לפניכם המחלקות First ו־Second, והמחלקה Tester:

public class First
{
    private static int count = 0;
    protected int x;
    protected int y;
    public First (int num) {
        this.x = num; this.y = num; count ++;
        Console.WriteLine ("First 1");
    }
    public First (int num1, int num2) {
        this.x = num1; this.y = num2; count ++;
        Console.WriteLine ("First 2");
    }
    public static int GetCount() { return count; }
    public int GetX() { return x; }
    public int GetY() { return y; }
    public virtual int Sum() { return this.x + this.y; }
    public virtual void Add (First other) {
        this.x += other.x; this.y += other.y;
        Console.WriteLine ("x = " +this.x + " y = " +this.y);
    }
}

public class Second : First
{
    private int z;
    public Second (int num) : base (num) {
        this.z = num;
        Console.WriteLine ("Second");
    }
    public override int Sum() { return base.Sum() + this.z; }
    public override void Add (First other) {
        this.x += other.GetX(); this.y += other.GetY();
        if (other is Second)
            this.z += ((Second)other).z;
        Console.WriteLine("x = "+ this.x + " y =" + this.y+ " z =" +this.z);
    }
}

class Tester
{
    static void Main(string[] args)
    {
        First f1 = new First (40);
        First f2 = new First (40, 50);
        First f3 = new Second (100);
        Second s1 = new Second (100);
        Second s2 = new Second (100);
        // ***
    }
}