What is use of Parent object instantiating with child class

Question

Please tell me what is of parent object instantiating with child class like:

 public class A
    {
        public A()
        {
            Console.WriteLine("A");
        }
        public virtual void method()
        {
            Console.WriteLine("AM");
        }

    }
    public class B : A
    {
        public B()
        {
            Console.WriteLine("B");

        }
        public new void method()
        {
            Console.WriteLine("BM");

        }
        public void method1()
        {
            Console.WriteLine("BM1");
        }
    }

 class Program
    {
        static void Main(string[] args)
        {
            A obj = new B();// what is use of it?
            obj.method();              
            Console.Read();
        }
        private void methodP1()
        {

        }
    }

please tell me what is use of Parent obj = new Child(); as i we can only call to only public methods of parent class which is possible using Parent obj = new Parent();

is it possible: Child obj = new Parent() ?

Answer 1

please tell me what is use of Parent obj = new Child(); as i we can only call to only public methods of parent class which is possible using Parent obj = new Parent();

This is the basis for polymorphism: Imagine you have several child classes that inherit from you parent class. You want to use all these child classes through the interface / methods defined on your parent class, without worrying about the implementation details in each child class (each might do something different, but with the same overall semantics).

This is possible because the child class has a IS A relationship with its parent class since child inherits from parent.

Answer 2

In your example, B is-a A, but A is-not-a B.

The above is of use when the code using the reference to B can only understand (or needs to understand) types of A. B is a specialisation of A. Imagine something like (pseudo-code)

Shape s;
if (x == 1) {
  s = new Square();
}
else {
  s = new Triangle();
}
// following code knows only about Shapes
s.draw();
s.calculateArea();

The following code doesn't need to know if s is a square or a triangle, just that it's a shape. What use is that ? If you call s.draw(), the shape itself determines how it's going to look. The code calling it doesn't know or care.

This is a key point of object-oriented programming. Asking objects to do things for you rather than determine yourself what's needed.

Note that your final question doesn't intuitively make sense when using this example.

Shape s = new Square(); // fine

vs

Square s = new Shape(); // can you instantiate a "shape" and why then do you decide it's a square?