using System;

public class Program

{

    public interface ICalculator

    {

        int Add(int a, int b);

        int Subtract(int a, int b);

        int Multiply(int a, int b);

        int Divide(int a, int b);

    }

​

    public class Calculator : ICalculator

    {

        public int Add(int a, int b)

        {

            return a + b;

        }

​

        public int Subtract(int a, int b)

        {

            return a - b;

        }

​

        public int Multiply(int a, int b)

        {

            return a * b;

        }

​

        public int Divide(int a, int b)

        {

            return a / b;

        }

    }

    public static void Main()

    {

        // Does compile and work but should be avoided.

        // The Calculator class is tightly coupled now.

        Calculator calculator = new Calculator();

        int res = calculator.Add(20, 50);

        var calculator2 = new Calculator();

        res = calculator2.Add(30, 40);

        Console.WriteLine(res);

        // Object type interface should be used

        // Loosly coupled, meaning that we can easily swop out the calculator class

        // with another class that implement the ICalculator interface

        ICalculator calc = new Calculator();

        int value = calc.Add(20, 30);

        Console.WriteLine(value);

​

    }

}