BankAccounts

public enum CustomerType
    {
        Individual,
        Company
    }
    public interface IRateable
    {
        double GetInterestAmount(int months);
    }
    public abstract class Account
    {
        public string CustomerFullName { get; set; }
        protected decimal Balance { get; set; }
        public CustomerType TypeOfCustomer { get; set; }

        public Account(string fullName, decimal balance, CustomerType typeCustomer)
        {
            this.CustomerFullName = fullName;
            this.Balance = balance;
            this.TypeOfCustomer = typeCustomer;
        }

        public void Deposit(decimal moneyToDeposit) { }
    }
    public class DepositAccount : Account, IRateable
    {
        public DepositAccount(string fullName, decimal balance, CustomerType typeCustomer)
            : base(fullName, balance, typeCustomer)
        { }

        public void Deposit(decimal money)
        {
            this.Balance += money;
            Console.WriteLine("Successful!");
        }

        public void WithDraw(decimal money)
        {
            if (money > this.Balance)
            {
                Console.WriteLine("Sorry! You haven't enough funds in your account! (-{0})", money - this.Balance);
                return;
            }
            else
            {
                this.Balance -= money;
                Console.WriteLine("Done.");
            }
        }

        public double GetInterestAmount(int months)
        {
            if (this.Balance >= 0 && this.Balance < 1000)
            {
                return 0;
            }
            else
            {
                return 1*months;
            }
        }
    }
    public class LoanAccount : Account, IRateable
    {
       private const int INDIVIDUAL_MONTHS_MINIMUM_REQUIRE = 3;
       private const int COMPANY_MONTHS_MINIMUM_REQUIRE = 2;

        public LoanAccount(string fullName, decimal balance, CustomerType typeCustomer)
            : base(fullName, balance, typeCustomer)
        { }

        public void Deposit(decimal money)
        {
            this.Balance += money;
            Console.WriteLine("Successful!");
        }

        public double GetInterestAmount(int months)
        {
            if (this.TypeOfCustomer == CustomerType.Individual && months <=INDIVIDUAL_MONTHS_MINIMUM_REQUIRE)
            {
                return 0;
            }
            else if (this.TypeOfCustomer == CustomerType.Individual && months > INDIVIDUAL_MONTHS_MINIMUM_REQUIRE)
            {
                return months-INDIVIDUAL_MONTHS_MINIMUM_REQUIRE;
            }
            if (this.TypeOfCustomer == CustomerType.Company && months <= COMPANY_MONTHS_MINIMUM_REQUIRE)
            {
                return 0;
            }
            else
            {
                return months - COMPANY_MONTHS_MINIMUM_REQUIRE;
            }
        }
    }
    public class MortgageAccount : Account, IRateable
    {
        private const int INDIVIDUAL_MONTHS_MINIMUM_REQUIRE = 6;
        private const int COMPANY_MONTHS_MINIMUM_REQUIRE = 12;

        private const double COMPANY_RATE = 1.5;

        public MortgageAccount(string fullName, decimal balance, CustomerType typeCustomer)
            : base(fullName, balance, typeCustomer)
        { }

        public void Deposit(decimal money)
        {
            this.Balance += money;
            Console.WriteLine("Successful!");
        }

        public double GetInterestAmount(int months)
        {
            if (this.TypeOfCustomer == CustomerType.Individual && months<=6)
            {
                return 0;
            }
            else if (this.TypeOfCustomer == CustomerType.Individual && months > 6)
            {
                return months-INDIVIDUAL_MONTHS_MINIMUM_REQUIRE;
            }
            if (this.TypeOfCustomer == CustomerType.Company && months <= 12)
            {
                return months*COMPANY_RATE;
            }
            else
            {
                return months-COMPANY_MONTHS_MINIMUM_REQUIRE;
            }
        }
    }