C#尝试访问对象的属性会引发异常

Question

所以我开始学习面向对象编程,我在理解所有这些元素之间的关系时遇到了一些麻烦.

我正在用RPG风格打造一个简单的文字游戏.

为此我有几个类,包括一个用于:

武器:

class Weapon
{
    private string _name;
    private int _mindamage;
    private int _maxdamage;

    public string Name { get { return _name; } set { _name = value; } }
    public int MaxDamage { get { return _maxdamage; } set { _maxdamage = value; } }
    public int MinDamage { get { return _mindamage; } set { _mindamage = value; } }

    public Weapon(int mindamage, int maxdamage, string name)
    {
        this._name = name;
        this._mindamage = mindamage;
        this._maxdamage = maxdamage;
    }
}

人物:

class Character
{
    private string _name;
    private int _life;
    private Weapon _weapon;

    public string Name { get { return _name; } set { _name = value; } }
    public int Life { get { return _life; } set { _life = value; } }
    public Weapon Weapon { get { return _weapon; } set { _weapon = value; } }

    public Character(string name, Weapon weapon)
    {
        _name = name;
        _weapon = weapon;
        _life = 100;
    }

而我正在努力的那个:它是一个代表列表中一组字符的类:

class Company
{

    private int _size;
    private List<Character> _members;

    public int Size { get { return _size; } set { _size = value; } }
    public List<Character> Members { get { return _members; } set { _members = value; } }

    public Company(string[] names, Weapon[] weapons)
    {
        int length = names.Length;
        List<Character> _members = new List<Character>();
        for (int i = 0; i < length; i++)
        {
            _members.Add(new Character(names[i], weapons[i]));
            Console.WriteLine("added {0} to character list", _members[i].Name);
        }
        Console.WriteLine("characters {0} and {1} exist after the for", _members[0].Name, _members[1].Name);
        _size = length;
    }

}

这段代码编译完全正常但是当它执行时,如果我尝试访问列表中的字符名称,它会在我的Program.cs类中抛出一个错误,例如:

static void Main(string[] args)
    {
        Weapon anduril = new Weapon(55, 80, "Anduril");
        Weapon hache = new Weapon(25, 60, "hache");
        Weapon lame = new Weapon(30, 50, "lame");
        Weapon arc = new Weapon(30, 40, "arc");
        string[] gentils = { "Aragorn", "Legolas", "Gimli" };
        string[] méchants = { "Lurtz", "Berserk", "UrukA" };
        Weapon[] armes = { anduril, arc, hache };
        Weapon[] armes2 = { lame, lame, arc };
        Company Communauté = new Company(gentils, armes);
        Company Uruks = new Company(méchants, armes2);
        Console.WriteLine(Communauté.Members[0].Name);

最后一行抛出了一个null对象错误.正如您所看到的,我Console.WriteLine在Company构造函数中调用了一些函数来确保构造函数实际上是添加Character到a中List,并且它可以正常工作.那问题是什么？

这是我为会员编写getter/setter的方式吗？

Answer 1

List<Character> _members = new List<Character>();

这声明了一个新的本地调用_members,但你打算做的是初始化字段_members.该字段永远不会被初始化,因此当您访问其成员时,null引用异常.

所以不要这样做.它应该是

this._members = new List<Character>();

甚至更好,在声明中初始化它.

但真正的要点是:今天是学习使用调试器的好日子.您对程序的功能有一定的假设.这些假设是错误的.通过观察调试器中的程序并在每个步骤验证您的假设是否有效,您将很快发现错误.与在调试器中运行代码相比,盯着代码一小时,放弃并询问Stack Overflow是一种非常糟糕的调试技术.打印出程序的状态是一个很好的第一步,但实际观看程序运行要好得多.