C#stream文件文本读取的问题

我这个是书上的例子,但这例子应该是印错了,在ButtonAdd的事件那,TextNewItem都没有申明是啥,我新手,想问问,TextNewItem应该怎么申明才能让这个读取文件内容到程序里的例子顺利运行

 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;
using System.IO;


namespace steamwriter
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        void FillItems()
        {
            ComboItems.Items.Clear();
            StreamReader sr = new StreamReader("Items.txt", Encoding.Default);
            try
            {
                while (!sr.EndOfStream)
                {
                    string NewItem = sr.ReadLine();
                    if (NewItem != "")
                    {
                        ComboItems.Items.Add(NewItem);

                    }
                }


            }
            finally
            {
                sr.Close();

            }
            ComboItems.Text = ComboItems.Items[0].ToString();


        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.Text = "文本文件的读写";
            FillItems();
        }

        private void ButtonAdd_Click(object sender, EventArgs e)
        {

            StreamWriter sw = new StreamWriter("item.txt ", true, Encoding.UTF8);

            if (TextNewItem.text != "")
            {
                sw.WriteLine(TextNewItem.text);

            }
            else
            {
                MessageBox.Show("不能添加空选项", "操作失败", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }

            sw.Close();
            FillItems();
            MessageBox.Show("选项添加成功!", "操作完成", MessageBoxButtons.OK, MessageBoxIcon.Information);

        }


    }
}

TextNewItem是一个控件的名称吧,比如是textBox的Name