I am currently trying to return an array which contains information about a seat at a theate such as Seat number, Name, Price and Status. I am using a combobox where I want to list all vacant or reserved seats based upon choice. When I choose reserved seats in my combobox, I call upon a method using AddRange. This method is supposed to loop through an array containing all seats and their information. If a seat is Vacant, I add it to an array. When all is done, I return this array. However, I am dealing with a ArgumentNullException.

MainForm

namespace Assignment4
{

    public partial class MainForm : Form
    {
       // private const int totNumberOfSeats = 240;
         private SeatManager seatMngr; 


         private const int columns = 10;
         private const int rows = 10;



         public enum DisplayOptions
         {
             AllSeats,
             VacantSeats,
             ReservedSeats
         }




        public MainForm()
        {
            InitializeComponent();

            seatMngr = new SeatManager(rows, columns);

            InitializeGUI();

        }




        /// <summary>
        /// Fill the listbox with information from the beginning, 
        /// let the user be able to choose from vacant seats.
        /// </summary>
        private void InitializeGUI()
        {

            rbReserve.Checked = true;
            txtName.Text = string.Empty;
            txtPrice.Text = string.Empty;

            lblTotalSeats.Text = seatMngr.GetNumOfSeats().ToString();

            cmbOptions.Items.AddRange(Enum.GetNames(typeof(DisplayOptions)));
            cmbOptions.SelectedIndex = 0;

            UpdateGUI();

        }




        /// <summary>
        /// call on methods ValidateName and ValidatePrice with arguments
        /// </summary>
        /// <param name="name"></param>
        /// <param name="price"></param>
        /// <returns></returns>
        private bool ValidateInput(out string name, out double price)
        {
            bool nameOK = ValidateName(out name);
            bool priceOK = ValidatePrice(out price);

            return nameOK && priceOK;
        }




        /// <summary>
        /// Validate name using inputUtility, show error if input is invalid
        /// </summary>
        /// <param name="name"></param>
        /// <returns></returns>
        private bool ValidateName(out string name)
        {
            name = txtName.Text.Trim();

            if (!InputUtility.ValidateString(name))
            {
                //inform user
                MessageBox.Show("Input of name is Invalid. It can not be empty, " + Environment.NewLine +
                    "and must have at least one character.", " Error!");

                txtName.Focus();
                txtName.Text = " ";
                txtName.SelectAll();
                return false;
            }
            return true;
        }




        /// <summary>
        /// Validate price using inputUtility, show error if input is invalid
        /// </summary>
        /// <param name="price"></param>
        /// <returns></returns>
        private bool ValidatePrice(out double price)
        {

            // show error if input is invalid
            if (!InputUtility.GetDouble(txtPrice.Text.Trim(), out price, 0))
            {
                //inform user
                MessageBox.Show("Input of price is Invalid. It can not be less than 0, " + Environment.NewLine +
                    "and must not be empty.", " Error!");

                txtPrice.Focus();
                txtPrice.Text = " ";
                txtPrice.SelectAll();
                return false;
            }
            return true;
        }



        /// <summary>
        /// Check if item is selected in listbox
        /// </summary>
        /// <returns></returns>
        private bool CheckSelectedIndex()
        {
            int index = lbSeats.SelectedIndex;
            if (index < 0)
            {
                MessageBox.Show("Please select an item in the box");
                return false;
            }
            else
                return true;
        }




        /// <summary>
        /// Call method ReserveOrCancelSeat when button OK is clicked
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnOK_Click(object sender, EventArgs e)
        {
            ReserveOrCancelSeat();
        }



        /// <summary>
        /// Reserve or cancel seat depending on choice the user makes. Update GUI after choice.
        /// </summary>
        private void ReserveOrCancelSeat()
        {
            if (CheckSelectedIndex() == true)
            {
                string name = string.Empty;
                double price = 0.0;
                int selectedSeat = lbSeats.SelectedIndex;
                bool reserve = false;
                bool cancel = false;

                if (rbReserve.Checked)
                {
                    DialogResult result = MessageBox.Show("Do you want to continue?", "Approve", MessageBoxButtons.YesNo);
                    if (result == DialogResult.Yes)
                    {
                        if (ValidateInput(out name, out price))
                        {
                            reserve = seatMngr.ReserveSeat(name, price, selectedSeat);
                            if (reserve == true)
                            {
                                MessageBox.Show("Seat has been reserved");
                                UpdateGUI();
                            }
                            else
                            {
                                MessageBox.Show("Seat has already been reserved");
                            }
                        }
                    }
                }
                else
                {
                    DialogResult result = MessageBox.Show("Do you want to continue?", "Approve", MessageBoxButtons.YesNo);
                    if (result == DialogResult.Yes)
                    {
                        cancel = seatMngr.CancelSeat(selectedSeat);
                        if (cancel == true)
                        {
                            MessageBox.Show("Seat has been cancelled");
                            UpdateGUI();
                        }
                        else
                        {
                            MessageBox.Show("Seat is already vacant");
                        }
                    }
                }
                UpdateGUI();
            }
        }




