Singleton Design Pattern - C#

Singleton pattern falls under Creational Pattern of Gang of Four (GOF) Design Patterns in .Net. It is pattern is one of the simplest design patterns. This pattern ensures that a class has only one instance. In this article, I would like share what is Singleton pattern and how is it work?

What is Singleton Pattern?

Singleton pattern is one of the simplest design patterns. This pattern ensures that a class has only one instance and provides a global point of access to it.

Singleton Pattern - UML Diagram & Implementation

The UML class diagram for the implementation of the Singleton design pattern is given below:

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

1. Singleton

   This is a class which is responsible for creating and maintaining its own unique instance.

C# - Implementation Code

//eager initialization of singleton
public class Singleton
{
 private static Singleton instance = new Singleton();
 private Singleton() { }
 
 public static Singleton GetInstance
 {
 get
 {
 return instance;
 }
 }
}
 
////lazy initialization of singleton
public class Singleton
{
 private static Singleton instance = null;
 private Singleton() { }
 
 public static Singleton GetInstance
 {
 get
 {
 if (instance == null)
 instance = new Singleton();
 
 return instance;
 }
 }
}
 
////Thread-safe (Double-checked Locking) initialization of singleton
public class Singleton
{
 private static Singleton instance = null;
 private Singleton() { }
 private static object lockThis = new object();
 
 public static Singleton GetInstance
 {
 get
 {
 lock (lockThis)
 {
 if (instance == null)
 instance = new Singleton();
 
 return instance;
 }
 }
 }
}

Singleton Pattern - Example

Who is what?

The classes and objects in the above class diagram can be identified as follows:

1. Singleton - Singleton class

C# - Sample Code

/// <summary>
/// The 'Singleton' class
/// </summary>
public class Singleton
{
 // .NET guarantees thread safety for static initialization
 private static Singleton instance = null;
 private string Name{get;set;}
 private string IP{get;set;}
 private Singleton()
 {
 //To DO: Remove below line
 Console.WriteLine("Singleton Intance");
 
 Name = "Server1";
 IP = "192.168.1.23";
 }
 // Lock synchronization object
 private static object syncLock = new object();
 
 public static Singleton Instance
 {
 get
 {
 // Support multithreaded applications through
 // 'Double checked locking' pattern which (once
 // the instance exists) avoids locking each
 // time the method is invoked
 lock (syncLock)
 {
 if (Singleton.instance == null)
 Singleton.instance = new Singleton();
 
 return Singleton.instance;
 }
 }
 }
 
 public void Show()
 {
 Console.WriteLine("Server Information is : Name={0} & IP={1}", IP, Name);
 }
 
}
 
/// <summary>
/// Singleton Pattern Demo
/// </summary>
/// 
class Program
{
 static void Main(string[] args)
 {
 Singleton.Instance.Show();
 Singleton.Instance.Show();
 
 Console.ReadKey();
 }
}

Singleton Pattern Demo - Output

When to use it?

1. Exactly one instance of a class is required.
2. Controlled access to a single object is necessary.