using System;

// -- case 1

public class A

{

    private int _a;

    protected A(int a){

        // init class   

        _a = a;

    }

    public void printA(){

        Console.WriteLine("_a = {0}", _a);

    }

}

​

public class B : A

{   

    private int _b;

    public B(int a, int b) : base(a){

        // do some another init stuff

        _b = b;

    }

    public void printB(){

        Console.WriteLine("_b = {0}", _b);

    }

}

​

​

public class Program {

    public void Main(){

        var b = new B(10, 20);

        b.printA();

        b.printB();

    }

}