public SudokuGrid(SudokuGrid grid) { if (grid == null) throw new ArgumentNullException(); _cells = new List<Cell>(9 * 9); for (int x = 0; x < 9; x++) { for (int y = 0; y < 9; y++) { Cell cell = new Cell(this, grid.Get(x, y).Value, x, y); _cells.Add(cell); } } }