Is-a relationships in Generics

April 25, 2023

Suppose we have generic classes G1<T> and G2<T>: G1<T>. And classes C1 and C2: C1.

Of the following which is not true?:

G2<C1> is-a G1<C1>,
G1<C2> is-a G1<C1>,
G2<C2> is-a G1<C1>

Aside from these is there any case that would surprise intuition?

>Solution :

It is easy to check:

Console.WriteLine(new G2<C1>() is G1<C1>); // True
Console.WriteLine(new G1<C2>() is G1<C1>); // False
Console.WriteLine(new G2<C2>() is G1<C1>); // False

class C1{}
class C2:C1{}
class G1<T>{}
class G2<T>:G1<T>{}

Demo

The last two in general case are quite easy to explain. Imagine G1 being:

class G1<T>
{
    public T Data { get; set; }
}

Then if new G1<C2>() is G1<C1> would be true then the next would be possible –
((G1<C1>)new G1<C2>()).Data = new C1(); which obviously breaks type safety.

Something can be done with variance support in generic interfaces, but it is limited to interfaces.