Mastering Design Patterns in C# and ASP.NET Core
Part 4.7 – Observer Design Pattern
Series: Design Patterns in C# and ASP.NET Core
Pattern Category: Behavioral Design Pattern
Difficulty: ⭐⭐⭐☆☆ Intermediate
Prerequisites: C#, OOP, Interfaces, Delegates, Events, Dependency Injection, ASP.NET Core
Table of Contents
Introduction
What is the Observer Design Pattern?
Why Do We Need the Observer Pattern?
Real-World Analogy
Subject and Observer Concepts
How the Observer Pattern Works
UML Class Diagram
Components of the Observer Pattern
Complete C# Console Application
Observer Pattern Using C# Events
IObservable<T>andIObserver<T>ASP.NET Core Implementation
Banking Notification Example
E-Commerce Order Notification Example
Stock Price Monitoring Example
Event-Driven Architecture
Observer vs Pub/Sub
Observer vs Mediator
Observer vs Event-Driven Architecture
Advantages
Disadvantages
Best Practices
Common Mistakes
Real-World Enterprise Scenarios
Interview Questions
Key Takeaways
Conclusion
Coming Up Next
1. Introduction
In real-world applications, one object often needs to notify multiple other objects when something changes.
For example:
Order Status Changed
|
+----> Email Notification
|
+----> SMS Notification
|
+----> Push Notification
|
+----> Audit Logging
|
+----> Analytics
A common question is:
How can one object notify multiple dependent objects without becoming tightly coupled to them?
The answer is the Observer Design Pattern.
The Observer Pattern is one of the most useful behavioral patterns in modern software development and has strong connections with:
C# events
Delegates
IObservable<T>Reactive programming
Notification systems
Event-driven applications
UI updates
Domain events
2. What is the Observer Design Pattern?
The Observer Design Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependent objects are automatically notified.
The object being observed is generally called the:
Subject
The objects receiving notifications are called:
Observers
The basic structure is:
Subject
|
State changes
|
+-------+-------+
| | |
↓ ↓ ↓
Observer Observer Observer
When the Subject changes:
Subject
|
| Notify()
|
+------> Observer 1
|
+------> Observer 2
|
+------> Observer 3
The Subject does not need to know the detailed implementation of each Observer.
3. Why Do We Need the Observer Pattern?
Consider an e-commerce application.
When an order status changes:
Order #1001
Status = Shipped
Several things may need to happen:
Order Status Changed
|
+---- Email Customer
|
+---- Send SMS
|
+---- Push Notification
|
+---- Update Analytics
|
+---- Write Audit Log
A poor implementation might look like:
public void UpdateOrderStatus()
{
SendEmail();
SendSms();
SendPushNotification();
UpdateAnalytics();
WriteAuditLog();
}
This creates strong coupling.
The OrderService now knows about:
Email service
SMS service
Push notification service
Analytics service
Audit service
If tomorrow we add:
WhatsApp Notification
the order service must change.
The Observer Pattern allows us to separate these responsibilities.
4. Real-World Analogy
Imagine subscribing to a YouTube channel.
You are an observer.
The YouTube channel is the subject.
When a new video is published:
YouTube Channel
|
| New Video
↓
Subscribers
|
+---- Subscriber A
+---- Subscriber B
+---- Subscriber C
+---- Subscriber D
The channel doesn't need to individually contact every subscriber using custom logic.
Subscribers register themselves.
When something happens, the subscribers receive a notification.
This is the basic idea behind the Observer Pattern.
5. Subject and Observer Concepts
The pattern usually consists of two main abstractions.
Subject
Responsible for:
Maintaining observers
Adding observers
Removing observers
Notifying observers
Example:
public interface ISubject
{
void Attach(IObserver observer);
void Detach(IObserver observer);
void Notify();
}
Observer
Responsible for responding to notifications.
public interface IObserver
{
void Update();
}
The Subject communicates with observers through the abstraction.
