在 Visual Studio 2019 中，类的 C# 实例返回空值，我不知道为什么

我正在开发一个 C# 表单应用程序。当我尝试使用从文本框中引入的值修改/更新现有对象的值时发生错误。我确信在构造函数中传递的数据类型是正确的。

在第二个屏幕截图中可以看到一条错误消息：

输入的字符串格式不正确

我在下面粘贴了 Holiday 对象的代码：

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

namespace AnthonyQuinnAssignmentObjectOrientatedProgram
{
    public class Holiday
    {
        public int HolidayNo { get; set; }
        public string Destination { get; set; }
        public decimal Cost { get; set; }
        public DateTime DepartureDate { get; set; }
        public int NoOfDays { get; set; }
        public int Available { get; set; }

        public Holiday() { }

        public Holiday(int holidayNo,string destination,decimal cost,DateTime departureDate,int noOfDays,int available)
        {
            HolidayNo = holidayNo;
            Destination = destination;
            Cost = cost;
            DepartureDate = departureDate;
            NoOfDays = noOfDays;
            Available = available;
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.SqlClient;
using System.Data;
using System.Windows.Forms;

namespace AnthonyQuinnAssignmentObjectOrientatedProgram
{
    public class HolidayDB
    {
        public HolidayDB()
        {
        }

        public static List<Holiday> GetHoliday()
        {
            SqlConnection connection = TravelDB.getconnection();
            List<Holiday> holidays = new List<Holiday>();

            try
            {
                connection.Open();

                string select = "SELECT *" + " FROM  tblHoliday";

                SqlCommand selectCommand = new SqlCommand(select,connection);

                SqlDataReader holidayReader = selectCommand.ExecuteReader();

                while (holidayReader.Read())
                {
                    Holiday holiday = new Holiday();

                    holiday.HolidayNo = Convert.ToInt32(holidayReader["HolidayNo"]);
                    holiday.Destination = holidayReader["Destination"].ToString();
                    holiday.Cost = decimal.Parse(holidayReader["Cost"].ToString());
                    holiday.DepartureDate = DateTime.Parse(holidayReader["DepartureDate"].ToString());
                    holiday.NoOfDays = Convert.ToInt32(holidayReader["NoOfDays"]);
                    holiday.Available = Convert.ToInt32(holidayReader["Available"]);

                    holidays.Add(holiday);
                }

                return holidays;
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.Message + "\n\n" + ex.GetType().ToString() + "\n\n" + ex.StackTrace,"Exception");
            }
            finally
            {
                connection.Close();
            }

            return null;
        }

        public static bool UpdateHoliday(Holiday oldHoliday,Holiday newHoliday)
        {
            SqlConnection connection = TravelDB.getconnection();

            string updateStatment =
                "UPDATE tblHoliday SET " +
                "Destination = @NewDestination," +
                "Cost = @NewCost" +
                "DepartureDate = @NewDepartureDate" +
                "NoOfDays = @NoOfDays" +
                "Availability = @Availability" +
                "WHERE Destination = @OldDestination" +
                "AND Destination = @OldDestination" +
                "AND Cost = @OldCost" +
                "AND DepartureDate = @OldDepartureDate" +
                "AND NoOfDays = @OldNoOfDays " +
                "AND Availability = @OldAvailability";

            SqlCommand updateCommand = new SqlCommand(updateStatment,connection);
            updateCommand.Parameters.AddWithValue("@NewDestination",newHoliday.Destination);
            updateCommand.Parameters.AddWithValue("@NewCost",newHoliday.Cost);
            updateCommand.Parameters.AddWithValue("@NewDepartureDate",newHoliday.DepartureDate);
            updateCommand.Parameters.AddWithValue("@NewNoOfDays",newHoliday.NoOfDays);
            updateCommand.Parameters.AddWithValue("@NewAvailability",newHoliday.Available);

            try
            {
                connection.Open();

                int count = updateCommand.ExecuteNonQuery();

                if (count > 0)
                    return true;
                else
                    return false;
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.Message + "\n\n" + ex.GetType().ToString() + "\n\n" + ex.StackTrace,"Exception");
            }
            finally
            {
                connection.Close();
            }

            return false;
        }

        public static bool AddHoliday(Holiday holiday)
        {
            SqlConnection connection = TravelDB.getconnection();
            string insertStatment =
                "INSERT tblHoliday" +
                "(HolidayNo,Destination,Cost,DepartureDate,NoOfDays,Available)" +
                "VALUES (@HolidayNo,@Destination,@Cost,@DepartureDate,@NoOfDays,@Available)";

            SqlCommand insertCommand = new SqlCommand(insertStatment,connection);
            insertCommand.Parameters.AddWithValue("@HolidayNo",holiday.HolidayNo);
            insertCommand.Parameters.AddWithValue("@Destination",holiday.Destination);
            insertCommand.Parameters.AddWithValue("@Cost",holiday.Cost);
            insertCommand.Parameters.AddWithValue("@DepartureDate",holiday.DepartureDate);
            insertCommand.Parameters.AddWithValue("@NoOfDays",holiday.NoOfDays);
            insertCommand.Parameters.AddWithValue("@Available",holiday.Available);

            try
            {
                connection.Open();

                int count = insertCommand.ExecuteNonQuery();

                if (count > 0)
                    return true;
                else
                    return false;
            }
            catch (SqlException ex)
            {
                throw ex;
                // MessageBox.Show(ex.Message + "\n\n" + ex.GetType().ToString() + "\n\n" + ex.StackTrace,"Exception");
            }
            finally
            {
                connection.Close();
            }
        }

        public static bool DeleteHoliday(Holiday holiday)
        {
            SqlConnection connection = TravelDB.getconnection();
            string deleteStatement =
                "DELETE  FROM tblHoliday" +
                "WHERE HolidayNo = @HolidayNo" +
                "AND Destination = @Destination" +
                "AND Cost = @Cost" +
                "AND DepartureDate = @DepartureDate " +
                "AND NoOfDays = @NoOfDays" +
                "AND Available = @Available";

            SqlCommand deleteCommand = new SqlCommand(deleteStatement,connection);
            deleteCommand.Parameters.AddWithValue("@HolidayNo",holiday.HolidayNo);
            deleteCommand.Parameters.AddWithValue("@Destination",holiday.Destination);
            deleteCommand.Parameters.AddWithValue("@Cost",holiday.Cost);
            deleteCommand.Parameters.AddWithValue("@DepartureDate",holiday.DepartureDate);
            deleteCommand.Parameters.AddWithValue("@NoOfDays",holiday.NoOfDays);
            deleteCommand.Parameters.AddWithValue("@Available",holiday.Available);

            try
            {
                connection.Open();

                int count = deleteCommand.ExecuteNonQuery();

                if (count > 0)
                    return true;
                else
                    return false;
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.Message + "\n\n" + ex.GetType().ToString() + "\n\n" + ex.StackTrace,"Exception");
                return false;
            }
            finally
            {
                connection.Close();
            }
        }
    }
}