1. In the base class, use the virtual keyword to make the methods overridable.
2. In every derived class, add the override keyword in the method definitions.

using System;
using System.Collections.Generic;

class Shape
{
    // Perimeter is the length of the 'outline' of a shape
    public float perimeter;

    // Change code below this line
    public virtual float getArea()
    {
        return 0.0f;
    }

    protected virtual float calculatePerimeter()
    {
        return 0.0f;
    }
    // Change code above this line
}

class Rectangle : Shape
{
    float width;
    float height;

    public Rectangle(float width, float height)
    {
        this.width = width;
        this.height = height;
        this.perimeter = this.calculatePerimeter();
    }

    // Change code below this line
    public override float getArea()
    {
        return width * height;
    }

    protected override float calculatePerimeter()
    {
        return width * 2 + height * 2;
    }
    // Change code above this line
}

class Square : Shape
{
    float length;

    public Square(float length)
    {
        this.length = length * length;
        this.perimeter = calculatePerimeter();
    }

    // Change code below this line
    public override float getArea()
    {
        return length * length;
    }

    protected override float calculatePerimeter()
    {
        return 4 * length;
    }
    // Change code above this line
}

class Circle : Shape
{
    float radius;

    public Circle(float radius)
    {
        this.radius = radius;
        this.perimeter = calculatePerimeter();
    }

    // Change code below this line
    public override float getArea()
    {
        return 3.14f * radius * radius;
    }

    protected override float calculatePerimeter()
    {
        return 2.00f * 3.14f * radius;
    }
    // Change code above this line
}

class ConsoleApp
{
    static void Main()
    {
        List<Shape> shapes = new List<Shape>();
        shapes.Add(new Rectangle(10f, 20f));
        shapes.Add(new Square(10f));
        shapes.Add(new Circle(10f));

        foreach (Shape shape in shapes)
        {
            Console.WriteLine(shape.getArea());
        }
    }
}