این کد هایی است که نوشتم اما فقط خط اول ذخیره میشه
SaveFileDialog saveDialog = newSaveFileDialog();
saveDialog.Filter = "Text File (*.txt)|*.txt";
saveDialog.Title = "Save File ";
if (saveDialog.ShowDialog() == DialogResult.OK)
{
string filename = saveDialog.FileName;
StreamWriter Writer = newStreamWriter(filename);
Writer.WriteLine(richTextBox1);
Writer.WriteLine(Writer.NewLine);
Writer.Close();
}