NullReferenceExceptionUnhandled

Question

我正在编写一个二十一点程序,但我的问题是创建一个具有其他对象列表的对象时的解决方案,但是当我首先创建对象时,列表为空.我想这就是我收到这个错误的原因.下面是我的代码,我将播放器初始化为在播放器的构造函数中有一个手的实例,但是当我调用方法playerHand.Hit(card)时,我得到"对象引用未设置为对象的实例"错误消息;

public class Player
{
    private string name;
    public Hand playerHand;

    public Player()
    { }

    public Player(string name)
    {
        this.Name = name;
        this.playerHand = new Hand();
    }

    public string Name
    {
        get { return name; }
        set { name = value; }
    }

    public void TakeCard(Card card) 
    { playerHand.Hit(card); }`

`

public class Hand
{
    public List<Card> hand;
    public Hand()
    {
        hand = new List<Card>();
    }
    public int handValue;`

    public void Hit(Card card)
    {
        hand.Add(card);
    }

    public int HandValue()
    {
        foreach (Card card in hand)
            handValue += (int)card.value;

        return handValue;
    }
}

public class Game
{
    public Game(IEnumerable<string> playerNames)
    {
        List<Player> players = new List<Player>();
        foreach (string name in playerNames)
            players.Add(new Player(name));
        Player dealer = new Dealer();
        players.Add(dealer);
        Deck playingDeck = new Deck();
        Deal(playingDeck, players);
        Play(playingDeck, players);
        DisplayResults(players);
    }       

    public void Deal(Deck deck, List<Player> players)
    {
        deck.Shuffle();
        for (int i = 1; i < 3; i++)
            foreach (Player player in players)
                player.TakeCard(deck.Deal());                
    }

Answer 1

你有这条线:

Player dealer = new Dealer();

你没有告诉我们代码Dealer.但是,从代码中可以明显看出Dealer : Player.请注意,您的默认构造函数Player永远不会初始化Player.playerHand.如果默认的构造函数Dealer调用默认的构造函数Player(和它在默认情况下),那么playerHand就会null对dealer.

所以,我的猜测就是playerHand.Hit(card);给你一个NullReferenceException什么时候调用,dealer因为你从来没有初始化Player.playerHand.

要修复它,只需提供一个初始化程序

public Hand playerHand = new Hand();

并从构造函数中删除相应的行Player(string).

最后,除此之外,您的类中有太多public变量.你正在"泄漏"你的班级消费者可以不受约束地修改的变量,并破坏你的不变量.封装是游戏的名称.