public void DrawRectangleAtCaret() { if (MainTextBox != null && showLineThing.IsChecked == true) { Rect p = MainTextBox.GetCaretLocation(); Thickness t = new Thickness(0, p.Y - 1, 17, MainTextBox.ActualHeight - p.Bottom - 1);; if (t.Top >= 0 && t.Bottom >= 0) { aditionalSelection.Visibility = Visibility.Visible; aditionalSelection.Margin = t; } else { aditionalSelection.Visibility = Visibility.Collapsed; } } else { aditionalSelection.Visibility = Visibility.Collapsed; } }