using System;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        CalculateClient minusClient = new CalculateClient(new Minus());

        Console.Write("<br />Minus: " + minusClient.Calculate(7, 1).ToString());

        CalculateClient plusClient = new CalculateClient(new Plus());

        Console.Write("<br />Plus: " + plusClient.Calculate(7, 1).ToString());

        Singleton s1 = Singleton.GetInstance();

        Singleton s2 = Singleton.GetInstance();

        // Test for same instance

        if (s1 == s2)

        {

            Console.WriteLine("Objects are the same instance");

        }

​

    }

}

​

public interface ICalculate

{

    int Calculate(int value1, int value2);

}

​

class Minus : ICalculate

{

    public int Calculate(int value1, int value2)

    {

        return value1 - value2;

    }

}

​

class Plus : ICalculate

{

    public int Calculate(int value1, int value2)

    {

        return value1 + value2;

    }

}

​

class CalculateClient

{

    private ICalculate calculateStrategy;

    public CalculateClient(ICalculate strategy)

    {

        calculateStrategy = strategy;

    }

​

    public int Calculate(int value1, int value2)

    {

        return calculateStrategy.Calculate(value1, value2);

    }

}

​

//singleton pattern

public class Singleton

{

    private static Singleton _instance;

    private Singleton()

    {

    }

​

    public static Singleton GetInstance()

    {

        if (_instance == null)

        {

            _instance = new Singleton();

        }

​

        return _instance;

    }

}