6. How the Observer Pattern Works
The lifecycle is:
Step 1 – Create Subject
StockPrice
Step 2 – Create Observers
Mobile App
Web App
Trading Dashboard
Step 3 – Subscribe
StockPrice
|
+---- Mobile App
+---- Web App
+---- Dashboard
Step 4 – State Changes
Stock Price
$150 → $155
Step 5 – Notify
Notify()
Step 6 – Observers React
Mobile App → Update
Web App → Update
Dashboard → Update
7. UML Class Diagram
A traditional Observer Pattern UML diagram looks like this:
+----------------------+
| Subject |
+----------------------+
| - observers |
+----------------------+
| + Attach() |
| + Detach() |
| + Notify() |
+----------+-----------+
|
|
+----------v-----------+
| IObserver |
+-----------------------+
| + Update() |
+----------+------------+
|
+-------------+-------------+
| |
+--------v---------+ +---------v--------+
| ConcreteObserver | | ConcreteObserver |
+------------------+ +------------------+
| + Update() | | + Update() |
+------------------+ +------------------+
The key relationship is:
Subject → many Observers
8. Components of the Observer Pattern
8.1 Subject
Maintains the collection of observers.
public interface ISubject
{
void Attach(IObserver observer);
void Detach(IObserver observer);
void Notify();
}
8.2 Concrete Subject
Contains the actual state.
public class Stock : ISubject
{
private readonly List<IObserver> _observers = new();
public decimal Price { get; private set; }
public void Attach(IObserver observer)
{
_observers.Add(observer);
}
public void Detach(IObserver observer)
{
_observers.Remove(observer);
}
public void Notify()
{
foreach (var observer in _observers)
{
observer.Update();
}
}
public void SetPrice(decimal price)
{
Price = price;
Notify();
}
}
8.3 Observer
public interface IObserver
{
void Update();
}
8.4 Concrete Observer
public class MobileApp : IObserver
{
public void Update()
{
Console.WriteLine(
"Mobile App received stock update.");
}
}
9. Complete C# Console Application
Let's build a complete stock price notification application.
Step 1 – Observer Interface
public interface IObserver
{
void Update(decimal price);
}
Step 2 – Subject Interface
public interface ISubject
{
void Attach(IObserver observer);
void Detach(IObserver observer);
void Notify();
}
Step 3 – Concrete Subject
public class Stock : ISubject
{
private readonly List<IObserver> _observers = new();
public string Symbol { get; }
public decimal Price { get; private set; }
public Stock(string symbol)
{
Symbol = symbol;
}
public void Attach(IObserver observer)
{
_observers.Add(observer);
}
public void Detach(IObserver observer)
{
_observers.Remove(observer);
}
public void Notify()
{
foreach (var observer in _observers)
{
observer.Update(Price);
}
}
public void SetPrice(decimal price)
{
Price = price;
Console.WriteLine(
$"{Symbol} price changed to {Price:C}");
Notify();
}
}
Step 4 – Concrete Observers
Mobile Application
public class MobileApp : IObserver
{
public void Update(decimal price)
{
Console.WriteLine(
$"Mobile App: Stock price updated to {price:C}");
}
}
Web Dashboard
public class WebDashboard : IObserver
{
public void Update(decimal price)
{
Console.WriteLine(
$"Web Dashboard: Stock price updated to {price:C}");
}
}
Trading System
public class TradingSystem : IObserver
{
public void Update(decimal price)
{
Console.WriteLine(
$"Trading System: Received price {price:C}");
}
}
Step 5 – Program
var stock = new Stock("MSFT");
var mobileApp = new MobileApp();
var webDashboard = new WebDashboard();
var tradingSystem = new TradingSystem();
stock.Attach(mobileApp);
stock.Attach(webDashboard);
stock.Attach(tradingSystem);
stock.SetPrice(150);
Console.WriteLine();
stock.SetPrice(155);
Console.WriteLine();
stock.Detach(webDashboard);
stock.SetPrice(160);
Possible output:
MSFT price changed to $150.00
Mobile App: Stock price updated to $150.00
Web Dashboard: Stock price updated to $150.00
Trading System: Received price $150.00
MSFT price changed to $155.00
Mobile App: Stock price updated to $155.00
Web Dashboard: Stock price updated to $155.00
Trading System: Received price $155.00
MSFT price changed to $160.00
Mobile App: Stock price updated to $160.00
Trading System: Received price $160.00
Notice what happened:
WebDashboard
|
↓
Detach()
It no longer receives notifications.
10. Observer Pattern Using C# Events
In modern C#, we don't always need to manually maintain:
List<IObserver>
C# provides:
Delegates
Events
These are frequently used to implement observer-like behavior.
