string str;
            openPicture.Filter = "rtf文件(*.rtf)|*.rtf";
            if (openPicture.ShowDialog() == DialogResult.OK && openPicture.FileName.Length > 0)
            {
                str = openPicture.FileName;
                FileStream fs = new FileStream(str, FileMode.Open, FileAccess.Read);
                StreamReader m_streamReader = new StreamReader(fs);
                //使用StreamReader类来读取文件
                m_streamReader.BaseStream.Seek(0, SeekOrigin.Begin);
                // 从数据流中读取每一行,直到文件的最后一行,并在richTextBox1中显示出内容
                this.richTextBox1.Text = "";
                string strLine = m_streamReader.ReadLine();
                while (strLine != null)
                {
                    this.richTextBox1.Text += strLine + "\n";
                    strLine = m_streamReader.ReadLine();
                }
                //关闭此StreamReader对象
                m_streamReader.Close();
            }
试试