Online Circular Chess in Silverlight, ASP.NET AJAX, WCF Web Services, and LINQ to SQL

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.Linq;
using System.Collections.Generic;
using CChess;

public partial class CChessPlayer
{
    /// <summary>
    /// It's the number of seconds that can pass without recieving a message.
    /// After that, the user is offline.
    /// </summary>
    public const int LEGAL_INACTIVITY_SECONDS = 30;

    public bool IsOnline()
    {
        CChessDataContext cdc = new CChessDataContext();
        int players_count = (from p in cdc.CChessPlayers
                             where p.player_id == this.player_id &&
                                 UnixDate.GetCurrentUnixTimestamp() - p.last_activity < 30
                             select new { ID = p.player_id }).Count();

        return (players_count > 0);
    }

    public bool IsInAGame()
    {
        CChessDataContext cdc = new CChessDataContext();
        int games_count = cdc.CChessGamesPlayers.Count(p => ((p.player_id == player_id) && (p.CChessGame.game_state != (byte)CChessGameStates.Finished)));

        return (games_count > 0);
    }

    partial void OnValidate(ChangeAction action)
    {
        if (player_nick == String.Empty)
        {
            throw new Exception("The nickname is empty.");
        }

        CChessDataContext cdc = new CChessDataContext();

        List<CChessPlayer> is_unique_player = (from players in cdc.GetOnlinePlayers(CChessPlayer.LEGAL_INACTIVITY_SECONDS, UnixDate.GetCurrentUnixTimestamp())
                                               where players.player_nick == player_nick
                                               select new CChessPlayer
                                               {
                                                   player_id = players.player_id
                                               }).ToList();

        if (is_unique_player.Count > 0)
        {
            throw new Exception("This username is currently being used by another player, please choose another one.");
        }
    }
}