For example:
public class Stock
{
public event EventHandler<StockPriceChangedEventArgs>?
PriceChanged;
public decimal Price { get; private set; }
public void SetPrice(decimal price)
{
Price = price;
PriceChanged?.Invoke(
this,
new StockPriceChangedEventArgs(price));
}
}
Event arguments:
public class StockPriceChangedEventArgs
: EventArgs
{
public decimal Price { get; }
public StockPriceChangedEventArgs(
decimal price)
{
Price = price;
}
}
Subscribe:
stock.PriceChanged +=
OnStockPriceChanged;
Handler:
void OnStockPriceChanged(
object? sender,
StockPriceChangedEventArgs e)
{
Console.WriteLine(
$"Price changed to {e.Price:C}");
}
Unsubscribe:
stock.PriceChanged -=
OnStockPriceChanged;
This is a very common implementation of observer-style notification in C#.
11. How C# Events Work
The relationship is:
Publisher
|
| event
↓
Delegate Invocation List
|
+---- Subscriber 1
+---- Subscriber 2
+---- Subscriber 3
When:
PriceChanged?.Invoke(...);
is executed, subscribed handlers are called.
Therefore:
Event
↓
Delegate
↓
Multiple Subscribers
This is why understanding the Observer Pattern helps developers understand C# events.
12. IObservable<T> and IObserver<T>
.NET also provides explicit observer abstractions:
IObservable<T>
IObserver<T>
The IObservable<T> represents the producer.
The IObserver<T> represents the subscriber.
The observer interface provides:
void OnNext(T value);
void OnError(Exception error);
void OnCompleted();
For example:
public class StockObserver
: IObserver<decimal>
{
public void OnNext(decimal value)
{
Console.WriteLine(
$"Price received: {value:C}");
}
public void OnError(Exception error)
{
Console.WriteLine(
$"Error: {error.Message}");
}
public void OnCompleted()
{
Console.WriteLine(
"Stock stream completed.");
}
}
This model is especially useful when working with streams of data and reactive programming concepts.
13. ASP.NET Core Implementation
Let's create a realistic notification example.
Suppose an order changes status:
Order
↓
Status Changed
↓
Notification Subscribers
We might have:
Order Service
|
+---- Email
|
+---- SMS
|
+---- Push Notification
|
+---- Audit Log
14. Order Event
Create an event model:
public record OrderStatusChangedEvent(
int OrderId,
string Status);
15. Observer Interface
public interface IOrderObserver
{
Task UpdateAsync(
OrderStatusChangedEvent orderEvent);
}
16. Email Observer
public class EmailNotificationObserver
: IOrderObserver
{
public Task UpdateAsync(
OrderStatusChangedEvent orderEvent)
{
Console.WriteLine(
$"Email sent for Order {orderEvent.OrderId}");
return Task.CompletedTask;
}
}
17. SMS Observer
public class SmsNotificationObserver
: IOrderObserver
{
public Task UpdateAsync(
OrderStatusChangedEvent orderEvent)
{
Console.WriteLine(
$"SMS sent for Order {orderEvent.OrderId}");
return Task.CompletedTask;
}
}
18. Audit Observer
public class AuditObserver
: IOrderObserver
{
public Task UpdateAsync(
OrderStatusChangedEvent orderEvent)
{
Console.WriteLine(
$"Audit entry created for Order " +
$"{orderEvent.OrderId}");
return Task.CompletedTask;
}
}
19. Order Notification Service
public interface IOrderNotificationService
{
Task NotifyAsync(
OrderStatusChangedEvent orderEvent);
}
Implementation:
public class OrderNotificationService
: IOrderNotificationService
{
private readonly IEnumerable<IOrderObserver>
_observers;
public OrderNotificationService(
IEnumerable<IOrderObserver> observers)
{
_observers = observers;
}
public async Task NotifyAsync(
OrderStatusChangedEvent orderEvent)
{
foreach (var observer in _observers)
{
await observer.UpdateAsync(orderEvent);
}
}
}
This is a powerful ASP.NET Core technique.
The service doesn't need to know:
Email
SMS
Audit
Push
It simply receives all registered observers.
