Casting object of inherited class to instance of inheriting type

Question

I have the following code.

class Program
{
    static void Main(string[] args)
    {
        Outer outer = new Outer();
        Inner inner = new Inner();

        Outer outer2 = new Inner();
        Inner inner2 = new Outer();
    }
}
public class Outer { }
public class Inner : Outer { }

I get the compiler error when assigning new Outer() to inner2 because it's of type Inner. I understand that it's to do with the inheritance but I can not figure out why because the contents are the same. Also, how can I force it to be cast to Inner.

I have tried

(inner)new Outer();
new Outer() as Inner;

But it threw an exception about conversion in the former and became null in the latter. Not sure what to google for to find out. I mean like this but the exact opposite.

Outer is not an Inner, and you can never force it to become an Inner when it isn't an Inner. The contents are irrelevant. — Bent Tranberg, Nov 16 '21 at 19:31
It's a lot easier if you name things `Base` and `Sub`, btw. A variable of type `Outer` can refer to an instance of an `Outer` or to an instance of any subclass of `Outer` (in this case, `Inner`). But, `Inner` is a _specialization_ of `Outer`, so a variable of `Inner` can only refer to an `Inner` object (or any subclasses of `Inner`). Just because `Inner` doesn't do anything special while it's specializing `Outer`, doesn't mean it's not a specialization. Consider the case where you have what you have, and it compiles, but then you add a member to `Inner`. It would suddenly break — Flydog57, Nov 16 '21 at 19:31
Yes, the names are super confusing. If just playing around learning C#, I'd suggest experimenting with a class hierarchy named like e.g. Animal -> Insect -> Butterfly — Bent Tranberg, Nov 16 '21 at 19:39

Adam B · Accepted Answer · 2021-11-16T20:07:00.763

A subclass is of type base class, but not the opposite. consider renaming them to

class Dog {}
class GermanShepard : Dog {}

This makes it clear that A German Sheppard is definitely a dog. But a dog is not necessarily a German Shepard.

writing the following is not possible, since the new dog could end up being another type of dog.

GermanShepard fred = new Dog();  //error!!

It might be a Shitzu or a Labrador. But you can always cast a German Shepard to a dog. Similarly you can always cast a Labrador to a dog.

Casting object of inherited class to instance of inheriting type

1 Answers1