        /// <summary>
        /// Update GUI with new information. 
        /// </summary>
        /// <param name="customerName"></param>
        /// <param name="price"></param>
        private void UpdateGUI()
        {
            lbSeats.Items.Clear();
            lbSeats.Items.AddRange(seatMngr.GetSeatInfoString());
            lblVacantSeats.Text = seatMngr.GetNumOfVacant().ToString();
            lblReservedSeats.Text = seatMngr.GetNumOfReserved().ToString();

            if (rbReserve.Checked)
            {
                txtName.Text = string.Empty;
                txtPrice.Text = string.Empty;
            }
        }




        /// <summary>
        /// set textboxes to false if cancel reservation button is checked
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void rbCancel_CheckedChanged_1(object sender, EventArgs e)
        {
            txtName.Enabled = false;
            txtPrice.Enabled = false;
        }




        /// <summary>
        /// set textboxes to true if reserved radiobutton is checked
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void rbReserve_CheckedChanged_1(object sender, EventArgs e)
        {
            txtName.Enabled = true;
            txtPrice.Enabled = true;
        }



        /// <summary>
        /// Make necessary changes on the list depending on what choice the user makes. Show only 
        /// what the user wants to see, whether its all seats, reserved seats or vacant seats only.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void cmbOptions_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (cmbOptions.SelectedIndex == 0 && rbReserve.Checked) //All seats visible.
            {
                UpdateGUI();
                txtName.Enabled = true;
                txtPrice.Enabled = true;
                btnOK.Enabled = true;
            }
            else if (cmbOptions.SelectedIndex == 0 && rbCancel.Checked)
            {
                UpdateGUI();
                txtName.Enabled = false;
                txtPrice.Enabled = false;
                btnOK.Enabled = true;
            }

            else if (cmbOptions.SelectedIndex == 1) //Only vacant seats visible.
            {
                lbSeats.Items.Clear();
                lbSeats.Items.AddRange(seatMngr.ReturnVacantSeats()); // Value cannot be null
                txtName.Enabled = false;
                txtPrice.Enabled = false;
                btnOK.Enabled = false;
            }

            else if (cmbOptions.SelectedIndex == 2) //Only reserved seats visible.
            {
                lbSeats.Items.Clear();
                lbSeats.Items.AddRange(seatMngr.ReturnReservedSeats()); // Value cannot be null
                txtName.Enabled = false;
                txtPrice.Enabled = false;
                btnOK.Enabled = false;
            }
        }

    }
}

SeatManager

namespace Assignment4
{
class SeatManager
{

    private string[,] nameList = null;
    private double[,] priceList = null;
    private string[,] seatList = null;
    private readonly int totCols;
    private readonly int totRows;




    /// <summary>
    /// Constructor with declarations of size for all arrays.
    /// </summary>
    /// <param name="totNumberOfSeats"></param>
    public SeatManager(int row, int cols)
    {

        totCols = cols;
        totRows = row;

        nameList = new string[row, cols];
        priceList = new double[row, cols];
        seatList = new string[row, cols];


        for (int rows = 0; rows < row; rows++)
        {
            for (int col = 0; col < totCols; col++)
            {
                seatList[rows, col] = "Vacant";
            }
        }
    }




    /// <summary>
    /// Check if index is within bounds of the array
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    private bool CheckIndex(int index)
    {
        if (index >= 0 && index < nameList.Length)
            return true;
        else
            return false;
    }




    /// <summary>
    /// Return total number of seats
    /// </summary>
    /// <returns></returns>
    public int GetNumOfSeats()
    {
        int count = 0;

        for (int rows = 0; rows < totRows; rows++)
        {
            for (int cols = 0; cols < totCols; cols++)
            {
                count++;
            }   
        }
        return count;

    }




    /// <summary>
    /// Calculate and return total number of reserved seats
    /// </summary>
    /// <returns></returns>
    public int GetNumOfReserved()
    {
        int totReservedSeats = 0;

        for (int rows = 0; rows < totRows; rows++)
        {
            for (int col = 0; col < totCols; col++)
            {
                if (!string.IsNullOrEmpty(nameList[rows, col]))
                {
                    totReservedSeats++;
                }
            }
        }
        return totReservedSeats;
    }




