Implement Sudoku using LINQ

Question

WiKi and GitHub.

Sudoku rules are being checked with the following tests:

    [TestMethod]
    public void Be_Valid()
    {
        var data = new int?[,]
        {
            { 5, 3, null },
            { 6, null, null },
            { null, 9, 8 }
        };

        var board = new Board(data);
        Assert.IsTrue(board.Valid);
    }

And:

    [TestMethod]
    public void Be_Invalid()
    {
        var data = new int?[,]
        {
            { 5, 3, 8 },
            { 6, null, null },
            { null, 9, 8 }
        };

        var board = new Board(data);
        Assert.IsFalse(board.Valid);
    }

Where actual implementation looks like:

public class Board
{
    public Board(int?[,] cells)
    {
        Cells = cells ?? throw new ArgumentNullException(nameof(cells));
        if(Cells.GetLength(0) != Cells.GetLength(1))
            throw new ArgumentNullException(nameof(cells), "Square board required.");
        if (Cells.GetLength(0) % 3 != 0)
            throw new ArgumentNullException(nameof(cells), "Board size should be multiple of 3.");
    }

    int? [,] Cells { get; }
    int N => Cells.GetLength(0);

    public bool Valid => ColumnsValid && RowsValid && GridsValid;

    bool RowsValid => Rows.All(AreUnique);
    bool ColumnsValid => Columns.All(AreUnique);
    bool GridsValid => Grids.All(AreUnique);

    IEnumerable<IEnumerable<int>> Rows => 
        Range(0, N).Select(r => Numbers(0, N - 1, r, r));
    IEnumerable<IEnumerable<int>> Columns => 
        Range(0, N).Select(c => Numbers(c, c, 0, N - 1));
    IEnumerable<IEnumerable<int>> Grids => 
        from r in Range(0, N / 3)
        from c in Range(0, N / 3)
        select Numbers(
            c * 3, c * 3 + 2, r * 3, r * 3 + 2);

    bool AreUnique(IEnumerable<int> numbers) =>
        numbers.Distinct().Count() == numbers.Count();

    IEnumerable<int> Numbers(int left, int right, int top, int bottom) =>
        from r in Range(top, bottom - top + 1)
        from c in Range(left, right - left + 1)
        where Cells[r, c].HasValue
        select Cells[r, c].Value;
}