Question

I have a problem that is very confusing for me. In the constructor of my class I create an instance of SeatManager.cs which holds two arrays (string and double). In the method btnReserveCancel_Click I’m trying to fill these two arrays with data. But when I later call the UpdateGUI() method another instance of SeatManager.cs is created (my teacher added that line of code when I needed help with another thing) and when that happens the all the data I just filled in the two arrays get lost! The wierd part is that if I delete the line which creates the new instance in UpdateGUI() the compiler yells at me that something is wrong.

Why does UpdateGUI() need a new instance of SeatManager.cs when btnReserveCancel_Click doesn’t? Why does UpdateGUI() need a new instance of SeatManager.cs when there is one available in the instance variables?

    private double revenue = 0.0;
    private const int totalNumOfSeats = 10;
    private int numOfReservedSeats = 0; //Increases every time a new reservation is made
    const double minLimit = 10;
    const double maxLimit = 50;
    private SeatManager seatMngr;

    public MainForm()
    {
        InitializeComponent();
        InitializeGUI();
        seatMngr = new SeatManager(totalNumOfSeats);//skapar en instans av klassen SeatManager
        UpdateGUI();
    }



    private void btnReserveCancel_Click(object sender, EventArgs e)
    {
        if (rbtnReserved.Checked == true)//Om radiobutton RESERVE är iklickad
        {
            string customerName = string.Empty;
            double seatPrice = 0.0;

            int selection = listBox1.SelectedIndex;
            if (selection == -1)
            {
                MessageBox.Show(string.Format("You must select which seat you want to reserve!"), "Select a seat.", MessageBoxButtons.OK, MessageBoxIcon.None);

            }
            else
            {
                string getSeatNumber = listBox1.SelectedItem.ToString();//Tar första bokstaven i den markerade strängen i listboxen och gör om till index.
                int seatNumber = int.Parse(getSeatNumber.Substring(0, 1));

                bool inputOk = ReadAndValidateInput(out customerName, out seatPrice);
                bool validSeats = CheckVacantSeats();

                if (inputOk && validSeats)
                {
                    if (seatMngr.ReserveSeat(customerName, seatPrice, seatNumber) != true)
                    {
                        var result = MessageBox.Show(string.Format("Do you wish to overwrite reservation? "), "Seat already registered", MessageBoxButtons.YesNo, MessageBoxIcon.None);
                        if (result == DialogResult.Yes)
                        {
                            double amount = seatMngr.GetPaidPrice(seatNumber);
                            MoneyBackWhenCancelOrOverwrite(amount);
                            seatMngr.ReserveSeatOverwrite(customerName, seatPrice, seatNumber);
                            revenue += seatPrice;
                        }
                    }
                    else
                    {
                        seatMngr.ReserveSeat(customerName, seatPrice, seatNumber);
                        numOfReservedSeats++;
                        revenue += seatPrice;
                        if (seatMngr.ReserveSeat(customerName, seatPrice, seatNumber) == true)
                        {
                            MessageBox.Show(string.Format("Det funkade "), "Sfgdfg", MessageBoxButtons.OK, MessageBoxIcon.None);
                        }
                    }
                }
            }
        }
        else if (rbtnCancel.Checked == true)//Om radiobutton CANCEL är iklickad.
        {
            string getSeatNumber = listBox1.SelectedItem.ToString();//Tar första bokstaven i den markerade strängen i listboxen och gör om till index.
            int seatNumber = int.Parse(getSeatNumber.Substring(0, 1));

            var result = MessageBox.Show(string.Format("Do you wish to cancel reservation? "), "Seat registered", MessageBoxButtons.YesNo, MessageBoxIcon.None);
            if (result == DialogResult.Yes)
            {
                double amount = seatMngr.GetPaidPrice(seatNumber);
                MoneyBackWhenCancelOrOverwrite(amount);
                seatMngr.CancelSeat(seatNumber);
                numOfReservedSeats--;
            }
            else { }
        }
        UpdateGUI();
    }



    private void UpdateGUI()
    {
        labelVacant.Text = (totalNumOfSeats - numOfReservedSeats).ToString();//Visar antal ledig platser.
        labelReserved.Text = numOfReservedSeats.ToString();//Visar antal reserverade platser.
        labelRevenue.Text = revenue.ToString();//Visar intäkter.
        labelSeats.Text = totalNumOfSeats.ToString();//Visar totalt antal platser. Värdet är konstant så det kan inte ändras.
        DisplayOptions choice = (DisplayOptions)comboBox1.SelectedIndex;

        string[] strSeatInfoStrings;


        //seatMngr = new SeatManager(totalNumOfSeats);

        int display = seatMngr.GetSeatInfoStrings(choice, out strSeatInfoStrings);
        listBox1.Items.Clear();
        if (strSeatInfoStrings == null)
        {
            listBox1.Items.Add("No seats where found");
        }
        else
        {
            listBox1.Items.AddRange(strSeatInfoStrings);
        }
    }

Answer 1

If you have a reference pointing to an object in memory and then you assign it a new instance, the object it was previously pointing to is “lost” (provided there are no more references to it) and eventually gets garbage collected. That’s why you are losing all previously filled data when you create the new instance inside UpdateGUI().

The correct version is obviously without that new instantiation if you plan to maintain state across method calls. What is the compiler error you are getting if you remove that line?

Edit: When you declare seatMngr try to also make the instantiation and remove it from the constructor:

private SeatManager seatMng = new SeatManager(totalNumOfSeats);