CompareTo for abstract class

/*
1.Создать абстрактный класс Figure с методами вычисления площади и периметра, а также методом, выводящим информацию о фигуре на экран.
2.Создать производные классы: Rectangle (прямоугольник), Circle (круг), Triangle (треугольник) со своими методами вычисления площади и периметра.
3.Реализовать метод CompareTo, для сравнения объектов по площади.
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using Microsoft.Win32;
using System.IO;

namespace prakt_10
{
    abstract public class Figure : IComparable
    {
        abstract public double Area();    // get area
        abstract public double Perimeter();    //get perimetr
        abstract public void Show();    //show information
        abstract public void ShowArea(); // return area only

        public int CompareTo(object obj)
        {
            Figure b = (Figure)obj;
            if (this.Area() == b.Area())
            {
                return 0;
            }
            else
            {
                if (this.Area() > b.Area())
                {
                    return 1;
                }
                else
                {
                    return -1;
                }
            }
        }
    }

    class Trangle : Figure
    {
        private readonly string name = "Trangle";

        private int x1, y1;
        private int x2, y2;
        private int x3, y3;

        public Trangle(int x1, int y1, int x2, int y2, int x3, int y3)
        {
            this.x1 = x1;
            this.y1 = y1;

            this.x2 = x2;
            this.y2 = y2;

            this.x3 = x3;
            this.y3 = y3;
        }

        public int lengthSegment(int x2, int x1, int y2, int y1)
        {
            return ((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
        }

        public override double Area()
        {
            int segment1 = lengthSegment(this.x2, this.x1, this.y2, this.y1);
            int segment2 = lengthSegment(this.x3, this.x2, this.y3, this.y2);
            int segment3 = lengthSegment(this.x1, this.x3, this.y1, this.y3);

            double pPerimetr = segment3 + segment1 + segment2;

            return pPerimetr * Math.Sqrt((pPerimetr - segment1) * (pPerimetr - segment2) * (pPerimetr - segment3));
        }

        public override double Perimeter()
        {
            int segment1 = lengthSegment(this.x2, this.x1, this.y2, this.y1);
            int segment2 = lengthSegment(this.x3, this.x2, this.y3, this.y2);
            int segment3 = lengthSegment(this.x1, this.x3, this.y1, this.y3);

            return segment2 + segment1 + segment3;
        }

        public override void Show()
        {
            Console.WriteLine("Name figure:{0}", this.name);

            Console.WriteLine("Point coordinates:");
            Console.WriteLine("Point 1: ({0};{1})", this.x1, this.y1);
            Console.WriteLine("Point 2: ({0};{1})", this.x2, this.y2);
            Console.WriteLine("Point 3: ({0};{1})", this.x3, this.y3);

            Console.WriteLine("Length of a segment: {0}, {1}, {2}, area: {3}, perimetr: {4}",
                this.lengthSegment(this.x2, this.x1, this.y2, this.y1),
                this.lengthSegment(this.x3, this.x2, this.y3, this.y2),
                this.lengthSegment(this.x1, this.x3, this.y1, this.y3),
                this.Area(), this.Perimeter());
            Console.WriteLine();
        }

        public override void ShowArea()
        {
            Console.WriteLine("Area {0} = {1}",this.name, this.Area());
        }
    }

    class Rectangle : Figure
    {
        private readonly string name = "Rectangle";

        private int x1, y1;
        private int x2, y2;
        private int x3, y3;
        private int x4, y4;

        public Rectangle(int x1, int y1, int x2, int y2, int x3, int y3, int x4, int y4)
        {
            this.x1 = x1;
            this.y1 = y1;

            this.x2 = x2;
            this.y2 = y2;

            this.x3 = x3;
            this.y3 = y3;

            this.x4 = x4;
            this.y4 = y4;
        }

        public int lengthSegment(int x2, int x1, int y2, int y1)
        {
            return ((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
        }

        public override double Area()
        {
            int segment1 = lengthSegment(this.x2, this.x1, this.y2, this.y1);
            int segment2 = lengthSegment(this.x3, this.x2, this.y3, this.y2);
            return (segment2 * segment1);
        }

        public override double Perimeter()
        {
            int segment1 = lengthSegment(this.x2, this.x1, this.y2, this.y1);
            int segment2 = lengthSegment(this.x3, this.x2, this.y3, this.y2);
            return ((segment2 + segment1) * 2);
        }

        public override void Show()
        {
            Console.WriteLine("Name figure:{0}", this.name);

