c# メモ

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

public class PlayerRepository
{
    private static readonly List<Player> Players = new List<Player>();

    public IReadOnlyList<Player> GetAll()
    {
        CheckPlayers();
        return new List<Player>(Players);
    }

    public void Regist(Player p)
    {
        CheckArguments(p);
        CheckRegistYet(p);
        Players.Add(p);
    }

    public void UnRegist(Player p)
    {
        CheckArguments(p);
        CheckRegistAlready(p);
        Players.Remove(p);
    }

    public bool Exists(Player p)
    {
        return Players.Contains(p);
    }

    private void CheckPlayers()
    {
        if (Players.Any(null))
            throw new Exception("Exist null in the list.");
    }

    private void CheckArguments(params object[] args)
    {
        foreach (var arg in args)
            CheckArgment(arg);
    }

    private void CheckArgment(object arg)
    {
        if (arg == null)
            throw new ArgumentNullException();
    }

    private void CheckRegistYet(Player p)
    {
        if (Exists(p))
            throw new Exception("Player is already registed");
    }

    private void CheckRegistAlready(Player p)
    {
        if (Exists(p))
            throw new Exception("Player is not registed");
    }
}