C# 教程第 11 課：索引器完整指南（建立、多載、多參數、字串索引）

學習目標

1. 理解索引器概念與用途
2. 實作單一整數索引器
3. 多載索引器（不同參數型別）
4. 多參數索引器

基本索引器：IntIndexer

using System;

class IntIndexer
{
    private string[] data;

    public IntIndexer(int size)
    {
        data = new string[size];
        for (int i = 0; i < size; i++) data[i] = "empty";
    }

    public string this[int pos]
    {
        get => data[pos];
        set => data[pos] = value;
    }

    static void Main()
    {
        var idx = new IntIndexer(10);
        idx[9] = "Some Value";
        idx[3] = "Another Value";
        idx[5] = "Any Value";

        Console.WriteLine("\nIndexer Output\n");
        for (int i = 0; i < 10; i++)
            Console.WriteLine($"myInd[{i}]: {idx[i]}");
    }
}

輸出

myInd[0]: empty
myInd[1]: empty
myInd[2]: empty
myInd[3]: Another Value
myInd[4]: empty
myInd[5]: Any Value
myInd[6]: empty
myInd[7]: empty
myInd[8]: empty
myInd[9]: Some Value

多載索引器：OvrIndexer

同一類別可定義多個索引器，參數型別/個數不同即可：

class OvrIndexer
{
    private string[] data;
    private int size;

    public OvrIndexer(int size)
    {
        this.size = size;
        data = new string[size];
        for (int i = 0; i < size; i++) data[i] = "empty";
    }

    // 整數索引器
    public string this[int pos]
    {
        get => data[pos];
        set => data[pos] = value;
    }

    // 字串索引器：get 回傳符合值的計數，set 將所有符合值替換
    public string this[string val]
    {
        get
        {
            int count = 0;
            for (int i = 0; i < size; i++)
                if (data[i] == val) count++;
            return count.ToString();
        }
        set
        {
            for (int i = 0; i < size; i++)
                if (data[i] == val) data[i] = value;
        }
    }

    static void Main()
    {
        var idx = new OvrIndexer(10);
        idx[9] = "Some Value";
        idx[3] = "Another Value";
        idx[5] = "Any Value";
        idx["empty"] = "no value"; // 呼叫字串索引器 set

        Console.WriteLine("\nIndexer Output\n");
        for (int i = 0; i < 10; i++)
            Console.WriteLine($"myInd[{i}]: {idx[i]}");

        Console.WriteLine($"\nNumber of \"no value\" entries: {idx["no value"]}"); // 呼叫 get
    }
}

輸出

myInd[0]: no value
myInd[1]: no value
myInd[2]: no value
myInd[3]: Another Value
myInd[4]: no value
myInd[5]: Any Value
myInd[6]: no value
myInd[7]: no value
myInd[8]: no value
myInd[9]: Some Value

Number of "no value" entries: 7

多參數索引器語法

public class Matrix
{
    private double[,] data = new double[10, 10];

    public double this[int row, int col]
    {
        get => data[row, col];
        set => data[row, col] = value;
    }
}

呼叫：matrix[2, 3] = 5.5;

合法索引器參數型別

• 整數型別 (int, long...)
• 列舉型別
• 字串 (string)
• 其他實值型別/參考型別（語意上需謹慎）

小結

索引器讓類別支援陣列語法存取內部資料，支援多載與多參數，極適合集合、矩陣、字典式查詢等場景。善用索引器可大幅提升 API 易用性。