Code Cộng trừ phân số với C# OPP

Code Cộng trừ phân số với C# OPP

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PhanSo
{
    class PhanSo
    {
        protected int Mau, Tu;
        //public PhanSo(int mau, int tu)
        //{
        //    this.Tu = tu;
        //    this.Mau = mau;
        //}
        public void Nhap()
        {
            int a = 0, b = 0;
            Console.Write("\nNhap vao phan so [a/b]:  ");
            NhapThem(ref a, ref b);
            this.Tu = a;
            this.Mau = b;
        }

        public void NhapThem(ref int a, ref int b)
        {
            string so = Console.ReadLine();
            string[] splitStr = so.Split('/');
            a = int.Parse(splitStr[0]);
            b = int.Parse(splitStr[1]);
        }



        public void Gan(int tu, int mau)
        {
            this.Tu = tu;
            this.Mau = mau;
        }

        public void Xuat()
        {
            Console.WriteLine("Phan so la: {0}/{1}", Tu, Mau);
            
        }

        public void Cong(int a, int b)
        {
            this.Tu = a * Mau + b * Tu;
            this.Mau = b * Mau;
        }
        public PhanSo Tru(PhanSo S1)
        {
            if (this.Tu != 0)
            {
                PhanSo ps1 = new PhanSo();
                ps1.Tu = Tu * S1.Mau - Mau * S1.Tu;
                ps1.Mau = Mau * S1.Mau;
                return ps1;
            }
            else {
                PhanSo ps = new PhanSo();
                ps.Tu = -S1.Tu;
                ps.Mau = S1.Mau;
                return ps;
            }
        }

        private int UCLN(int a, int b)
        {

            if (a != 0)
            {
                if (a > 0)
                {
                    while (a != b)
                    {
                        if (a > b) a -= b;
                        else b -= a;
                    }
                    return a;
                }
                else
                {
                    a = -a;
                    while (a != b)
                    {
                        if (a > b) a -= b;
                        else b -= a;
                    }
                    return a;
                }
            }
            else return 0;
            
        }

        public void rutGon()
        {
            int a = UCLN(this.Tu, this.Mau);

            if (a != 0)
            {
                    if (a > 0)
                    {
                        this.Tu = this.Tu / a;
                        this.Mau = this.Mau / a;
                    }
                    else
                    {
                        this.Tu = this.Tu / (-a);
                        this.Mau = this.Mau / (a);
                    }
            }
            else 
            {
                Gan(0, 0);
            }
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            int a = 0, b = 0;
            PhanSo ps = new PhanSo();
            tag:
            ps.Nhap();
            ps.rutGon();
            ps.Xuat();

            
            Console.WriteLine("\nXin moi xhon chuc nang: ");
            Console.WriteLine("Bam 1 de cong phan so[a/b]");
            Console.WriteLine("Bam 2 de tru phan so[a/b]");
            Console.WriteLine("Bam 3 Exit.");
            int choose = int.Parse(Console.ReadLine());
            switch (choose){
                case 1 :
                        Console.Write("\nNhap vao phan so: ");
                        ps.NhapThem(ref a, ref b);
                        ps.Cong(a,b);
                        ps.rutGon();
                        ps.Xuat();
                        goto tag;
                        break;
                       

                case 2:
                        Console.Write("\nNhap vao phan so: ");
                        PhanSo ps1 = new PhanSo();
                        ps1.Nhap();
                        ps = ps.Tru(ps1);
                        ps.rutGon();
                        ps.Xuat();
                        goto tag;
                        break;

                case 3:
                        break;
            }

           
        }
    }
}