Get it in to listbox

Question

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Booking
{
    public partial class MainForm : Form
    {
        private double revenue = 0.0;
        private const int totalNumOfSeats = 240;
        private int numOfReservedSeats = 0;
        public MainForm()
        {
            InitializeComponent();
            InitializeGUI();

        }
        private void InitializeGUI()
        {
            rbtnReserve.Checked = true;
            lstSeats.Items.Clear();
            txtName.Text = string.Empty;
        }
        private bool ReadAndValidateName(out string name)
        {
            name = "";
            if (txtName.Text.Length > 0)
            {
                name = txtName.Text;
                return true;
            }
           else
            {
           MessageBox.Show("Enter Letters Only", "Invalid Character", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtName.Focus();
                return false;
            }  
        }

        private bool ReadAndValidatePrice(out double price)
        {
            price = 0;
            double converted;
            converted = Convert.ToDouble(txtPrice.Text);

            if (converted >= 0.0)
            {
                price = Double.Parse(txtPrice.Text);
                return true;
            }
            else
            {
                MessageBox.Show("Enter Numbers Only", "Invalid Character", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtPrice.Focus();
                return false;
            }
        }
         
        private bool ReadAndValidateInput(out string name ,out double price)
        {
            return ReadAndValidateName(out name) & ReadAndValidatePrice(out price);
           
        }
        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void groupBox2_Enter(object sender, EventArgs e)
        {

        }

        private void txtPrice_TextChanged(object sender, EventArgs e)
        {

        }

        private void btnOK_Click(object sender, EventArgs e)
        {
            string costumerName = string.Empty;
            double seatPrice = 0.0;

            bool inputOk = ReadAndValidateInput(out costumerName, out seatPrice);
            if (inputOk)
            {
                numOfReservedSeats++;
                revenue += seatPrice;

            }
        }

        private void txtName_TextChanged(object sender, EventArgs e)
        {

        }

        private void lstSeats_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void txtPrice_TextChanged_1(object sender, EventArgs e)
        {

        }

        private void rbtnReserve_CheckedChanged(object sender, EventArgs e)
        {
            txtName.Enabled = true;
            txtPrice.Enabled = true;
            btnOK.Enabled = true;
        }

        private void rbtnCancel_CheckedChanged(object sender, EventArgs e)
        {
            txtName.Enabled = false;
            txtPrice.Enabled = false;
            btnOK.Enabled = false;
        }
 
    }
}
   
I wondering if anyone can help me to get it in to a listbox when i click on a button. I know i can do like lstSeats.Items.Add(string.Format("\t\t{0} \t{1}", txtPrice.Text, txtName.Text)); but then the error massage never com up so thats way i need help whit it on the butten i have know and the code in it