The Observer Pattern

The Observer Pattern


The Observer Pattern is a powerful tool for establishing one-to-many relationships between objects, enabling a change in one object to automatically notify and update its dependents.

This pattern promotes loose coupling and flexibility, allowing for flexible and dynamic interactions between objects.

Understanding the Observer Pattern

The Observer Pattern revolves around the concept of a Subject and its Observers.

A Subject is the object that manages the state that observers are interested in.

Observers register with the Subject to receive updates when the Subject’s state changes.

Benefits of the Observer Pattern

The Observer Pattern offers several advantages, including:

  • Loose Coupling: It promotes loose coupling between the Subject and its Observers, reducing dependencies and improving modularity.
  • Flexibility: It allows for dynamic addition and removal of Observers, enabling flexible and extensible systems.
  • Decoupling Notification from Update: It decouples the notification of changes from the actual updates, promoting modularity and code readability.
  • Efficient Communication: It promotes efficient communication between Subject and Observers, minimising the need for direct object references.
  • Support for Complex Event Handling: It can handle complex event-handling scenarios, such as propagating updates to multiple levels of observers.

Types of Observer Patterns

The Observer Pattern encompasses several variations, each with its specific characteristics:

  • Simple Observer: This pattern uses a direct reference from the Subject to its Observers.
  • Publish-Subscribe: This pattern uses a central event bus to decouple Publishers and Subscribers.
  • Chain of Responsibility: This pattern allows Observers to handle specific types of events or delegate them to other Observers in the chain.

Implementing the Observer Pattern in C#

To illustrate the implementation of the Observer Pattern in C#, consider a simplified scenario of tracking stock prices:

Subject Class:

The subject class represents the stock market and provides methods for updating its stock prices and notifying its observers.

public class StockMarket
    private List<Observer> observers = new List<Observer>();

    private double[] stockPrices = { 100.0, 80.0, 120.0 };

    public void UpdateStockPrice(int index, double price)
        stockPrices[index] = price;

    public void RegisterObserver(Observer observer)

    public void UnregisterObserver(Observer observer)

    private void NotifyObservers()
        foreach (Observer observer in observers)

Observer Class:

The observer class represents the price monitor and provides a method for receiving stock price updates.

public interface IObserver
    void UpdateStockPrices(double[] stockPrices);

Specific Observer Classes:

Concrete observer classes can be implemented to represent specific types of price monitors, such as:

  • StockPriceDisplay: Displays current stock prices.
  • PriceAlerter: Sends email alerts when stock prices exceed certain thresholds.

Using the Observer Pattern:

To use the observer pattern for tracking stock prices:

StockMarket stockMarket = new StockMarket();

StockPriceDisplay display = new StockPriceDisplay();

PriceAlerter alerter = new PriceAlerter();

stockMarket.UpdateStockPrice(0, 150.0);

This example demonstrates a basic implementation of the Observer Pattern in C# for tracking stock prices. The observer pattern can be extended to handle more complex event-handling scenarios.


The Observer Pattern is a versatile tool for establishing loosely coupled and dynamic communication between objects.

It promotes flexibility, reusability, and maintainability in software designs. Developers should consider using the Observer Pattern when they need to:

  • Establish asynchronous communication between objects.
  • Manage complex event-handling scenarios.
  • Decouple data updates from UI updates.
  • Implement data dissemination and monitoring systems.

By leveraging the Observer Pattern, developers can create robust and adaptable applications that efficiently handle events and maintain flexibility in their design.


Hi, my name is Stephen Finchett. I have been a software engineer for over 30 years and worked on complex, business critical, multi-user systems for all of my career. For the last 15 years, I have been concentrating on web based solutions using the Microsoft Stack including ASP.Net, C#, TypeScript, SQL Server and running everything at scale within Kubernetes.