为什么当我尝试从父类（向下转换）创建子 class 时，我得到 null object

Question

namespace Test
{  
    public class Basic
    {

        public virtual void WriteIt()
        {
            Console.WriteLine("Hi");
        }
    }

    public class Not_So_Basic : Basic
    {
        public override void WriteIt()
        {
            Console.WriteLine("Hi2");
        }
    }
    class Program
    {

        static void Main(string[] args)
        {

            Not_So_Basic obj = new Basic() as Not_So_Basic;
            if(obj==null)
                Console.WriteLine("Error");
            else
            obj.WriteIt();
        }



    }
}

So basically I created these two classes, Basic and Not_So_Basic which is a child class of the Basic class.

When I try to create an object of the Not_So_Basic class using an object of its parent class by Downcasting it, I always get a null object.

I understand by doing this, I'm just referencing my child class object to the parent class object and I get that "child class view", right? I'm not really creating a separate space in heap of a child class based off of the parent class.

I'm kind of new at programming so please be gentle, don't roast me. I'm sure there is a rule that I'm breaking. Btw I'm trying to learn to program, I'm not working on a project, just trying to wrap my head around a rule that I'm breaking.

The code is written in in c#.

Answer 1

The as operator attempts to cast, but gives you a null object if it can't. So this means the cast failed.

You're casting a Basic object to Not_So_Basic . You cannot do that because Basic does not inherit from Not_So_Basic .

To illustrate, all foxes are animals, but not all animals are foxes, so you can't cast an Animal to a Fox .

That said, if you have an actual Not_So_Basic object that's just represented as a Basic object, then you can cast it to Not_So_Basic . For example:

Basic basic = new Not_So_Basic();

Not_So_Basic not_so_basic = (Not_So_Basic) basic; //this works

Answer 2

Downcasting is where you take a base class and then try and turn it into a more specific class. This can be accomplished with using as like this:

Not_So_Basic notSoBasic = basic as Not_So_Basic;

The cast will be successful only if basic is referring to an instance of Not_So_Basic

//successful cast
Basic basic = new Not_So_Basic();
Not_So_Basic notSoBasic = basic as Not_So_Basic;

OR basic is referring to an instance of Not_So_Basic child.

public class Not_So_Basic_Child : Not_So_Basic
    {
        public override void WriteIt()
        {
            Console.WriteLine("Hi from child");
        }
    }

//another successful cast
Basic basic = new Not_So_Basic_Child();
Not_So_Basic notSoBasic = basic as Not_So_Basic;

If you are trying to cast an instance of a base class to an instance of a child class, as operator will return null

//unsuccessful cast
Basic basic = new Basic();
Not_So_Basic notSoBasic = basic as Not_So_Basic; //is null

Answer 3

Creating an Object of Child Class with reference to Parent class, result will be always null. We can not create a child class object like that.

//unsuccessful cast
Parent parent = new Parent();
Child child = parent as Child; //is null

Correct way:

Parent parent= new Child();
Child  child = (Child) parent; //this works