读取txt文件字符串转化问题~~~~~~~~~~~~~~~~~

1、先是去读txt文件StreamReader sr = new StreamReader(textBox4.Text, Encoding.GetEncoding("gb2312"));
string _txtContent = sr.ReadToEnd();
richTextBox1.Text = _txtContent;//显示正常2、对文件分割保存，例如分割一个txt文件成50份                            Byte[] bs = Encoding.GetEncoding("gb2312").GetBytes(txtContent.Substring(k * zhengshu, zhengshu).ToCharArray());
                            string txtContentSub = Encoding.GetEncoding("gb2312").GetString(bs);
                            StreamWriter sw = new StreamWriter(textBox1.Text + "\\" + textBox4.Text.Substring(textBox4.Text.LastIndexOf("\\") + 1, (textBox4.Text.LastIndexOf(".") - textBox4.Text.LastIndexOf("\\") - 1)) + (k + 1).ToString() + ".txt", false);
                            if (k == n - 1)
                            {
                                sw.WriteLine(txtContent.Substring((n - 1) * zhengshu));
                            }
                            else
                            {
                                sw.WriteLine(txtContentSub);
                            }
                            sw.Close();进行上面的代码后，新生成的txt文件显示正常
但是，当用程序打开生成之后的txt文件时显示乱码，这是什么原因？？应该怎么修改
读取通过程序分割之后的txt文件StreamReader sr = new StreamReader(textBox4.Text, Encoding.GetEncoding("gb2312"));
string _txtContent = sr.ReadToEnd();
richTextBox1.Text = _txtContent;//显示乱码