//convert as numberbox

List<NumberBox> numberBoxes = elements_in_col_1.ConvertAll(x => (NumberBox)x);

/*check Nr

            FrameworkElement cell_element = get_cell_child(rowNr, 0);

            if (cell_element == null) return;   //if no nr field : error

            //*check empty nr*

            NumberBox ctlNr=(NumberBox)cell_element;

            string sNr =  ctlNr.Text;//get_cell_value(rowNr, 0);

            if(sNr=="")

            {

                //get elements of grid

                var children = grid_folders.Children.Cast<FrameworkElement>();

                //filter elements form column 1

                List<FrameworkElement> elements_in_col_1 = children.Where(item => (Grid.GetColumn(item) == 0 && Grid.GetRow(item) > 0)).ToList();

                //convert as numberbox

                List<NumberBox> numberBoxes = elements_in_col_1.ConvertAll(x => (NumberBox)x);

                //find max as integer from numberBox.text

                int maxNr = numberBoxes.findMax();

                //increase

                maxNr=maxNr+1;

                ctlNr.Text = maxNr.ToString();

            }