20. Register Observers with Dependency Injection
In Program.cs:
builder.Services.AddScoped<
IOrderObserver,
EmailNotificationObserver>();
builder.Services.AddScoped<
IOrderObserver,
SmsNotificationObserver>();
builder.Services.AddScoped<
IOrderObserver,
AuditObserver>();
builder.Services.AddScoped<
IOrderNotificationService,
OrderNotificationService>();
When we inject:
IEnumerable<IOrderObserver>
ASP.NET Core provides all registered implementations.
Therefore:
IEnumerable<IOrderObserver>
|
+---- EmailNotificationObserver
+---- SmsNotificationObserver
+---- AuditObserver
This is an excellent example of combining:
Observer Pattern
Dependency Injection
Open/Closed Principle
21. ASP.NET Core Controller
[ApiController]
[Route("api/orders")]
public class OrdersController : ControllerBase
{
private readonly IOrderNotificationService
_notificationService;
public OrdersController(
IOrderNotificationService notificationService)
{
_notificationService = notificationService;
}
[HttpPost("{id}/ship")]
public async Task<IActionResult> ShipOrder(
int id)
{
var orderEvent =
new OrderStatusChangedEvent(
id,
"Shipped");
await _notificationService
.NotifyAsync(orderEvent);
return Ok(new
{
OrderId = id,
Status = "Shipped"
});
}
}
The controller doesn't know about:
Email
SMS
Audit
It only interacts with the notification abstraction.
22. Banking Example
Consider a banking application.
A transaction occurs:
$500 deposited
Several components may need to react:
Bank Transaction
|
+---- Account Balance
|
+---- Email Notification
|
+---- SMS Notification
|
+---- Fraud Detection
|
+---- Audit Log
|
+---- Analytics
Instead of putting all logic into the transaction service, observers can subscribe to transaction events.
For example:
public record TransactionCompletedEvent(
string AccountNumber,
decimal Amount,
string TransactionType);
Observers:
EmailObserver
SmsObserver
FraudDetectionObserver
AuditObserver
AnalyticsObserver
This makes the system easier to extend.
23. E-Commerce Order Example
Suppose:
Order #1001
Status = Delivered
Observers might include:
CustomerNotificationObserver
InventoryObserver
AnalyticsObserver
AuditObserver
LoyaltyPointsObserver
Flow:
Order Service
|
Order Delivered
|
+-----------+-----------+
| | |
↓ ↓ ↓
Customer Inventory Analytics
Notify Update Update
Adding another subscriber does not necessarily require changing the core order logic.
24. Stock Price Monitoring
Another common example is financial market monitoring.
Stock Price Service
|
| Price Changed
|
+---- Trading Dashboard
|
+---- Mobile Application
|
+---- Alert Service
|
+---- Analytics
If the price becomes:
$150 → $155
all subscribers can receive the update.
25. Event-Driven Architecture
Observer Pattern concepts are closely related to event-driven architecture, but they are not identical.
A simple in-process observer system:
Object
↓
Event
↓
Observers
An enterprise event-driven architecture may look like:
Order Service
|
| OrderCreated
↓
Message Broker
|
+---- Notification Service
|
+---- Inventory Service
|
+---- Analytics Service
|
+---- Shipping Service
The important difference is that enterprise systems often use:
Message brokers
Queues
Topics
Durable messages
Retry policies
Dead-letter queues
Distributed consumers
Examples include Azure Service Bus, RabbitMQ, Kafka, and other messaging technologies.
The Observer Pattern provides the conceptual foundation for one-to-many notification, while distributed messaging provides a different architectural mechanism for implementing event-driven systems.
26. Observer vs Pub/Sub
These concepts are related but not exactly the same.
| Observer | Pub/Sub |
|---|---|
| Subject knows observers | Publisher generally doesn't know subscribers |
| Often in-process | Commonly distributed |
| Direct notification | Usually broker/topic based |
| Simple communication | Supports larger distributed systems |
| Often synchronous | Can be asynchronous |
Observer:
Subject
|
+---- Observer A
+---- Observer B
Pub/Sub:
Publisher
|
↓
Message Broker
|
+---- Subscriber A
+---- Subscriber B
27. Observer vs Mediator
These patterns can look similar.
Observer
Focuses on:
One-to-many notification
Example:
Order
|
+---- Email
+---- SMS
+---- Audit
Mediator
Focuses on:
Centralized communication between multiple components
Example:
Component A
|
Component B → Mediator ← Component C
|
Component D
The Observer pattern is primarily about notification.
