C# 索引子：讓類別像陣列一樣存取內部集合

基本語法與整數索引子

using System;

class MyClass {
    private float[] fs = new float[3] { 1.1f, 2.2f, 3.3f };

    // 一般屬性
    public int Length {
        get { return fs.Length; }
        set { fs = new float[value]; }
    }

    // 索引子：this[int n]
    public float this[int n] {
        get { return fs[n]; }
        set { fs[n] = value; }
    }
}

class Program {
    static void Main() {
        MyClass obj = new MyClass();
        for (int i = 0; i < obj.Length; i++) Console.WriteLine(obj[i]); // 1.1, 2.2, 3.3

        for (int i = 0; i < obj.Length; i++) obj[i] += 5.5f;
        for (int i = 0; i < obj.Length; i++) Console.WriteLine(obj[i]); // 6.6, 7.7, 8.8

        obj.Length = 5; // 觸發 set，重新配置陣列
        for (int i = 0; i < obj.Length; i++) Console.WriteLine(obj[i]); // 0, 0, 0, 0, 0

        Console.ReadKey();
    }
}

多型索引子：字串、多維、複合鍵

using System;

class WeatherOfWeek {
    // 整數索引：0=Sun, 5=Fri
    public string this[int index] {
        get {
            switch (index) {
                case 0: return "Today is cloudy!";
                case 5: return "Today is thundershower!";
                default: return "Today is fine!";
            }
        }
    }

    // 字串索引：曜日名稱
    public string this[string day] {
        get {
            switch (day) {
                case "Sunday": return "Today is cloudy!";
                case "Friday": return "Today is thundershower!";
                default: return "Today is fine!";
            }
        }
    }
}

class Program {
    static void Main() {
        var w = new WeatherOfWeek();
        for (int i = 0; i < 6; i++) Console.WriteLine(w[i]);
        foreach (var d in new[] { "Sunday", "Monday", "Friday" })
            Console.WriteLine(w[d]);
        Console.ReadKey();
    }
}

索引子進階特性

實務應用場景

1. 自訂集合封裝：List<T>、Dictionary<K,V> 皆大量使用索引子
2. 資料列存取：DataTable.Rows[i]["ColumnName"]
3. 矩陣/圖像處理：bitmap[x, y]、matrix[row, col]
4. 設定/快取查詢：config["section:key"]、cache[key]

設計建議

• 索引子應表達「存取元素」語意，避免在 get/set 中執行複雜業務邏輯
• 提供 Count/Length 屬性配合迴圈使用
• 參數驗證：越界拋 ArgumentOutOfRangeException，null 拋 ArgumentNullException
• 考慮實作 IEnumerable<T> 支援 foreach