Effective C# 原則 7：選擇不可變的原子實值型別（完）

編輯：關於C語言

防禦性複製：保護不可變型別內的參考型別欄位

public readonly struct PhoneList
{
    private readonly Phone[] _phones;

    public PhoneList(Phone[] phones)
    {
        _phones = new Phone[phones.Length];
        phones.CopyTo(_phones, 0); // 防禦性複製：Phone 為 struct 故複製值
    }

    public IEnumerator GetEnumerator() => ((IEnumerable)_phones).GetEnumerator();
}

// 使用
var phones = new Phone[10];
// 初始化 phones...
var list = new PhoneList(phones);
phones[5] = Phone.Generate(); // 不影響 list 內部陣列

屬性存取也需防禦性複製

public Phone[] Phones
{
    get
    {
        var copy = new Phone[_phones.Length];
        _phones.CopyTo(copy, 0);
        return copy;
    }
}

建立不可變物件的三種模式

模式	說明	範例
建構函式	單次呼叫完整初始化	`new Address("1F", "Taipei", "100")`
工廠方法	靜態方法封裝建立邏輯	`Color.FromArgb(r,g,b)`, `Color.FromKnownColor`
建構器/伴隨類別	多步驟建構，最後產出不可變物件	`StringBuilder → string`

Address 不可變結構完整實作

public readonly struct Address
{
    public string Line1 { get; }
    public string Line2 { get; }
    public string City { get; }
    public string State { get; }
    public int ZipCode { get; }

    public Address(string line1, string line2, string city, string state, int zipCode)
    {
        Line1 = line1 ?? throw new ArgumentNullException(nameof(line1));
        Line2 = line2 ?? "";
        City = city ?? throw new ArgumentNullException(nameof(city));
        State = state ?? throw new ArgumentNullException(nameof(state));
        ZipCode = zipCode;
        ValidateState(state);
        ValidateZip(zipCode);
    }

    // 修改城市 → 回傳新實例
    public Address WithCity(string newCity) => new Address(Line1, Line2, newCity, State, ZipCode);
}

使用方式

var a1 = new Address("111 S. Main", "", "Anytown", "IL", 61111);
// 修改城市
var a2 = a1.WithCity("Ann Arbor"); // a1 不變，a2 為新實例
// 或使用 with 運算式 (C# 9+)
var a3 = a1 with { City = "Ann Arbor", State = "MI", ZipCode = 48103 };