IPaymentService.cs

namespace ddd_patterns.Step6
{
    public interface IPaymentService
    {
        public bool Pay(decimal amount, decimal vat);
    }
    
}

ITicketRepository.cs

using System;

namespace ddd_patterns.Step6
{

    public interface ITicketRepository
    {
        public int HowManyAreAvailable();

        public Guid Add(Guid attendee);

        public void Remove(Guid tId);
        
        void Payed(Guid ticket);
        
        DateTime BookingDateFor(Guid ticket);
    }
}

IVoucherService.cs

namespace ddd_patterns.Step6
{
    public interface IVoucherService
    {
        public int DiscountFor(string voucher);
    }
    
}

OnlineTickets.cs

using System;

namespace ddd_patterns.Step6
{
    public class OnlineTickets
    {
        private readonly ITicketRepository _tickets;
        private readonly IVoucherService _vouchers;
        private readonly IPaymentService _paymentService;
        private readonly DateTime _earlyBirdFrom;
        private readonly DateTime _earlyBirdUntil;
        private readonly DateTime _blindBirdFrom;
        private readonly DateTime _blindBirdUntil;
        private readonly decimal _blindBirdPrice;
        private readonly decimal _earlyBirdPrice;
        private readonly decimal _normalBirdPrice;

        public OnlineTickets(
            ITicketRepository tickets, 
            IVoucherService vouchers,
            IPaymentService paymentService,
            DateTime earlyBirdFrom, 
            DateTime earlyBirdUntil, 
            DateTime blindBirdFrom, 
            DateTime blindBirdUntil, 
            decimal blindBirdPrice, 
            decimal earlyBirdPrice, 
            decimal normalBirdPrice)
        {
            _tickets = tickets;
            _vouchers = vouchers;
            _paymentService = paymentService;
            _earlyBirdFrom = earlyBirdFrom;
            _earlyBirdUntil = earlyBirdUntil;
            _blindBirdFrom = blindBirdFrom;
            _blindBirdUntil = blindBirdUntil;
            _blindBirdPrice = blindBirdPrice;
            _earlyBirdPrice = earlyBirdPrice;
            _normalBirdPrice = normalBirdPrice;
        }

        public Guid BookFor(Guid attendee)
        {
            var availableTickets = _tickets.HowManyAreAvailable();

            if (availableTickets <= 0)
            {
                throw new Exception("No Ticket available");
            }

            return _tickets.Add(attendee);
        }
        

        public void Cancel(Guid ticket)
        {
            _tickets.Remove(ticket);
        }


        public bool PayFor(Guid ticket, string voucher)
        {
            var bookingDate = _tickets.BookingDateFor(ticket);

            var amount = _normalBirdPrice;

            if (bookingDate >= _blindBirdFrom && bookingDate <= _blindBirdUntil)
            {
                amount = _blindBirdPrice;
            }
            else if (bookingDate >= _earlyBirdFrom && bookingDate <= _earlyBirdUntil)
            {
                amount = _earlyBirdPrice;
            }

            amount -= _vouchers.DiscountFor(voucher);

            var vat = amount * 0.19m;
            
            if (!_paymentService.Pay(amount, vat))
            {
                return false;
            }

            _tickets.Payed(ticket);
            
            return true;
        }
    }
}

SqlTicketRepository.cs

using System;
using System.Data.SqlClient;

namespace ddd_patterns.Step6
{


    public class SqlTicketRepository : ITicketRepository
    {
        private readonly string _connectionString;

        public SqlTicketRepository(string connectionString)
        {
            _connectionString = connectionString;
        }

        public int HowManyAreAvailable()
        {
            var availableTickets = 0;
            const string sql1 = @"SELECT get number of available tickets from db";

            using var connection = new SqlConnection(_connectionString);
            var command = new SqlCommand(sql1, connection);
            connection.Open();

            var reader = command.ExecuteReader();

            while (reader.Read())
            {
                availableTickets = reader.GetInt32(0);
            }

            reader.Close();
            return availableTickets;
        }

        public Guid Add(Guid attendee)
        {
            using var connection = new SqlConnection(_connectionString);
            var tId = Guid.NewGuid();
            const string sql2 = @"Insert into tickets... tId = @tId, aId = @aId";
            var command2 = new SqlCommand(sql2, connection);
            command2.Parameters.AddWithValue("@tId", tId);
            command2.Parameters.AddWithValue("@tad", attendee);
            command2.ExecuteNonQuery();
            return tId;
        }

        public void Remove(Guid tId)
        {
            using var connection = new SqlConnection(_connectionString);

            const string sql = @"Delete FROM tickets WHERE tId = @tId";
            var command = new SqlCommand(sql, connection);
            command.Parameters.AddWithValue("@tId", tId);
            command.ExecuteNonQuery();
        }

        public void Payed(Guid ticket)
        {
            throw new NotImplementedException();
        }

        public DateTime BookingDateFor(Guid ticket)
        {
            throw new NotImplementedException();
        }
    }
}