Overriding interface method return type with derived class in implementation

Question

I am trying to implement (C#) an interface method in a class, returning a derived type instead of the base type as defined in the interface:

interface IFactory {     BaseCar GetCar(); }  class MyFactory : IFactory {     MyCar GetCar()     {     } } 

Where, of course:

class MyCar : BaseCar {  } 

However, the following error happens:

'MyFactory' does not implement interface member 'IFactory.GetCar()'. 'MyFactory.BaseCar()' cannot implement IFactory.GetCar()' because it does not have the matching return type of 'BaseCar'.

Can anyone point me as to why this fails, and how would be the best way to work around it?

Answer 1

Use Generics

interface IFactory<T> where T: BaseCar {     T GetCar(); }  class MyFactory : IFactory<MyCar> {     MyCar GetCar()     {     } }   class MyCar : BaseCar {  } 

Answer 2

There are 2 ways to accomplish this. You can either use generics or explicitly implement interface members.

Generics

interface IFactory<T> where T: BaseCar {     T GetCar(); }  class MyFactory : IFactory<MyCar> {     MyCar GetCar()     {     } } 

Explicitly implemented members

interface IFactory {     BaseCar GetCar(); }  class MyFactory : IFactory {     BaseCar IFactory.GetCar()     {         return GetCar();     }      MyCar GetCar()     {     } }