Observer Design Pattern in C#

 Introduction

    In Observer design pattern an object 'subject' maintains the list of its dependent objects , observers, and notify them whenever there is a change in its state. It is a behavior design pattern.

It deals with below problems

- Define One to many dependency between objects without making them tightly coupled.

- Whenever there is a change in one object, other objects should get notified or updated.

UML diagram

Basic code example

public class Subject
{ 

    private IList<IObserver> _observers;
    private int state;

    // On change of this property, observers will be notified
    public int State{
        get{return state;}
        set{state = value; this.Notify();}
    }

    public Subject()
    {
        _observers = new List<IObserver>();
    }

    // Register Observer to get notification

    public void RegisterObserver(IObserver observer)

    {
        _observers.Add(observer);
    }

    //Remove Observer to stop getting notification
    public void RemoveObserver(IObserver observer)
    {
        _observers.Remove(observer);
    }

    //Send notification

    private void Notify()

{

var eventArg = new EventArg(){Value=4};

foreach(var observer in _observers)

observer.Execute(eventArg);

}

}

//Observer interface
public interface IObserver
{
    void Execute(EventArg eventarg);
}

public sealed class EventArg

{

public int Value{get;set;}

}

//Concrete ObserverA implementation of IObserver

public class ObserverA:IObserver
{
    public void Execute(EventArg eventArg)

{

Console.WriteLine($"ObserverA is execute & eventArg : {eventArg.Value}");

}

}

//Concrete ObserverB implementation of IObserver
public class ObserverB:IObserver
{
    public void Execute(EventArg eventArg)

{

Console.WriteLine($"ObserverB is execute & eventArg: {eventArg.Value}");

}

}

Program 

using System;

using System.Collections.Generic;

public class Program

{

public static void Main()

{

var subject = new Subject();

subject.RegisterObserver(new ObserverA());

subject.RegisterObserver(new ObserverB());

subject.State = 4;

}

}

//Since subject State property is changed when it is assigned with value 4, Observers get notification

//Output

    ObserverA is execute & eventArg : 4

   ObserverB is execute & eventArg: 4

In DotNet for this purpose there is Delegate & Event Type to achieve the same.