string FileName = "";
SaveFD1.InitialDirectory = "C:";
SaveFD1.FileName = "";
SaveFD1.Title = "Save File As";
SaveFD1.Filter = "Text Files|*.txt|All Files|*.xls|*.*";
if (SaveFD1.ShowDialog() == DialogResult.OK)
{
FileName = SaveFD1.FileName;
richTextBox1.SaveFile(FileName, RichTextBoxStreamType.PlainText);
}