abstract class Geometry 
{
    public string Color {get; set;}
}


class Ring : Geometry 
{
    public double Radius {get; set;}
}

class Triangle : Geometry 
{

}

class Square : Geometry 
{

}

I want to add property radius only to class ring, but I have to declare radius also in geometry class if I want instances of Ring to have access to Radius property in Main, and then Main instances of triangle and square also have access to radius - which I dont want.

The same thing is with methods

I declare classes in Main like this:

Geometry ring1 = new Ring();
Geometry triangle1 = new Triangle();
Geometry square1 = new Square();

I know that it works with Ring ring1 = new Ring(); but what if I want to declare it like this: Geometry ring1 = new Ring(); to put everything in an array of geometry objects?

CodePudding user response：

If you place everything in an array of type Geometry you don't want access to the radius. You want to handle all objects as a Geometry and since that class doesn't implement the radius is not visible without casting.

This becomes interesting when you have methods or properties that span all classes e.g. a method for calculating the area of a shape. They each can have their own implementation with the circle relying on the radius while a quare uses the length of a side.

After all you create an array of the shapes because they have something in common that you want to make use of.

CodePudding user response：

Maybe you should read more about inheritance and polymorphism.

You can access derived properties through the base class via conversion, e.g.:

Geometry ring= new Ring();
var radius = (ring as Ring).Radius;