public void SaveMyFile()
{
   // Create a SaveFileDialog to request a path and file name to save to.
   SaveFileDialog saveFile1 = new SaveFileDialog();

   // Initialize the SaveFileDialog to specify the RTF extension for the file.
   saveFile1.DefaultExt = "*.rtf";
   saveFile1.Filter = "RTF Files|*.rtf";

   // Determine if the user selected a file name from the saveFileDialog.
   if(saveFile1.ShowDialog() == System.Windows.Forms.DialogResult.OK &&
      saveFile1.FileName.Length > 0) 
   {
      // Save the contents of the RichTextBox into the file.
      richTextBox1.SaveFile(saveFile1.FileName, RichTextBoxStreamType.PlainText);
   }
}