Observer Design Pattern - C#

Shailendra Chauhan  Print   2 min read  
23 Feb 2017

Observer Design Pattern falls under Behavioral Pattern of Gang of Four (GOF) Design Patterns in .Net. The command pattern is commonly used in the menu systems of many applications such as Editor, IDE etc. In this article, I would like share what is observer pattern and how is it work?

What is Observer Design Pattern?

This pattern is used when there is one to many relationship between objects such as if one object is modified, its dependent objects are to be notified automatically.

This pattern is allows a single object, known as the subject, to publish changes to its state and other observer objects that depend upon the subject are automatically notified of any changes to the subject's state.

Observer Design Pattern - UML Diagram & Implementation

The UML class diagram for the implementation of the Observer Design Pattern is given below:

Observer Design Pattern C#

The classes, interfaces and objects in the above UML class diagram are as follows:

  1. Subject

    This is a class that contains a private collection of the observers that are subscribed to a subject for notification by using Notify operation.

  2. ConcreteSubject

    This is a class that maintains its own state. When a change is made to its state, the object calls the base class's Notify operation to indicate this to all of its observers.

  3. Observer

    This is an interface which defines an operation Update, which is to be called when the subject's state changes.

  4. ConcreteObserver

    This is a class that implements Observer interface and examines the subject to determine which information has changed.

C# - Implementation Code

public abstract class Subject
 private ArrayList observers = new ArrayList();

 public void Attach(IObserver o)

 public void Detach(IObserver o)

 public void Notify()
 foreach (IObserver o in observers)

public class ConcreteSubject : Subject
 private string state;

 public string GetState()
 return state;

 public void SetState(string newState)
 state = newState;

public interface IObserver
 void Update();

public class ConcreteObserver : IObserver
 private ConcreteSubject subject;

 public ConcreteObserver(ConcreteSubject sub)
 subject = sub;

 public void Update()
 string subjectState = subject.GetState();

Real Life Example:

Real Life Example of Observer Design Pattern C#

When to use it?

  1. Changes in state of an object need to be notified to a set of dependent objects, not all of them.

  2. Notification capability is required.

  3. The object sending the notification does not need to know about the receivers objects.

What do you think?

I hope you will enjoy the Observer Design Pattern while designing your software. I would like to have feedback from my blog readers. Your valuable feedback, question, or comments about this article are always welcome.

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