C#用抽象類定義幾何圖形

using System;
/*using System.Data;*/
namespace tx
{
    abstract class tx
    {
        public double chang;
        public double kuan;
        public double r;
        public double pai;
        public tx(double chang, double kuan)
        {
            this.chang = chang;
            this.kuan = kuan;
           /* this.r = r;
            this.pai = pai*/;
        }
        public abstract void mj();
    }
    class Cfx:tx
    {
        public Cfx(double chang, double kuan) : base(chang,kuan) { }
        public override void mj()
        {
            Console.WriteLine("這個圖形是長方形面積為"+chang*kuan+"m^2");
        }
    }
    class zfx : tx
    {
        public zfx(double chang, double kuan) : base(chang,kuan) { }
        public override void mj()
        {
            Console.WriteLine("這個圖形是正方形"+chang*kuan+"m^2");
        }
    }
    class Yx : tx
    {
        /*public int k;*/
        
        public Yx(double r) : base(r,0) { }
        public override void mj()
        {
            Console.WriteLine("這圖形是圓形面積是"+Math.PI*chang*chang + "m^2");
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Cfx c = new Cfx(10, 12);
            zfx z = new zfx(5, 5);
            Yx y = new Yx(4);
            c.mj();
            z.mj();
            y.mj();
        }
    }
}