form1.cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Chapter3
{
    public partial class Form1 : Form
    {

        Akun akun1 = new Akun(20000, "hiya");
        public Form1()
        {
            InitializeComponent();
            lblSaldo.Text = akun1.Saldo.ToString();
        }

        private void TarikSaldo(int total)
        {
            akun1.Tarik(total);
            lblSaldo.Text = akun1.Saldo.ToString();
        }

        private void SetorSaldo(int total)
        {
            akun1.Setor(total);
            lblSaldo.Text = akun1.Saldo.ToString();
        }

        private void btnLanjut_Click(object sender, EventArgs e)
        {
            if(akun1.PasswordTervalidasi(tbPassword.Text))
            {
                if (rbSetor.Checked)
                    SetorSaldo(int.Parse(tbUang.Text));
                else if (rbTarik.Checked)
                    TarikSaldo(int.Parse(tbUang.Text));
            }
            else
            {
                MessageBox.Show("Password salah");
            }
                
            
           
        }
    }
}

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

namespace Chapter3
{
    public class Akun
    {
        private const int maxSaldo = 2000000;
        private int _saldo;
        protected string password;
        

       
        public int Saldo
        {
            get
            {
                return _saldo;
            }
            set
            {
                if (value < 0)
                    throw new InvalidOperationException("Saldo tidak bisa kurang dari 0 rupiah");
                else if (value > maxSaldo)
                    throw new InvalidOperationException("Saldo tidak boleh melebihi 2000000 rupiah");
                else
                    _saldo = value;
            }
        }

        public Akun(int saldo, string password)
        {
           
            _saldo = saldo;
            this.password = password;
        }

        public  void Setor(int jumlah)
        {
            Saldo += jumlah;
        }

        public  void Tarik(int jumlah)
        {
            Saldo -= jumlah;
        }

        public bool PasswordTervalidasi(string password)
        {
            if (this.password == password)
                return true;
            else
                return false;
        }
    }
}