It is necessary to do so that the file is saved without displaying the save dialog box with the same name and extension in the same folder. Now it works like this.
try { TextRange conversionText = ConversionText; using (MemoryStream stream = new MemoryStream()) { conversionText.Save(stream, System.Windows.DataFormats.Rtf); stream.Seek(0, SeekOrigin.Begin); using (StreamReader reader = new StreamReader(stream)) { IRtfDocument rtfDocument = RtfInterpreterTool.BuildDoc(reader); RtfHtmlConverter htmlConverter = new RtfHtmlConverter(rtfDocument); textBox.Text = htmlConverter.Convert(); } System.Windows.Forms.SaveFileDialog save = new System.Windows.Forms.SaveFileDialog(); if (save.ShowDialog() == System.Windows.Forms.DialogResult.OK) { using (StreamWriter sw = new StreamWriter(save.FileName)) { sw.Write(textBox.Text); } } } } catch (Exception exception) { System.Windows.MessageBox.Show(this, "Error " + exception.Message, Title, MessageBoxButton.OK, MessageBoxImage.Error); }