            Console.WriteLine("Point coordinates:");
            Console.WriteLine("Point 1: ({0};{1})", this.x1, this.y1);
            Console.WriteLine("Point 2: ({0};{1})", this.x2, this.y2);
            Console.WriteLine("Point 3: ({0};{1})", this.x3, this.y3);
            Console.WriteLine("Point 4: ({0};{1})", this.x4, this.y4);

            Console.WriteLine("Length of a segment: length {0}, width: {1}, area: {2}, perimetr: {3}",
                this.lengthSegment(this.x2, this.x1, this.y2, this.y1),
                this.lengthSegment(this.x3, this.x2, this.y3, this.y2),
                this.Area(), this.Perimeter());
            Console.WriteLine();
        }

        public override void ShowArea()
        {
            Console.WriteLine("Area {0} = {1}", this.name, this.Area());
        }
    }

    class Circle : Figure
    {
        private readonly string name = "Circle";

        private int x, y;
        private double radius;

        public Circle(int x, int y, int radius)
        {
            this.x = x;
            this.y = y;
            this.radius = radius;
        }

        public override double Area()
        {
            return Math.PI * this.radius * this.radius;
        }

        public override double Perimeter()
        {
            return Math.PI * this.radius * 2;
        }

        public override void Show()
        {
            Console.WriteLine("Name figure:{0}", this.name);

            Console.WriteLine("Point coordinates:");
            Console.WriteLine("Point: ({0};{1})", this.x, this.y);

            Console.WriteLine("Area: {0}, perimetr: {1}", this.Area(), this.Perimeter());
            Console.WriteLine();
        }

        public override void ShowArea()
        {
            Console.WriteLine("Area {0} = {1}", this.name, this.Area());
        }
    }


    class Programm
    {
        static void Main(string[] args)
        {
            Console.Write("Enter n: ");
            int n = int.Parse(Console.ReadLine());

            Figure[] array = new Figure[n];
            using (StreamReader file = new StreamReader("../../data.txt"))
            {
                for (int i = 0; i < n; i++)
                {
                    string[] line = file.ReadLine().Split(' ');
                    switch (line[0])
                    {
                        case "Rectangle":
                            array[i] = new Rectangle(
                                int.Parse(line[1]),
                                int.Parse(line[2]),
                                int.Parse(line[3]),
                                int.Parse(line[4]),
                                int.Parse(line[5]),
                                int.Parse(line[6]),
                                int.Parse(line[7]),
                                int.Parse(line[8]));
                            break;

                        case "Trangle":
                            array[i] = new Trangle(
                                int.Parse(line[1]),
                                int.Parse(line[2]),
                                int.Parse(line[3]),
                                int.Parse(line[4]),
                                int.Parse(line[5]),
                                int.Parse(line[6]));
                            break;

                        case "Circle":
                            array[i] = new Circle(
                                int.Parse(line[1]),
                                int.Parse(line[2]),
                                int.Parse(line[3]));
                            break;

                        default:
                            Console.WriteLine("Keywords not found.");
                            break;
                    }
                }

                for (int i = 0; i < n; i++)
                {
                    array[i].ShowArea();
                }

                Console.WriteLine();
                Array.Sort(array);

                for (int i = 0; i < n; i++)
                {
                    array[i].ShowArea();
                }

                Console.ReadKey();
            }
        }
    }
}