将RichTextBox的内容直接写入数据库：
private void button1_Click(object sender, EventArgs e)
{
   System.IO.MemoryStream mstream = new System.IO.MemoryStream();
   this.richTextBox1.SaveFile(mstream, RichTextBoxStreamType.RichText);
   //将流转换成数组
   byte[] bWrite = mstream.ToArray();
   //将数组写入数据库
   System.Data.SqlClient.SqlParameter[] pram ={
          sqlHelper.MakeInParam("@XX",System.Data.SqlDbType.Image)
   };
   pram[0].Value = http://www.mamicode.com/bWrite;
   sqlHelper.RunSql("insert into XXX (XX) values (@XX)", pram);
}

将数据库中的RTF读出并填充到RichTextBox
private void button2_Click(object sender, EventArgs e)
{
   //从数据库中读出数据
   DataTable dt=sqlHelper.GetDataTable("select XX from XXX where .....");
   byte[] bWrite = (byte[])dt.Rows[0][0];
   //将数组转换成stream
   System.IO.MemoryStream mstream = new System.IO.MemoryStream(bWrite, false);
   //将stream填充到RichTextBox
   this.richTextBox1.LoadFile(mstream, RichTextBoxStreamType.RichText);
}