?.Invoke(message);
    }
}

// Subscriber class
public class Subscriber
{
    private string subscriberName;

    public Subscriber(string name)
    {
        subscriberName = name;
    }

    // Method to handle the event
    public void OnMessageReceived(string message)
    {
        Console.WriteLine($"{subscriberName} received: {message}");
    }
}

The event keyword creates a special kind of delegate that only the publisher can invoke.

Subscribers add their methods to the event to listen for messages.

Examples

If no one subscribes, sending a message does nothing.

C Sharp (C#)

// Example: No subscribers
Publisher publisher = new Publisher();
publisher.SendMessage("Hello"); // No output because no one listens

One subscriber listens and prints the message.

C Sharp (C#)

// Example: One subscriber
Publisher publisher = new Publisher();
Subscriber subscriber = new Subscriber("Alice");
publisher.MessageReceived += subscriber.OnMessageReceived;
publisher.SendMessage("Hello Alice");

Multiple subscribers get the message and each prints it.

C Sharp (C#)

// Example: Multiple subscribers
Publisher publisher = new Publisher();
Subscriber alice = new Subscriber("Alice");
Subscriber bob = new Subscriber("Bob");
publisher.MessageReceived += alice.OnMessageReceived;
publisher.MessageReceived += bob.OnMessageReceived;
publisher.SendMessage("Hello everyone");

After unsubscribing, the subscriber no longer receives messages.

C Sharp (C#)

// Example: Unsubscribe
Publisher publisher = new Publisher();
Subscriber alice = new Subscriber("Alice");
publisher.MessageReceived += alice.OnMessageReceived;
publisher.SendMessage("First message");
publisher.MessageReceived -= alice.OnMessageReceived;
publisher.SendMessage("Second message");

Sample Program

This program shows a publisher sending messages to two subscribers. Both receive the first message. Then Bob unsubscribes and only Alice receives the second message.

C Sharp (C#)

using System;

public delegate void MessageReceivedHandler(string message);

public class Publisher
{
    public event MessageReceivedHandler MessageReceived;

    public void SendMessage(string message)
    {
        Console.WriteLine($"Publisher sends: {message}");
        MessageReceived?.Invoke(message);
    }
}

public class Subscriber
{
    private string subscriberName;

    public Subscriber(string name)
    {
        subscriberName = name;
    }

    public void OnMessageReceived(string message)
    {
        Console.WriteLine($"{subscriberName} received: {message}");
    }
}

public class Program
{
    public static void Main()
    {
        Publisher newsPublisher = new Publisher();

        Subscriber alice = new Subscriber("Alice");
        Subscriber bob = new Subscriber("Bob");

        // Subscribers subscribe to the publisher's event
        newsPublisher.MessageReceived += alice.OnMessageReceived;
        newsPublisher.MessageReceived += bob.OnMessageReceived;

        Console.WriteLine("-- Sending first message --");
        newsPublisher.SendMessage("Breaking news: Publisher-subscriber model works!");

        // Bob unsubscribes
        newsPublisher.MessageReceived -= bob.OnMessageReceived;

        Console.WriteLine("-- Sending second message --");
        newsPublisher.SendMessage("Update: Bob has unsubscribed.");
    }
}

Important Notes

Time complexity: Sending a message calls all subscriber methods, so it is O(n) where n is the number of subscribers.

Space complexity: Storing subscribers uses O(n) space.

Common mistake: Forgetting to unsubscribe can cause memory leaks or unexpected behavior.

Use this model when you want loose coupling between parts of your program.

Summary

The publisher-subscriber model lets one part send messages to many listeners without knowing them.

Subscribers register methods to receive messages and can unsubscribe when needed.

This model helps keep code organized and flexible for changes.

Practice

1. In the publisher-subscriber model in C#, what is the main role of the subscriber?

easy

A. To send messages to multiple listeners

B. To register methods that respond to events from the publisher

C. To create new events for the publisher

D. To directly modify the publisher's data

Solution

1. Step 1: Understand the roles in the model

   The publisher sends messages or events, while the subscriber listens and reacts.

2. Step 2: Identify subscriber's responsibility

   Subscribers register their methods to handle events raised by the publisher.

