Understanding polymorphism

If you write a piece of code , Reflect polymorphism , How would you write ？

How do you define polymorphism , There are probably several different explanations ：

1、 The ability of a behavior to have multiple forms or forms of expression . Polymorphism based on inheritance .

2、 It refers to providing a unified interface for entities of different data types .

3、 Use the reference of the parent class to point to the object of the child class .

If I choose without thinking, I will choose the first answer , But after careful consideration, I chose the third explanation . Let's start with a piece of code

public static void Main(string[] args)
        {
            Operation oper;
            oper = OperationFactory.createOperate("+");
            oper.NumberA = 1;
            oper.NumberB = 2;
            double result=oper.GetResult();
            Console.WriteLine(" The result is ："+result);
            Console.ReadKey();
        }

 class OperationAdd:Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA + NumberB;
            return result;
           
        }
    }

class OperationMul : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA * NumberB;
            return result;
        }
    }

  The following two codes show that addition and multiplication are inherited from Operation,Operation Only attributes and a virtual method are defined .

 public class Operation
    {// Define two variables , Two attributes , A virtual method 
        private double _numberA = 0;
        private double _numberB = 0;

        public double NumberA
        {
            get { return _numberA; }
            set { _numberA = value; }
        }
        public double NumberB
        {
            get { return _numberB; }
            set { _numberB = value; }
        }
        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }

    }
}

From the code above , What can best reflect polymorphism is the following sentence

oper = OperationFactory.createOperate("+");

Let me talk about my understanding ：

Why the parent class reference points to the child class object reflects polymorphism ？

What is declared is the parent object , But when compiling, it is a subclass object , When a subclass is instantiated, it will compile the parent class as well !

When calling a method , After the parent class declaration is forcibly converted to a child class object , Is a method that can be called to subclasses , So the concrete implementation is the subclass method , The same behavior is realized （ Method ） The ability to have many different manifestations or forms . In fact, the use of polymorphism , Not just to extend the functionality of the parent class , But by overriding the parent method , Override or add subclass specific functions , To achieve the implementation method of calling the method of the parent class and finally pointing to the specific subclass .

I wonder if I have the same question , The above code seems to reflect the Richter replacement principle more , Is there any difference between polymorphism and Richter substitution ？

The polymorphism mentioned above is more about rewriting the parent class method , Override or add subclass specific functions . And the use of Richter substitution , More is to extend the function of the parent class , But not inclined to change the original function of the parent class , Just add some new methods .