c# 读取及写入BOM 和不带BOM的文本文件

private void button6_Click_3(object sender, EventArgs e)
        {
            string filename = AppDomain.CurrentDomain.BaseDirectory + "test.json";
            try
            {
                FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read);
                int n = (int)fs.Length;
                byte[] b = new byte[n];
                int r = fs.Read(b, 0, n);
                fs.Close();
                rtxFlow.Text = Encoding.UTF8.GetString(b, 0, n);
                MessageBox.Show("读取成功", "bom.txt");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "错误提示");
            }

        }

        private void button7_Click_1(object sender, EventArgs e)
        {
            string filename = AppDomain.CurrentDomain.BaseDirectory + "sql.xml";
            try
            {
                FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read);
                int n = (int)fs.Length;
                byte[] b = new byte[n];
                int r = fs.Read(b, 0, n);
                fs.Close();
                rtxFlow.Text = Encoding.UTF8.GetString(b, 0, n);
                MessageBox.Show("读取成功", "non-bom.txt");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "错误提示");
            }
        }

        private void button8_Click(object sender, EventArgs e)
        {
            string filename = AppDomain.CurrentDomain.BaseDirectory + "bom.txt";
            try
            {
                using (StreamWriter writer = new StreamWriter(filename, false, Encoding.UTF8))
                {
                    writer.Write(rtxFlow.Text);
                }
                MessageBox.Show("写入成功", "bom.txt");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "错误提示");
            }

        }

        private void button9_Click(object sender, EventArgs e)
        {
            try
            {
                string filename = AppDomain.CurrentDomain.BaseDirectory + "nob-bom.txt";
                using (StreamWriter writer = new StreamWriter(filename, false, new UTF8Encoding(false)))
                {
                    writer.Write(rtxFlow.Text);
                }
                MessageBox.Show("写入成功", "nob-bom.txt");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "错误提示");
            }
        }

写入的bom.txt ,带有"EF BB BF "标志