Publisher-subscriber execution model in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Publisher-subscriber execution model

Publisher creates event

↓

Subscribers register handlers

↓

Publisher triggers event

↓

All subscribers receive notification

↓

Subscribers execute their handlers

↓

Event handling complete

The publisher creates an event and subscribers register their handlers. When the publisher triggers the event, all subscribers get notified and run their handlers.

Execution Sample

C Sharp (C#)

using System;

class Publisher {
  public event Action OnChange;
  public void Notify() => OnChange?.Invoke();
}

class Subscriber {
  public void Respond() => Console.WriteLine("Subscriber notified");
}

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

This code shows a publisher with an event and a subscriber that registers a handler. When the publisher notifies, the subscriber prints a message.

Execution Table

Step	Action	Event State	Subscribers Registered	Output
1	Create Publisher instance	No event triggered	None
2	Create Subscriber instance	No event triggered	None
3	Subscriber registers Respond handler	No event triggered	Respond handler added
4	Publisher calls Notify()	Event triggered	Respond handler added	Subscriber notified
5	Event handling complete	No event triggered	Respond handler remains

💡 Notify() finished, all subscribers notified

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
pub.OnChange	null	Respond handler added	Respond handler added	Respond handler added
sub	null	Subscriber instance	Subscriber instance	Subscriber instance

Key Moments - 3 Insights

Why does the event call use OnChange?.Invoke() instead of just OnChange()?

The ?. operator checks if any subscribers are registered before calling Invoke. This prevents errors if no handlers exist, as shown in step 4 of the execution_table.

What happens if multiple subscribers register to the same event?

All registered handlers are called in order when the event triggers. The execution_table shows one subscriber, but more would be called similarly.

Does the event keep subscribers after Notify() is called?

Yes, subscribers remain registered until explicitly removed. Step 5 shows the handler still registered after Notify().

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of pub.OnChange after step 3?

A Respond handler added

B null (no handlers)

C Event triggered

D Empty delegate

💡 Hint

Check the 'Event State' and 'Subscribers Registered' columns at step 3

At which step does the subscriber print 'Subscriber notified'?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Output' column in the execution_table

If no subscriber registers, what happens when Notify() is called?

A NullReferenceException error

B Event triggers but no output

C Nothing happens, safely ignored

D Program crashes

💡 Hint

Refer to the use of ?.Invoke() in the code and step 4 explanation

Concept Snapshot

Publisher-subscriber model:
- Publisher defines event (e.g., event Action OnChange)
- Subscribers register handlers (+=)
- Publisher triggers event (Invoke())
- All subscribers notified in order
- Use ?.Invoke() to avoid errors if no subscribers

Full Transcript

The publisher-subscriber execution model involves a publisher that creates an event and subscribers that register their handlers to this event. When the publisher triggers the event by calling Invoke, all registered subscribers receive the notification and execute their handlers. The ?.Invoke() syntax ensures safe calling even if no subscribers are registered. Subscribers remain registered until removed. This model allows decoupled communication where publishers do not need to know about subscribers directly.