private void c_formatting_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {

            switch (c_formatting.SelectedIndex)
            {
                case 0:
                    _previewBox.Document.Selection.CharacterFormat.Bold = FormatEffect.On;
                    break;

                case 1:
                    _previewBox.Document.Selection.CharacterFormat.Bold = FormatEffect.On;
                    break;

                case 2:
                    break;

                case 3:
                    break;

                default:
                    break;
            }
        }