how to call derived function using base class object

Question

class Parent
{
    public int GetNo()
    {
        return 1;
    }
}

class Child : Parent
{        
    public Child()
    {

    }

    public int GetNo()
    {           


        

Answer 1

You can cast it:

((Child)p).GetNo();

or

if(p is Child)
    (p as Child).GetNo();