public class SqlHelper<T, Command>

        where T : DbConnection

        where Command : DbCommand, new() {

        public object ReadValue(T connection, string selectQuery, params DbParameter[] pars) {

            using(var sql = CreateCommand(selectQuery, connection)) {

                sql.CommandTimeout = ;

                if(pars != null)

                    sql.Parameters.AddRange(pars);

                try {

                    return CheckDbNull(sql.ExecuteScalar());

                }

                catch { return null; }

            }

        }

        object CheckDbNull(object value) {

            if(value == null) return null;

            if(Object.ReferenceEquals(value, DBNull.Value)) return null;

            return value;

        }

        static DbCommand CreateCommand(string selectQuery, T connection) {

            return new Command() { CommandText = selectQuery, Connection = connection };

        }

        public List<object[]> ReadValues(T connection, string selectQuery, params DbParameter[] pars) {

            List<object[]> res = new List<object[]>();

            try {

                using(var sql = CreateCommand(selectQuery, connection)) {

                    sql.CommandTimeout = ;

                    if(pars != null) sql.Parameters.AddRange(pars);

                    using(DbDataReader reader = sql.ExecuteReader()) {

                        if(!reader.HasRows) return res;

                        while(reader.Read()) {

                            object[] values = new object[reader.FieldCount];

                            reader.GetValues(values);

                            res.Add(values);

                        }

                    }

                }

            }

            catch { }

            return res;

        }

        public string GetString(object val) {

            if(val == DBNull.Value || val == null) return string.Empty;

            return val.ToString();

        }

        public DateTime GetDateInv(object val) {

            if(val == DBNull.Value || val == null) return DateTime.MinValue;

            return DateTime.ParseExact(val.ToString(), "d/M/yyyy", DateTimeFormatInfo.InvariantInfo);

        }

        public int GetInt(object val) {

            if(val == DBNull.Value || val == null) return ;

            return Convert.ToInt32(val);

        }

        public DateTime GetDate(object value) {

            if(value == null || value == DBNull.Value) return DateTime.MinValue;

            if(value is DateTime) return (DateTime)value;

            return GetDateInv(value);

        }

        public decimal GetDecimal(object value) {

            if(value == null || value == DBNull.Value) return ;

            return (decimal)Convert.ChangeType(value, typeof(decimal));

        }

        public Guid GetGuid(object value) {

            if(value == null || value == DBNull.Value) return Guid.Empty;

            return new Guid(value.ToString());

        }

        public bool GetBool(object value) {

            if(value == null || value == DBNull.Value) return false;

            if(value is bool) return (bool)value;

            if(value is int) return (int)value == ;

            return false;

        }

    }

   using (var connection = new OleDbConnection(connectionString))

            {

                connection.Open();

                //new OleDbCommand("delete * from sales", connection).ExecuteNonQuery(); //FOR TEST PURPOSES

                var helper = new SqlHelper<OleDbConnection, OleDbCommand>();

                DateTime minDate = helper.GetDate(helper.ReadValue(connection, "select min(sale_date) from sales"));

                DateTime maxDate = helper.GetDate(helper.ReadValue(connection, "select max(sale_date) from sales"));

                this.maxId = helper.GetInt(helper.ReadValue(connection, "select max(id) from sales"));

                DateTime startDate = minRequiredDate;

                if (minDate > startDate && maxDate != DateTime.MinValue)

                {

                    startDate = maxDate.AddDays();

                }

                if (startDate > DateTime.Today.AddDays())

                    return true;

                double daysCount = currentDate.Subtract(startDate).TotalDays;

                RaiseStart();

                try { Generate(connection, startDate, (int)daysCount); }

                finally { RaiseComplete(); }

                connection.Close();

            }