textbox 光标定位问题

       private void button1_Click(object sender, EventArgs e)

        {

            OpenFileDialog openFileDialog = new OpenFileDialog();

            // openFileDialog.InitialDirectory = "c:\\";//注意这里写路径时要用c:\\而不是c:\

            openFileDialog.Filter = "文本文件|*.*|C#文件|*.cs|所有文件|*.*";

            openFileDialog.RestoreDirectory = true;

            openFileDialog.FilterIndex = 1;

            if (openFileDialog.ShowDialog() == DialogResult.OK)

            {

                try

                {

                    

                    textBox1.Text = System.IO.File.ReadAllText(openFileDialog.FileName, Encoding.Default);



                  

                }

                catch (Exception ex)

                {

                    MessageBox.Show(ex.Message);

                }

            }

        }

如何在打开文件后 把光标定位到指定的一行 和某个位置  就累世与书签   谢谢大神们