Mediator is primarily about coordinating communication.
28. Observer vs Event-Driven Architecture
Observer:
Subject
↓
Observers
Event-driven architecture:
Producer
↓
Event Broker
↓
Consumers
Observer is often an object-level design pattern.
Event-driven architecture is an architectural approach.
29. Advantages of Observer Pattern
1. Loose Coupling
The Subject depends on an abstraction instead of concrete observers.
2. Open/Closed Principle
New observers can often be added without modifying the Subject.
Existing Subject
+
New Observer
3. Supports One-to-Many Communication
One state change can notify many components.
4. Extensibility
Adding:
Email
SMS
Push
Audit
Analytics
can be straightforward.
5. Separation of Responsibilities
The Subject handles state.
Observers handle their respective reactions.
6. Natural Fit for Notifications
Examples:
Order Status
Stock Price
Bank Transaction
System Alerts
30. Disadvantages of Observer Pattern
1. Too Many Observers
If hundreds of observers are registered, a single notification can trigger substantial work.
2. Notification Order
Unless explicitly designed, observers may not have a guaranteed execution order.
3. Hidden Dependencies
It may not be obvious from reading the Subject that multiple observers will execute when an event occurs.
4. Error Handling
One observer throwing an exception can potentially affect other observers, depending on how notification is implemented.
5. Memory Leaks with Events
Long-lived publishers holding references to subscribers can prevent subscribers from being garbage collected.
This is particularly important in long-running applications.
6. Synchronous Processing Can Become Slow
If every observer executes synchronously:
Subject
↓
Observer 1 → 100 ms
↓
Observer 2 → 500 ms
↓
Observer 3 → 300 ms
the total operation can become slow.
For distributed or long-running work, asynchronous messaging or background processing may be more appropriate.
31. Best Practices
1. Depend on Abstractions
Use:
IOrderObserver
instead of concrete implementations.
2. Keep Observers Focused
An observer should ideally have one clear responsibility.
For example:
EmailObserver
should focus on email notification.
3. Avoid Heavy Synchronous Work
Don't perform long-running operations inside a request thread unnecessarily.
Consider:
Event
↓
Queue
↓
Background Worker
when appropriate.
4. Handle Observer Failures Carefully
Don't allow one optional observer to unintentionally prevent all other observers from running.
5. Unsubscribe When Necessary
For event-based systems:
publisher.Event -= Handler;
is important when the subscriber's lifetime is shorter than the publisher's lifetime.
6. Use Immutable Event Data
Records are convenient:
public record OrderStatusChangedEvent(
int OrderId,
string Status);
The event payload should generally represent a stable fact.
7. Don't Use Observer Everywhere
If a simple direct method call is sufficient, introducing Observer may add unnecessary complexity.
32. Common Mistakes
Mistake 1 – Strong Coupling
Avoid:
public class OrderService
{
private readonly EmailService _email;
private readonly SmsService _sms;
private readonly AuditService _audit;
}
when the business requirement is better represented as extensible notification subscribers.
Mistake 2 – Too Much Logic in the Subject
The Subject should not perform every observer's business responsibility.
Mistake 3 – Forgetting to Unsubscribe
With event-based subscriptions, improper lifetime management can create memory retention problems.
Mistake 4 – Ignoring Exceptions
If multiple observers are notified synchronously, determine what should happen when one fails.
Mistake 5 – Confusing Observer with Distributed Messaging
An in-memory event does not automatically provide:
Durability
Retry
Cross-service communication
Guaranteed delivery
Dead-letter handling
Those requirements typically need messaging infrastructure.
Mistake 6 – Assuming Notification Order
Don't rely on a specific observer execution order unless the application explicitly guarantees it.
33. Real-World Enterprise Scenarios
The Observer Pattern can be useful in:
Banking
Transaction
↓
Notifications
Fraud Detection
Audit
Analytics
E-Commerce
Order Created
↓
Inventory
Payment
Notification
Shipping
Analytics
Monitoring
System Alert
↓
Email
SMS
Dashboard
Logging
Stock Market Applications
Price Changed
↓
Mobile
Web
Trading
Alerts
User Management
User Registered
↓
Welcome Email
Audit
Analytics
Profile Initialization
Workflow Systems
Workflow State Changed
↓
Notification
Audit
Task Assignment
Analytics
34. Observer Pattern and SOLID
Observer works particularly well with several SOLID principles.
