Do C# 8 default interface implementations allow for multiple inheritanceAsk Questions

 Posted on 09/11/2019

According to https://blogs.msdn.microsoft.com/dotnet/2018/11/12/building-c-8-0/, one of the new features coming in C# 8 is the default implementation of interfaces. Will this new feature also implicitly allow for multiple inheritance? If not, what exactly will happen if I try the following:

public interface A { int Foo() => 1; }

public interface B { int Foo() => 2; }

public class C : A, B { }

 

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Asked by Praful Chauhan on 09/11/2019 Score: 116 points
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Answered by hardik chaudhary on 9/28/2019 3:46:36 PM Score: 553 points

Your question is answered by Mads Torgersen in the blog post you linked to:

Actually interfaces are still quite far from abstract classes. Classes don’t inherit members from interfaces, so if a class leaves a member M implemented by the interface, the class does not have a member M! It’s like an explicit implementation today; you have to convert to the interface in order to get at such members.

So with your example:

public interface A { int Foo() => 1; }

public interface B { int Foo() => 2; }

public class C : A, B { }

You cannot do this:

var something = new C();

var x = something.Foo(); /* does not compile */

You can do the following:

var something = new C();

var x = ((A)something).Foo(); /* calls the implementation provided by A */

var y = ((B)something).Foo(); /* calls the implementation provided by B */

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Answered by Will Smith on 9/12/2019 8:09:21 AM Score: 54 points

Note that a class does not inherit members from its interfaces; that is not changed by this feature:

Thus, it seems reasonable (although impossible to confirm with 100% certainty until it is shipped) that:

public interface A { int Foo() => return 1; }
public interface B { int Foo() => return 2; }
public class C : A, B { }

will work fine.

Just as below shows:

new C().M(); // error: class 'C' does not contain a member 'M'

then we can assume, your version:

new C().Foo();

will also not compile.

IA i = new C();
i.M();

as valid, which is equivalent to your:

A i = new C();
i.Foo();

Since i is declared as type A there is no reason to assume the same would not work if A was changed to B - there are no collisions to speak of.

The entire point of this feature is to allow interfaces to be extended in a safe way. If this only worked if you implemented one interface, that seems contrary to the objective of the feature. And given the feature appears to be implemented in a way roughly akin to explicit interface implementation (which is why we can't invoke C.Foo() directly), I think we can reasonably assume that it will most likely allow for multiple interface implementation.

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Actually interfaces are still quite far from abstract classes. Classes don’t inherit members from interfaces, so if a class leaves a member M implemented by the interface, the class does not have a member M! It’s like an explicit implementation today; you have to convert to the interface in order to get at such members.
12/09/2019   08:10:17   Praful Chauhan
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