Understanding Polymorphism, inheritance in C#.

Question

Hello, I am little confused about polymorphism, inheritance in C#.

 

While learning about polymorphism, I found something like this ParentClass P = new ChildClass(); which is a bit confusing. Why would someone create type of parent class to store the object of child class? However it is valid in c# I just want to understand 3 things:

 

1. What is the purpose of doing this?

 

2. What are the advantages of doing this?

 

3. When we should create object like this?

 

Below is my code for reference:

using System;
class ParentClass
{
public void Display()
{
Console.WriteLine("Display method in Parent class");
}
}
class ChildClass : ParentClass
{
public void Display()
{
Console.WriteLine("Display method in Child class");
}
}
public class Program
{
public static void Main()
{
ParentClass P = new ChildClass();
P.Display();
}
}

The output of the code is:

 

Display method in Parent class.

 

Now if someone has to call method in parent class why not simply create

 

ParentClass P = new ParentClass();

 

Or if someone has to call method in Child class why not simply create

 

ChildClass C = new ChildClass();

 

Have gone through lot of forums, still not found the answer I want. Please if anyone can explain in detail. It would be great. Thanks.