C#用抽象类定义几何图形

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("这个图形是长方形面积为"+chang*kuan+"m^2");
        }
    }
    class zfx : tx
    {
        public zfx(double chang, double kuan) : base(chang,kuan) { }
        public override void mj()
        {
            console.writeline("这个图形是正方形"+chang*kuan+"m^2");
        }
    }
    class yx : tx
    {
        /*public int k;*/
        
        public yx(double r) : base(r,0) { }
        public override void mj()
        {
            console.writeline("这图形是圆形面积是"+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();
        }
    }
}