c# 文本文档 字体格式保存问题

我用C#窗体应用程序，做了一个文本文档，我想把富文本框中的文字（包括字体）保存，但是只能保存文字，字体不能保存
下面是打开文件按钮的代码，
private void 打开OToolStripMenuItem_Click(object sender, EventArgs e)
{
openfile_dialog.Filter = "(*.txt,*.rtf)|*txt;*rtf";
DialogResult dr = openfile_dialog.ShowDialog();
string text = null;
if (dr == DialogResult.OK)
{
file_path = openfile_dialog.FileName;
FileStream fstream = File.OpenRead(file_path); //创建文件流
StreamReader sreader = new StreamReader(fstream,Encoding.Default);
fstream.Seek(0, SeekOrigin.Begin);
while (sreader.Peek() > -1)
{
text += sreader.ReadLine() + "\r\n";
}
sreader.Close(); //关闭文件流
fstream.Close();
this.保存_tsbtn.Enabled = true;
this.保存_tsmi.Enabled = true;
rtxtbx.Text = text;
}
}
下面是我保存文件按钮的代码
private void 保存_tsmi_Click(object sender, EventArgs e)
{
change = false;
FileStream fstream = File.OpenWrite(file_path);
StreamWriter swriter = new StreamWriter(fstream,Encoding.Default);
swriter.WriteLine(rtxtbx.Text);
swriter.Close();
fstream.Close();
}
感谢大家不吝赐教