Why I am getting null in object casting?

Question

I have such example

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Boo();
        }

        interface IBASE_TYPE { };

        interface IFOO<TYPE>: IBASE<TYPE>
            where TYPE: IBASE_TYPE
        { };

        interface IBASE<TYPE> { };

        class A : IBASE_TYPE { }

        class B : IBASE_TYPE { }

        class OneChild : IFOO<A> { }

        class SecondChild : IFOO<B> { }

        void Execute(IFOO<IBASE_TYPE> item)
        {
            if (item == null)
            {
                Console.WriteLine("item is null");
            }
        }

        void Boo()
        {
            var oneChild = new OneChild();
            var secondChild = new SecondChild();

            Execute(oneChild as IFOO<IBASE_TYPE>);
            Execute(secondChild as IFOO<IBASE_TYPE>);
        }
    }

And log here is

item is null
item is null

It is not obvious for me... What is the problem here?

I don't expect that item is null...

P.S.

Look at this, it is like the same example, but here it is works

interface ONE { }

        class G : ONE { }

        class Y : ONE { }

        void SomeMethod()
        {
            var g = new G();
            var Y = new Y();
            More(g);
            More(Y);
        }

        void More(ONE item)
        {
            if (item == null)
            {
                Console.WriteLine("item is null");
            }
        }

So, what is the diff?

`IFOO` is not the same as `IFOO`. you have to use [covariant interface](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/out-generic-modifier) using the `out` keyword, like `IBASE` — oleksa, Jan 09 '20 at 08:51
@oleksa but `A` implements `IBASE_TYPE`... So, we can say that it is ok, right? — Sirop4ik, Jan 09 '20 at 08:51
No, we cannot say that. `IFOO` is **not** a `IFOO` (while `A` "is-a" `IBASE_TYPE`), so the `as` operator results in `null`. See [Covariance and Contravariance in Generics](https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance) — Fildor, Jan 09 '20 at 08:52
@Fildor but A implements IBASE_TYPE... So, we can say that it is ok, right? — Sirop4ik, Jan 09 '20 at 08:53
`interface IFOO : IBASE where TYPE : IBASE_TYPE { };` `interface IBASE { };` please, note `out` — Dmitry Bychenko, Jan 09 '20 at 08:54
@AlekseyTimoshchenko: You're going in circles. **Your assumption that this "A is B" logic remains correct when used as a generic parameter is not correct**. If you don't believe me, try casting a `List` to a `List — Flater, Jan 09 '20 at 08:54
@AlekseyTimoshchenko That's a common misconception. Have a look into the link in my above comment. — Fildor, Jan 09 '20 at 08:55
The most popular analogon to this question is this: imagine a `List`. If you **could** cast that to `List`, because `Giraffe` surely derives from `Animal`, you could then do this as well: `(List).Add(new Lion())`, which pretty sure isn´t what you want, is it? Having said this you cannot cast a `Generic` to `Generic — MakePeaceGreatAgain, Jan 09 '20 at 08:56
@AlekseyTimoshchenko Your second (counter-)example has nothing to do with your problem, since it is lacking _Generics_. That's the point. `G` "is-a" `ONE` and `Y` "is-a" `ONE`. But a `List` is not a `List` is not a `List`. — Fildor, Jan 09 '20 at 08:59
... like a Box of Apples is not a Box of Oranges, while Apples and Oranges are both Fruits and both Boxes are Boxes. :D Your second example is more like "A juicer can take fruit and will output juice. If you input apples, it will make apple juice, if you input Oranges, it will make orange juice." It is "generic" in a sense, but not in the C#/.Net sense of "Generics". — Fildor, Jan 09 '20 at 09:15
@Fildor Your example has the caveat that a box of apples surely is a box of fruits. OP does not want to convert different boxes in one another, but just to the base-box, which in some way is a different picture. — MakePeaceGreatAgain, Jan 09 '20 at 09:21
@HimBromBeere Granted, I just wanted to have a picture for the difference between Generics and Non-Generics. But of course, you are right. — Fildor, Jan 09 '20 at 09:44

Why I am getting null in object casting?

0 Answers0