Single Responsibility Principle
Each observer handles one responsibility.
EmailObserver → Email
SmsObserver → SMS
AuditObserver → Audit
Open/Closed Principle
New observers can be introduced without modifying the existing Subject.
Dependency Inversion Principle
The Subject depends on:
IObserver
instead of:
ConcreteObserver
35. Observer Pattern and Dependency Injection
ASP.NET Core makes Observer implementations particularly clean with:
IEnumerable<IOrderObserver>
For example:
public OrderNotificationService(
IEnumerable<IOrderObserver> observers)
{
_observers = observers;
}
The service automatically receives all registered implementations.
This means the architecture can evolve from:
Email
to:
Email
SMS
Push
Audit
Analytics
without changing the notification service's core structure.
36. Observer Pattern and Asynchronous Processing
Suppose an order API receives:
POST /api/orders
If the application synchronously performs:
Email
SMS
Push
Analytics
Audit
the API response may become slower.
An alternative is:
API
↓
Order Created Event
↓
Message Queue
↓
Consumers
For example:
Order Service
|
↓
OrderCreated
|
↓
Message Broker
/ | \
/ | \
↓ ↓ ↓
Email Inventory Analytics
Service Service Service
This is no longer simply an in-process Observer Pattern; it becomes distributed event-driven architecture.
37. Observer Pattern in Modern .NET
There are several ways to implement Observer-like behavior in .NET:
Option 1 – Interfaces
IObserver
Useful for explicit pattern implementation.
Option 2 – Events
event EventHandler
Very common for in-process notifications.
Option 3 – IObservable<T>
Useful for observable streams.
Option 4 – Application Events
Useful for decoupling application components.
Option 5 – Domain Events
Useful for business events inside domain-driven applications.
Option 6 – Message Brokers
Useful for distributed event-driven systems.
38. Interview Questions
Beginner
1. What is the Observer Design Pattern?
The Observer Pattern defines a one-to-many dependency where observers are notified when the subject changes.
2. What are the main components?
Subject
Observer
Concrete Subject
Concrete Observer
3. What is a Subject?
The object whose state changes and which notifies observers.
4. What is an Observer?
An object that receives notifications and reacts to changes.
5. What problem does Observer solve?
It reduces coupling between an object that generates changes and objects that need to react to those changes.
Intermediate
6. How is Observer implemented in C#?
Common approaches include:
Interfaces
Delegates
Events
IObservable<T>
7. How do C# events relate to Observer?
C# events provide a built-in mechanism for one-to-many notification and are frequently used to implement Observer-style behavior.
8. What is IObservable<T>?
It represents a producer of observable values that can be subscribed to by implementations of IObserver<T>.
9. What are the methods in IObserver<T>?
OnNext()
OnError()
OnCompleted()
10. What is the difference between Observer and Pub/Sub?
Observer usually represents direct object-level notification, while Pub/Sub commonly introduces an intermediary such as a message broker.
39. Advanced Interview Questions
11. What is the difference between Observer and Mediator?
Observer focuses on one-to-many notification.
Mediator centralizes communication and coordination between multiple components.
12. Can Observer be asynchronous?
Yes.
Observers can expose:
Task UpdateAsync(...)
or an asynchronous event-processing architecture can be introduced.
13. What happens if one observer throws an exception?
It depends on the implementation.
A robust notification system should define whether:
Processing stops
Other observers continue
Errors are logged
Failed work is retried
14. Can Observer cause memory leaks?
Yes, especially with events.
A long-lived publisher can keep references to subscribers that should otherwise be eligible for garbage collection.
15. How can you avoid event-related memory leaks?
Unsubscribe appropriately:
publisher.Event -= Handler;
Also consider subscription lifetime and ownership carefully.
16. How would you implement Observer in ASP.NET Core?
One approach is:
IObserver
↓
Multiple Implementations
↓
Dependency Injection
↓
IEnumerable<IObserver>
17. Is Observer suitable for microservices?
The Observer concept can inspire event-driven communication, but direct in-memory Observer implementations generally do not provide reliable cross-service communication.
For microservices, consider:
Message brokers
Events
Queues
Topics
Durable messaging
18. What is a Domain Event?
A domain event represents something meaningful that happened within the business domain.
