NP50 Commenting out altering the otuput

Question

Hi Guys

 

Commenting out “if(instance == null)” is altering the otuput. Anyone knows please explain the reason.

 

Thank you

 

using System;

class Singleton

{

    private static Singleton instance;

    private static int numOfReference;

    private string code;

 

    private Singleton()

    {

        numOfReference = 0;

        code = "Maasoom Faraz";

    }

    public static Singleton GetInstance()

    {

        //if (instance == null) Commenting out this will change the output

        instance = new Singleton();

 

        numOfReference++;

 

        return instance;

    }

    public static int Reference

    {

        get { return numOfReference; }

    }

    public string Code

    {

        get { return code; }

        set { code = value; }

    }

}

 

public class SingletonClientCode

{

    static void Main(string[] args)

    {

        //Singleton a = new Singleton();

        //will result in error because constructor is private

 

        Singleton b = Singleton.GetInstance();

        Console.WriteLine(b.Code);

        Console.WriteLine(Singleton.Reference);

 

        Singleton c = Singleton.GetInstance();

        Console.WriteLine(c.Code);

        Console.WriteLine(Singleton.Reference);

 

        c.Code = "Modified " + c.Code;

 

        Singleton d = Singleton.GetInstance();

        Console.WriteLine(d.Code);

        Console.WriteLine(Singleton.Reference);

 

        if (b == c && c == d && d == b)

            Console.WriteLine("Instances are equal");

        else

            Console.WriteLine("Instances are not equal");

 

        Console.ReadLine();

    }

}

 

/*

Output

 

Maasoom Faraz

1

Maasoom Faraz

1

Maasoom Faraz

1

Instances are not equal

*/