Why can't a derived class override base class' interface type abstract property with a type that extends that interface?

Question

In the following code, compiler complains about B not implementing TestProperty of abstract class A. ITest2 is derived from ITest1 so it implements everything ITest1 has. Why is this not possible?

public interface ITest1 { }
public interface ITest2 : ITest1 { }

public abstract class A
{
    public abstract ITest1 TestProperty { get; set; }
}

public class B:A
{
    public override ITest2 TestProperty { get; set; }
}

If you are going to down-vote so quickly, can you at least care to explain? — PersyJack, Feb 17 '15 at 19:41
What you are asking for is called covariance. See http://stackoverflow.com/questions/5709034 and http://en.wikipedia.org/wiki/Covariance_and_contravariance_%28computer_science%29#Inheritance_in_object_oriented_languages — Moby Disk, Feb 17 '15 at 20:59

score 8 · Accepted Answer · answered Feb 17 '15 at 19:43

8

This wouldn't be safe since you could do:

interface ITest3 : ITest1 { }
public class Test3 : ITest3 { }

A b = new B();
b.TestProperty = new Test3();

however Test3 does not implement ITest2 as required by B.

answered Feb 17 '15 at 19:43

Lee

142,018
20
234
287

To put it another way: Every dog is an animal, but not every animal is a dog. – Daniel Mann Feb 17 '15 at 19:44
Oh I see, with your example, it makes much more sense now. +1 for that. – PersyJack Feb 17 '15 at 19:50

score 2 · Answer 2 · answered Feb 17 '15 at 19:54

2

Make A class generic

Public abstract class A<TTest>
    Where TTest : ITest1
 {
    Public abstract TTest TestProperty {get; set;}
 }

 Public class B : A<ITest2>
 {
 ....
 }

answered Feb 17 '15 at 19:54

Evgeny Korolyuk

124
6

That's what I had initially, I wanted to make the class non-generic. – PersyJack Feb 17 '15 at 20:18

Why can't a derived class override base class' interface type abstract property with a type that extends that interface?

2 Answers2