Dynamic polymorphism

Question

I have the following program and my need is call the parent class method using parent class reference which points to child class object.

 

class aa

{

public virtual void test()

{

Console.WriteLine("test in parent class");

}

}

class chi:aa

{

public override void test()

{

Console.WriteLine("test in child class");

}

}

class Program:chi

{

static void Main(string[] args)

{

aa ref1 =new chi();

ref1.test();

}

}

}

I need to class the test method  of class "aa" using the reference ref1. how to do this?

Please help