How to satisfy the compiler when only partially implementing an interface with an abstract class?

Question

I have an interface here named IFish. I want to derive it with an abstract class (WalkingFishCommon) which provides an incomplete implementation, so that classes derived from WalkingFishCommon do not have to implement the CanWalk property:

interface IFish {     bool Swim();     bool CanWalk { get; } }  abstract class WalkingFishCommon : IFish {     bool IFish.CanWalk { get { return true; } }      // (1) Error: must declare a body, because it is not marked     // abstract, extern, or partial     // bool IFish.Swim();      // (2) Error: the modifier 'abstract' is not valid for this item     // abstract bool IFish.Swim();      // (3): If no declaration is provided, compiler says      // "WalkingFishCommon does not implement member IFish.Swim()"     // {no declaration}      // (4) Error: the modifier 'virtual' is not valid for this item     // virtual bool IFish.Swim();      // (5) Compiles, but fails to force derived class to implement Swim()     bool IFish.Swim() { return true; } } 

I've not yet discovered how to make the compiler happy, while still achieving the goal of forcing classes derived from WalkingFishCommon to implement the Swim() method. Particularly baffling is the delta between (1) and (2), where the compiler alternates between complaining that Swim() isn't marked abstract, and in the next breath complains that it can't be marked abstract. Interesting error!

Any help?

Answer 1

Just declare Swim as abstract and don't try to use explicit interface declaration for it (i.e. remove IFish).

abstract class WalkingFishCommon : IFish {     public bool CanWalk { get { return true; } }     public abstract bool Swim(); }