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C# - Upward Transition

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In the process of learning C#, some of the basics like data types, control structures, arrays, etc. are much the same as the previous exposure to the language and can be understood, but this is the first time I have encountered the concept of upward transformation.

After studying a bit it is roughly understood as follows.

Upward transformation is the transformation of a subclass into a parent class, and the object can be used as its own type.

Let's start with a piece of code.

public class Animal
        public virtual void Eat()
            Console.WriteLine("I'm an animal., I want to eat something.!");
    public class Bird : Animal
        public override void Eat()
            Console.WriteLine(" I'm a bird., I want to eat something.!");
    class Test
        static void Main(string[] args)
             Animal animal = new Bird();   /upward transition

In the above example we have defined an animal class (which has an eating method), the bird class inherits from the animal class (overriding the eating method in the base class), and the main function has a life animal as an object of type Animal. ".

For this example. When declaring an animal, the type of the animal is Animal. But the instance pointed to by animal is a subclass of Animal, i.e. Bird.

Of course there is an upward transition and then there is a downward transition, in the opposite way of course, i.e. converting the parent class to a child class.

I saw a question on the internet, someone said "if the required function in the child class exists in the parent class, use upward transition; if the required function in the parent class exists in the child class, use downward transition", but as mentioned above, Bird is a subclass of Animal, Bird has inherited the methods in Animal, why should we use downward transition? I.e. why define Animal with "Animal animal=new Bird(); " instead of "Bird animal=new Bird(); "What about?

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