C#richTextBox的一个保存问题

实例化的richTextBox 不能存储内容吗??

控件:
richTextBox1 comboBox1 button1

把下面的 public static void BindCom(RichTextBox txt,ComboBox com, string path)

改为

public static void BindCom(ComboBox com, string path)

去掉注释
//RichTextBox txt = new RichTextBox(); ← 注意这实例化

这样的话,保存内容是什么都没有,断点后看了一下,明明有内容的,却为何保存不了呢?求解惑!!!

        //绑定下拉框数据

        public static void BindCom(RichTextBox txt,ComboBox com, string path)

        {

            //RichTextBox txt = new RichTextBox();   ← 注意这实例化

            string[] typeBooks;

            if (!File.Exists(path))

            {

                typeBooks = new string[]{"选择类型","type0","type1",

                    "type2","type3","type4","type5","type6"};

                int count = typeBooks.Count();

                int len = 1;

                foreach (var str in typeBooks)

                {

                    txt.Text += str;

                    

                    if (len < typeBooks.Count())

                    { txt.Text += "\n"; len++; }

                }

                txt.SaveFile(path, RichTextBoxStreamType.PlainText);

            }

            FileStream fs = new FileStream(path,

                FileMode.Open, FileAccess.Read, FileShare.None);

            StreamReader sr = new StreamReader(fs, Encoding.Default);

            txt.Text = sr.ReadToEnd();

            sr.Close();

            sr.Dispose();

            typeBooks = txt.Lines;

            foreach (var str in typeBooks)

            {

                if (str != "")

                    com.Items.Add(str);

            }

            com.SelectedIndex = 0;

        }

        private void button1_Click(object sender, EventArgs e)

        {

            //typeBooks = new string[]{"选择类型","type0","type1",

            //    "type2","type3","type4","type5","type6"};

            //int count = typeBooks.Count();

            //int len = 1;

            //foreach (var str in typeBooks)

            //{

            //    richTextBox1.Text += str;

            //    if (len < typeBooks.Count())

            //    { richTextBox1.Text += "\n"; len++; }

            //}

            //richTextBox1.SaveFile("Type.txt", RichTextBoxStreamType.TextTextOleObjs);



            BindCom(richTextBox1 ,comboBox1, "Type.txt");

        }