3. Final Answer:

   To register methods that respond to events from the publisher -> Option B

4. Quick Check:

   Subscriber = method registration [OK]

Hint: Subscribers listen and react by registering methods [OK]

Common Mistakes:

• Confusing publisher and subscriber roles
• Thinking subscriber sends messages
• Assuming subscriber modifies publisher directly

2. Which of the following is the correct syntax to subscribe a method named OnDataReceived to an event DataReceived in C#?

easy

A. DataReceived += OnDataReceived;

B. DataReceived = OnDataReceived();

C. DataReceived -= OnDataReceived;

D. OnDataReceived += DataReceived;

Solution

1. Step 1: Recall event subscription syntax

   In C#, subscribing to an event uses the += operator with the method name (without parentheses).

2. Step 2: Match correct syntax

   DataReceived += OnDataReceived; correctly subscribes the method.

3. Final Answer:

   DataReceived += OnDataReceived; -> Option A

4. Quick Check:

   Subscribe event = += method [OK]

Hint: Use += to subscribe methods to events [OK]

Common Mistakes:

• Using = instead of +=
• Calling method with parentheses when subscribing
• Confusing subscription with unsubscription (-=)

3. Consider the following C# code snippet:

public class Publisher {
    public event Action Notify;
    public void Raise() {
        Notify?.Invoke();
    }
}

public class Subscriber {
    public void Respond() {
        Console.WriteLine("Event received");
    }
}

var pub = new Publisher();
var sub = new Subscriber();
pub.Notify += sub.Respond;
pub.Raise();

What will be the output when pub.Raise() is called?

medium

A. No output

B. NullReferenceException

C. Event received

D. Compile-time error

Solution

1. Step 1: Understand event subscription and invocation

   The subscriber's Respond method is subscribed to the Notify event, so it will be called when Raise() invokes Notify.

2. Step 2: Analyze output of Respond method

   Respond prints "Event received" to the console, so this text will appear.

3. Final Answer:

   Event received -> Option C

4. Quick Check:

   Subscribed method runs and prints message [OK]

Hint: Subscribed methods run on event invoke [OK]

Common Mistakes:

• Forgetting to subscribe the method
• Expecting no output if event is null
• Confusing event invocation syntax

4. The following code tries to unsubscribe a method from an event but causes a runtime error:

public class Publisher {
    public event Action Notify;
}

var pub = new Publisher();
pub.Notify -= () => Console.WriteLine("Hello");

What is the main issue causing the error?

medium

A. Lambda expressions cannot be used with events

B. Events cannot be unsubscribed in C#

C. The event Notify is not initialized

D. Cannot unsubscribe a lambda expression that was never subscribed

Solution

1. Step 1: Understand event unsubscription rules

   In C#, events start as null delegates. Unsubscribing (-=) from a null event throws NullReferenceException.

2. Step 2: Identify cause of runtime error

   No prior subscription means Notify is null, so pub.Notify -= lambda throws NullReferenceException. The lambda was never subscribed.

3. Final Answer:

   Cannot unsubscribe a lambda expression that was never subscribed -> Option D

4. Quick Check:

   Null event -= throws NullRef [OK]

Hint: Unsubscribe only delegates you subscribed before [OK]

Common Mistakes:

• Assuming unsubscribing any lambda works
• Thinking events cannot be unsubscribed
• Confusing null event invocation with unsubscription

5. You want to implement a publisher-subscriber system where multiple subscribers can register to receive messages, but some subscribers should only receive messages if a condition is true (e.g., message contains a keyword). Which approach best applies this conditional filtering in C#?

hard

A. Use event subscription with wrapper methods that check the condition before handling

B. Modify the publisher to send messages only to specific subscribers directly

C. Use multiple events for each condition and subscribe accordingly

D. Use a single event and let subscribers unsubscribe if condition is false

Solution

1. Step 1: Understand conditional filtering in subscriber model

   The publisher sends events to all subscribers; filtering is best done in subscriber methods.

2. Step 2: Apply wrapper methods for condition

   Subscribers can register methods that first check the condition and only proceed if true, keeping publisher simple.

3. Step 3: Evaluate other options

   Modifying publisher to target subscribers breaks decoupling; multiple events add complexity; unsubscribing dynamically is inefficient.

4. Final Answer:

   Use event subscription with wrapper methods that check the condition before handling -> Option A

5. Quick Check:

   Filter in subscriber methods for flexibility [OK]

Hint: Filter messages inside subscriber methods [OK]

Common Mistakes:

• Trying to filter inside publisher breaking decoupling
• Creating many events for each condition
• Unsubscribing subscribers dynamically instead of filtering