using System;

public class Program

{

    public static void Main()

    {

        AClass a = new AClass();

        BaseClass b = a;

        b.num = 5;

        Console.WriteLine(a.num);

    }

}

​

public class BaseClass{

    public int num {get;set;}

}

​

public class AClass : BaseClass {

    public  int num {get;set;}

}