C#•2y ago

❔ Covariant Interface Return

Is there some reason why this doesn't work? I understand we can get this behavior with virtual methods and inheritance, but it feels like this should work with interfaces.

interface IFooReturner { Foo GetFoo(); }
class Foo {}
class FooBar : Foo {}
class FooBarGetter : IFooReturner { FooBar GetFoo(); }

interface IFooReturner { Foo GetFoo(); }
class Foo {}
class FooBar : Foo {}
class FooBarGetter : IFooReturner { FooBar GetFoo(); }

If FooBar is a Foo, this implementation should be valid no?

7 Replies

ero•2y ago

no, interfaces are contracts any implementing classes must fulfill them exactly

SirCarterOP•2y ago

It kinda does though

ero•2y ago

it doesn't FooBar is not Foo

SirCarterOP•2y ago

class FooReturner { virtual Foo GetFoo(); }
class Foo {}
class FooBar : Foo {}
class FooBarGetter : FooReturner { override FooBar GetFoo(); }

class FooReturner { virtual Foo GetFoo(); }
class Foo {}
class FooBar : Foo {}
class FooBarGetter : FooReturner { override FooBar GetFoo(); }

So something like this works, we can override the return type and it still works because FooBar is a Foo, it still matches the contract of the original class but with more specificity I'm not sure I'm understanding why they can't do the same things with an interface, if I cass FooBarGetter to IFooReturner, I'd still get a Foo from GetFoo

reflectronic•2y ago

the feature is deliberately only implemented for classes the implementation is much more complex for interfaces so they did not do it

SirCarterOP•2y ago

Sad but that makes sense

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Covariant Interface Return