TicTacToe

using System;
using System.Collections.Generic;
using System.Linq;

class MainClass {
    static void Main() {
        var board = new Board();
        PlayState move;
        
        move = TicTacToe.Next(board, Player.X, 0, 0);
        Console.WriteLine(move);
        
        move = TicTacToe.Next(board, Player.O, 1, 1);
        Console.WriteLine(move);
        
        move = TicTacToe.Next(board, Player.X, 1, 0);
        Console.WriteLine(move);
        
        move = TicTacToe.Next(board, Player.O, 2, 1);
        Console.WriteLine(move);
        
        move = TicTacToe.Next(board, Player.X, 2, 0);
        Console.WriteLine(move);
    }
}

public enum Player {
    NoPlayerSet,
    X,
    O,
}

public class Board {
    public Player[,] Grid = new Player[3, 3];
    public Player CurrentPlayer = Player.X;
}

public enum PlayState {
    Illegal,
    Continue,
    Draw,
    X_Winner,
    O_Winner,
}

public static class TicTacToe {
    public static PlayState Next(Board board, Player player, int x, int y) {
        if (board.CurrentPlayer != player) {
            return PlayState.Illegal;
        }
        
        if (board.Grid[x, y] != Player.NoPlayerSet) {
            return PlayState.Illegal;
        }
        
        board.Grid[x, y] = board.CurrentPlayer;
        board.CurrentPlayer = board.CurrentPlayer == Player.X
            ? Player.O
            : Player.X;
        
        return PlayState.Continue;
    }
}