Programming - Singleton Design Pattern

Ensure a class has only one instance and provide a global point of access to it.

and

The classes and/or objects participating in this pattern are:

Singletonand
and

defines an Instance operation that lets clients access its unique instance. Instance is a class operation.
responsible for creating and maintaining its own unique instance.

// Singleton pattern -- Structural example

                using
              
              System;

               namespace

DoFactory.GangOfFour.Singleton.Structural
{

// MainApp test application

class MainApp
{

static void Main()
{
// Constructor is protected -- cannot use new
Singleton s1 = Singleton.Instance();
Singleton s2 = Singleton.Instance();

if (s1 == s2)
{
Console.WriteLine("Objects are the same instance");
}

// Wait for user
Console.Read();
}
}

// "Singleton"

class Singleton
{
private static Singleton instance;

// Note: Constructor is 'protected'
protected Singleton()
{
}

public static Singleton Instance()
{
// Use 'Lazy initialization'
if (instance == null)
{
instance = new Singleton();
}

return instance;
}
}
}

and

Output expected:

Objects are the same instance

and