How to change values of constant or static variables when deriving a class?

Question

I have a question similar to this one: Can I have a base class where each derived class has its own copy of a static property?, but the answer there didn't seem very conclusive to me.

Basically, I have a class that has a constant property, and I want to derive a class that has all the same methods but uses a different value for the variable. For a simple analogy, imagine that I had a class that looked, in part, like this:

public class Triangle
{
    public const NumSides = 3;
    int sideLength;

    public int Perimeter()
    {
        return NumSides * sideLength;
    }
}

and now I wanted to make a Square class that was exactly like the Triangle class, except NumSides was now 4.

In other words, I want a class that now looks, in its entirety, something like:

public class Square : Triangle
{
    public const NumSides = 4;
}

but that still has access to all the methods of the Triangle class. (I can handle writing a new constructor if necessary, but I don't want to have to copy over the dozen or so methods from the original class. They're all supposed to behave exactly the same, but with NumSides now having a value of 4.)

I can't figure out how to do this, either with NumSides being a const or as a static property with only a get function. If I try to override the constant, I get "The modifier 'override' is not valid for this item." When I try to override the static property version, I get the message equivalent of "A static member Triangle.NumSides cannot be marked as override, virtual, or abstract."

And, yes, I've tried creating a Shape class, putting all the functionality in there, and then having both Triangle and Square inherit from that class. I run into the same problem with NumSides though. How can I change the effective value of a constant or static variable/property when deriving a new class?

A square is a type of triangle? I think my head just exploded a little bit. — Daniel Mann, Jul 15 '14 at 00:38

score 4 · Accepted Answer · answered Jul 15 '14 at 00:41

And, yes, I've tried creating a Shape class, putting all the functionality in there, and then having both Triangle and Square inherit from that class.

This is the right approach. A Square is not a Triangle, and shouldn't subclass it.

I run into the same problem with NumSides though. How can I change the effective value of a constant or static variable/property when deriving a new class?

You can't use static variables. You need to use non-statics, though they can be properties backed by a local constant if you chose. That being said, given the simplicity here, I'd just do:

public abstract class Shape
{
   public abstract int NumSides { get; }
   // Other functionality...
}

Override via:

public override int NumSides { get { return 3; } } // For triangle

This is good, thank you. The only problem is that it completely messes up most of my static methods, since I can't now reference the constant in those methods anymore. That's a little beyond the scope of the question I asked, but it is a significant side effect to this solution, which does bear mentioning. — Galendo, Jul 15 '14 at 02:36

Guffa · Answer 2 · 2014-07-15T00:56:04.493

To override the member it has to be virtual, and a constant or static can't be virtual. Make it a property instead:

public class Triangle
{
    public virtual int NumSides { get { return 3; } }

    int sideLength;

    public int Perimeter()
    {
        return NumSides * sideLength;
    }
}

public class Square : Triangle
{
    public override int NumSides { get { return 4; } }
}

The same approach would work for a Shape class, but you might want to make it abstract insetad of virtual to avoid having an implementation of the property in the Shape class.

How to change values of constant or static variables when deriving a class?

2 Answers2