For example:
OrderPlaced
PaymentCompleted
OrderShipped
CustomerRegistered
Observers or event handlers can react to those events.
19. What is the difference between an Event and an Event Handler?
The event represents something that happened.
The event handler contains the logic that reacts to that event.
Event
↓
Handler
20. When should you avoid Observer?
Avoid it when:
There are only two tightly related components.
A direct method call is simpler.
Notification behavior is difficult to understand.
The observer chain becomes excessively complicated.
A distributed messaging solution is actually required.
40. Practical Enterprise Architecture
A modern .NET application might evolve like this:
Order Service
|
↓
OrderPlaced Event
|
+-----------+-----------+
| | |
↓ ↓ ↓
Email Audit Analytics
Observer Observer Observer
For a distributed architecture:
Order Service
|
↓
OrderPlaced
|
↓
Message Broker
/ | \
/ | \
↓ ↓ ↓
Notification Inventory Analytics
Service Service Service
The first is an in-process Observer-style design.
The second is an event-driven distributed architecture.
41. Observer Pattern – Complete Conceptual Flow
The complete lifecycle is:
1. Create Subject
↓
2. Create Observers
↓
3. Attach / Subscribe
↓
4. Subject State Changes
↓
5. Subject Raises Notification
↓
6. Observers Receive Notification
↓
7. Each Observer Performs Its Responsibility
↓
8. Observer Can Unsubscribe
In code:
subject.Attach(observer);
subject.ChangeState();
subject.Notify();
subject.Detach(observer);
Or with C# events:
publisher.Event += Handler;
publisher.ChangeState();
publisher.Event -= Handler;
42. Key Takeaways
Remember these important points:
1. Observer is a Behavioral Design Pattern
It focuses on communication between objects.
2. It represents a one-to-many relationship
One Subject
↓
Many Observers
3. It reduces coupling
The Subject does not need to know the concrete implementation of observers.
4. C# events are closely related to Observer
event EventHandler
is a common way to implement one-to-many notification.
5. IObservable<T> is another .NET mechanism
It provides an explicit observable/subscriber model.
6. Dependency Injection works very well with Observer
ASP.NET Core can inject:
IEnumerable<IOrderObserver>
to provide multiple observers.
7. Observer is not the same as Pub/Sub
Observer is usually direct object-level notification.
Pub/Sub typically uses an intermediary.
8. Observer is not the same as distributed event-driven architecture
For microservices, reliable communication usually requires messaging infrastructure.
9. Watch observer lifetime
Especially when using C# events.
10. Keep observers focused
Each observer should ideally have a clear responsibility.
Conclusion
The Observer Design Pattern is one of the most practical behavioral patterns for modern software development.
It provides a clean way to establish:
One-to-Many Communication
without tightly coupling the publisher to every consumer.
The basic architecture is:
Subject
|
State Changed
|
+--------+--------+
| | |
↓ ↓ ↓
Observer Observer Observer
In C#, this concept appears naturally through:
Interfaces
Delegates
Events
IObservable<T>
In ASP.NET Core applications, it can be combined with:
Dependency Injection
Domain Events
Application Events
Notification Handlers
And in distributed systems, the same fundamental one-to-many notification concept can evolve into:
Producer
↓
Event / Message
↓
Message Broker
↓
Multiple Consumers
The most important lesson is:
Use the Observer Pattern when multiple components need to react to a change without making the component that produces the change tightly coupled to every consumer.
It is particularly valuable for notification systems, order processing, stock monitoring, banking events, UI updates, audit logging, analytics, and event-driven application designs.
🚀 Coming Up Next: Part 4.8 – State Design Pattern
In the next article, we'll explore the State Design Pattern, including:
What is the State Pattern?
Why do we need it?
State-dependent behavior
Eliminating large
if-elseandswitchstatementsContext and State concepts
UML Class Diagram
Complete C# Console Application
Order processing example
Banking account state example
ASP.NET Core implementation
State transitions
State machines
Real-world enterprise examples
State vs Strategy
State vs Command
State vs Chain of Responsibility
Advantages and disadvantages
Best practices
Common mistakes
Interview questions
The State Pattern is particularly useful in enterprise applications where an object's behavior changes significantly based on its current state—for example, Order Pending → Confirmed → Shipped → Delivered → Cancelled.
