SaveFileDialog saveFile1 = new SaveFileDialog();
saveFile1.DefaultExt = "*.txt";
saveFile1.Filter = "RTF Files|*.rtf|TXT Files|*.txt";
saveFile1.FileName = "please select a file extension at the bottom BEFORE saving.";
if (saveFile1.ShowDialog() == System.Windows.Forms.DialogResult.OK &&
saveFile1.FileName.Length > 0)
{
richtextBox1.SaveFile(saveFile1.FileName, RichTextBoxStreamType.PlainText);
}
hope i helped