Struggling to understand interfaces and abstract base classes

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Despite my head hurting with the stuff I have read, I am struggling to understand specifically what is wrong with the following:

C#

public interface IAnimal
{
    int Age();
    int NumLegs();
}

public interface IBird : IAnimal
{
    bool CanFly();
}

public interface IMammal : IAnimal
{
    int RunningSpeed();
}

public abstract class AnimalBase : IAnimal
{
    public abstract int Age();
    public abstract void PrintAge();
    public override int NumLegs() { return 4; } // Error!
};

public class Sparrow : AnimalBase, IBird
{
    public override void PrintAge() { Console.WriteLine( $"Sparrow is {Age()} years old." ); }

    public override bool CanFly() { return true; } // Error!

    public override int Age() { return 3; }
}

I have a couple of errors (so far):

In my class AnimalBase, I get an error saying 'AnimalBase.NumLegs() - No suitable method found to override'. Why? AnimalBase implements IAnimal so why can't it see the method declaration in the interface?

And why is the abstract Age() in AnimalBase okay? I am guessing here that I am simply declaring a brand new method that has nothing to do with the declaration in the interface, but shouldn't I get some kind of warning that I am kind-of 'hiding' the interface declaration?

The second error is exactly the same 'Sparrow.CanFly() - No suitable method found to override'. Again why? Sparrow implements IBird, so why can't it see IBird.CanFly()?

Any help understanding this might save my remaining hair.

What I have tried:

I have read and read, and I have found lots of examples that do work but no explanation of why what I am trying to do is apparently wrong.

Posted 20-Dec-16 4:20am

Patrick Skelton

Updated 20-Dec-16 4:40am

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OriginalGriff · Accepted Answer · 2016-12-20T04:33:00

Well, your abstract class declares an override member NumLegs - which has no base class implementation to override. What you need is to declare it as virtual which allows derived classes to override it:

C#

public virtual int NumLegs() { return 4; }

If they do, then the "new" version will be called, if they don't, then the base class version will be used, and 4 will be returned.

The same problem exists with CanFly: interface methods cannot be overridden as they don;t have a base class implementation to override. In this case, just remove the keyword:

C#

public bool CanFly() { return true; }

and your errors will disappear.