为什么我写的那个代码在测试的时候，打开后，文件里边的内容要么是显示出来是缺少几行，或者是显示不出中文什么的，显示乱码。

对了，我写的代码：
 private void OpenToolStripMenuItem_Click(object sender, EventArgs e)
        {
            OpenFileDialog Open = new OpenFileDialog();
            Open.Filter = "(*.txt)|*.txt|(*.*)|*.*";
            Open.FilterIndex = 1;
            if (Open.ShowDialog() != DialogResult.OK)
                return;
            try
            {
                StreamReader sw = new StreamReader(Open.FileName, true);
                string a = sw.ReadLine();
                while (sw.ReadLine() != null)
                {
                    richTextBox1.Text +=a;
                    richTextBox1.Text += "\r\n";
                    a = sw.ReadLine();
                    
                }
                sw.Close();
            }
            catch(IOException ex)
            {
                MessageBox.Show(ex.Message);
                return;
            }
        }

要怎么改，能不能指点一下，我现在学这个GUI ，学得头都大了

谢谢啊，我今天晚上回去看看行不行