    /// <summary>
    /// Calculate and return total number of vacant seats
    /// </summary>
    /// <returns></returns>
    public int GetNumOfVacant()
    {
        int totVacantSeats = 0;

        for (int rows = 0; rows < totRows; rows++)
        {
            for (int col = 0; col < totCols; col++)
            {
                if (string.IsNullOrEmpty(nameList[rows, col]))
                {
                    totVacantSeats++;
                }
            }
        }
        return totVacantSeats;
    }




    /// <summary>
    /// Return formated string with info about the seat, name, price and its status
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    public string GetSeatInfoAt(int index)
    {
        int cols = ReturnColumn(index);
        int rows = ReturnRow(index);

        string strOut = string.Format("{0,2} {1,10} {2,17} {3,20} {4,35:f2}",
                                       rows+1, cols+1, seatList[rows, cols], nameList[rows, cols], priceList[rows, cols]);
        return strOut;
    }




    /// <summary>
    /// Send an array containing all seats in the cinema
    /// </summary>
    /// <returns></returns>
    public string[] GetSeatInfoString()
    {
        int count = totRows * totCols;

        if (count <= 0)
            return null;

        string[] strSeatInfoStrings = new string[count];
        for (int i = 0; i < totRows * totCols; i++)
        {
            strSeatInfoStrings[i] = GetSeatInfoAt(i);
        }
        return strSeatInfoStrings;
    }




    /// <summary>
    /// Reserve seat if seat is vacant
    /// </summary>
    /// <param name="name"></param>
    /// <param name="price"></param>
    /// <param name="index"></param>
    /// <returns></returns>
    public bool ReserveSeat(string name, double price, int index)
    {
        int cols = ReturnColumn(index);
        int rows = ReturnRow(index);

        if (string.IsNullOrEmpty(nameList[rows, cols]))
        {
            nameList[rows, cols] = name;
            priceList[rows, cols] = price;
            seatList[rows, cols] = "Reserved";
            return true;
        }
        else
            return false;
    }




    public string[] ReturnVacantSeats()
    {

        int totVacantSeats = int.Parse(GetNumOfVacant().ToString());
        string[] vacantSeats = new string[totVacantSeats];
        for (int i = 0; i < vacantSeats.Length; i++)
        {
            if (GetSeatInfoAt(i) == "Vacant")
            {
                vacantSeats[i] = GetSeatInfoAt(i);
            }
        }
        return vacantSeats;
    }




    public string[] ReturnReservedSeats()
    {
        int totReservedSeats = int.Parse(GetNumOfReserved().ToString());

        string[] reservedSeats = new string[totReservedSeats];
        for (int i = 0; i < reservedSeats.Length; i++)
        {

            if (GetSeatInfoAt(i) == "Reserved")
            {
                reservedSeats[i] = GetSeatInfoAt(i);
            }
        }
        return reservedSeats;
    }




    /// <summary>
    /// Cancel seat if seat is reserved
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    public bool CancelSeat(int index)
    {
        int cols = ReturnColumn(index);
        int rows = ReturnRow(index);

        if (!string.IsNullOrEmpty(nameList[rows, cols]))
        {
            nameList[rows, cols] = "";
            priceList[rows, cols] = 0.0;
            seatList[rows, cols] = "Vacant";
            return true;
        }
        else
        {
            return false;
        }
    }



    /// <summary>
    /// Convert index to row and return value
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    public int ReturnRow(int index)
    {
        int vectorRow = index;
        int row;
        row = (int)Math.Ceiling((double)(vectorRow / totCols));
        return row;
    }



    /// <summary>
    /// Convert index to column and return value
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    public int ReturnColumn(int index)
    {
        int row = index;
        int col = row % totCols;
        return col;
    }
}
}

In MainForm, this is where I get ArgumentNullException:

lbSeats.Items.AddRange(seatMngr.ReturnVacantSeats());

And this is the method where the array is to be returned containing all vacant seats:

public string[] ReturnVacantSeats()
    {

        int totVacantSeats = int.Parse(GetNumOfVacant().ToString());
        string[] vacantSeats = new string[totVacantSeats];
        for (int i = 0; i < vacantSeats.Length; i++)
        {
            if (GetSeatInfoAt(i) == "Vacant")
            {
                vacantSeats[i] = GetSeatInfoAt(i);
            }
        }
        return vacantSeats;
    }