GameState.Tick.cs

private CellState[,] _nextGeneration = null;

public void Tick()
{
    _nextGeneration = _nextGeneration ?? new CellState[Size, Size];

    for (int row = 0; row < Size; row++)
    {
        for (int column = 0; column < Size; column++)
        {
            var aliveNeighbors = CountAliveNeighbors(row, column);
            var currentState = Cells[row, column];

            CellState nextState = Cells[row, column];

            if (currentState == CellState.Alive && aliveNeighbors < 2)
            {
                // Rule 1 - Underpopulation
                nextState = CellState.Dead;
            }
            else if (
                currentState == CellState.Alive && 
                (aliveNeighbors == 2 || aliveNeighbors <= 3))
            {
                // Rule 2
                nextState = CellState.Alive;
            }
            else if (currentState == CellState.Alive && aliveNeighbors > 3)
            {
                // Rule 3 - Overpopulation
                nextState = CellState.Dead;
            }
            else if (currentState == CellState.Dead && aliveNeighbors == 3)
            {
                // Rule 4 - Reproduction
                nextState = CellState.Alive;
            }

            _nextGeneration[row, column] = nextState;
        }
    }

    // Swap!
    (Cells, _nextGeneration) = (_nextGeneration, Cells);
}

private int CountAliveNeighbors(int row, int column)
{
    int aliveCount = 0;
    for (var rowOffset = -1; rowOffset <= 1; rowOffset++)
    {
        for (var columnOffset = -1; columnOffset <= 1; columnOffset++)
        {
            if (rowOffset == 0 && columnOffset == 0)
            {
                continue;
            }
            var targetRow = row + rowOffset;
            var targetColumn = column + columnOffset;
            if (targetRow >= 0 && targetColumn >= 0 &&
                targetRow < Size && targetColumn < Size)
            {
                aliveCount += (int)Cells[targetRow, targetColumn];
            }
        }
    }

    return aliveCount;
}