Private Assembly

create a class library -> build 
create a console app->solution explorer ->right click on project ->add->project referneces->add library

library:
using System;

namespace MyLibrary
{
    public class Calculator
    {
        public int Add(int a, int b)
        {
            return a + b;
        }

        public int Multiply(int a, int b)
        {
            return a * b;
        }
    }
}


class:
using System;
using MyLibrary; // Namespace of the private assembly

class Program
{
    static void Main()
    {
        Calculator calc = new Calculator();

        int sum = calc.Add(10, 20);
        int product = calc.Multiply(5, 6);

        Console.WriteLine("Sum: " + sum);
        Console.WriteLine("Product: " + product);
    }
}