Using instantiated class members as variables in abstract class methods

Question

I'm trying to figure out how to make a method in a base abstract class capable of using member values of instantiated objects derived from the class. For example, let's say I have a class called Vehicle that inherits an interface called IVehicle ;

public abstract class Vehicle : IVehicle
{
    public int Wheels = 4;

    public int CountWheels()
    {
        return Wheels;
    }
}

Let's say I then had another class called Motorcycle that inherited Vehicle as such;

public class Motorcycle : Vehicle, IVehicle
{
    public int Wheels = 2;
}

If I make a new Motorcycle called newMotorcycle and I call newMotorcycle.CountWheels() , I get 4 instead of 2. I'm assuming this is an issue with my Wheels definition in the Motorcycle class, but I've tried just about everything I can think of to get this to work and it always seems to want to ignore child class definitions. I know that I could make the CountWheels() method abstract and define it in the Motorcycle class, but I'm trying to keep the base Vehicle class as generic as possible so I can implement a ton of other child classes without having to redefine the same method over and over.

My first question; is what I'm trying to do even possible? If so, what part of my member definition needs to change for this to work the way I'm intending for it to?

I promise I scoured the internet before I posted this here and I couldn't find enough information to help myself...

Answer 1

You will notice that there is a warning in the code you posted:

Motorcycle.Wheels hides inherited member Vehicle.Wheels...

Hence your problem, Vehicle has no idea that a derived class has hidden one of its members (hence why hiding is a bad idea).

Since you can't have virtual fields, you should change it to a virtual (or in this case, abstract so everyone has to define it) property :

public abstract class Vehicle : IVehicle
{
    public abstract int Wheels {get;}

    public int CountWheels()
    {
        return Wheels;
    }
}

public class Motorcycle : Vehicle, IVehicle
{
    public int Wheels => 2;
}

Now every derived class will have to have a Wheels property that the base class can use.

Answer 2

You need to make the Wheels member virtual and override it, otherwise you're just "hiding" the base member. You'll also need to change it to a property in order to make it virtual :

public abstract class Vehicle 
{
    public virtual int Wheels { get { return 4; } }

    public int CountWheels()
    {
        return Wheels;
    }
}

public class Motorcycle : Vehicle
{
    public override int Wheels { get { return 2; } }
}

Answer 3

Look up virtual properties and methods ( https://msdn.microsoft.com/en-us/library/9fkccyh4.aspx )

You need Wheels to be public virtual int Wheels {get; set;} public virtual int Wheels {get; set;} in base and public override int Wheels {get; set;} public override int Wheels {get; set;} in derived

Then you can initialize the value as you need in each class.

Answer 4

Here's another option. Rather than defining Wheels as a virtual property on Vehicle , and overriding that property on Motorcycle , you can define Wheels as an ordinary property and then set it in the constructor for Motorcycle :

public abstract class Vehicle : IVehicle
{
    public int Wheels { get; protected set; }

    public Vehicle()
    {
        Wheels = 4;
    }

    public int CountWheels()
    {
        return Wheels;
    }
}

public class Motorcycle : Vehicle, IVehicle
{
    public Motorcycle()
    {
        Wheels = 2;
    }
}