in Sudoku/Board.cs [38:44]
private Board(Cell[,] cells, int activeRow, int activeCol, int activeNumber, int emptyCellsCount)
{
Cells = cells;
_emptyCellsCount = emptyCellsCount;
Cell(activeRow, activeCol).Number = activeNumber;
UpdateCellsAffectedBy(activeRow, activeCol, activeNumber);
}