using System;

public class Program

{

    public static void Main()

    {

        Parent instance = null;

        var rand = new System.Random();

        if(rand.Next(2) > 0){

            instance = new Child();

        }else{

            instance = new Parent();

        }

        instance.Build("Hello!");

        Console.WriteLine(instance.Greeting);

    }

}

​

​

public class Parent{

    public string Greeting { get; protected set; }

    public virtual void Build(string greeting){

        this.Greeting = greeting;

    }

}

public class Child : Parent{

    public override void Build(string greeting){

        base.Build(greeting);

        this.Greeting = "Child says: " + this.Greeting;

    }

}