[C#] ハッシュテーブル(連想配列)とは？使い道や基本的な使い方を解説

using System;
using System.Collections;
class Program
{
    static void Main()
    {
        // Hashtableの初期化
        Hashtable hashtable = new Hashtable();
        // キーと値の追加
        hashtable["apple"] = "りんご";
        hashtable["banana"] = "バナナ";
        hashtable["orange"] = "オレンジ";
        // 値の取得
        Console.WriteLine(hashtable["apple"]); // りんご
    }
}

りんご

using System;
using System.Collections;
class Program
{
    static void Main()
    {
        Hashtable hashtable = new Hashtable();
        // キーと値の追加
        hashtable["key1"] = "value1";
        hashtable["key2"] = "value2";
        // 値の取得
        Console.WriteLine(hashtable["key1"]); // value1
        // キーと値の削除
        hashtable.Remove("key1");
        Console.WriteLine(hashtable.ContainsKey("key1")); // False
    }
}

value1
False

using System;
using System.Collections;
class Program
{
    static void Main()
    {
        // 初期容量を10に設定
        Hashtable hashtable = new Hashtable(10);
        // 要素の追加
        for (int i = 0; i < 5; i++)
        {
            hashtable["key" + i] = "value" + i;
        }
        // 要素の数を表示
        Console.WriteLine("要素数: " + hashtable.Count); // 要素数: 5
    }
}

要素数: 5

using System;
using System.Collections;
class Program
{
    static void Main()
    {
        Hashtable hashtable = new Hashtable();
        hashtable["key1"] = "value1";
        // キーの存在確認
        if (hashtable.ContainsKey("key1"))
        {
            Console.WriteLine("key1は存在します。");
        }
        else
        {
            Console.WriteLine("key1は存在しません。");
        }
    }
}

key1は存在します。

using System;
using System.Collections;
class Program
{
    static void Main()
    {
        Hashtable hashtable = new Hashtable();
        hashtable["key1"] = "value1";
        // 値の存在確認
        if (hashtable.ContainsValue("value1"))
        {
            Console.WriteLine("value1は存在します。");
        }
        else
        {
            Console.WriteLine("value1は存在しません。");
        }
    }
}

value1は存在します。

using System;
using System.Collections;
class Program
{
    static void Main()
    {
        Hashtable hashtable = new Hashtable();
        hashtable["key1"] = "value1";
        hashtable["key2"] = "value2";
        // 全要素の列挙
        foreach (DictionaryEntry entry in hashtable)
        {
            Console.WriteLine($"キー: {entry.Key}, 値: {entry.Value}");
        }
    }
}

キー: key1, 値: value1
キー: key2, 値: value2

using System;
using System.Collections.Generic;
class User
{
    public string UserId { get; set; }
    public string Name { get; set; }
    public override int GetHashCode()
    {
        return UserId.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        return obj is User user && UserId == user.UserId;
    }
}
class Program
{
    static void Main()
    {
        Dictionary<User, string> userDictionary = new Dictionary<User, string>();
        User user1 = new User { UserId = "001", Name = "山田" };
        userDictionary[user1] = "ユーザー情報1";
        Console.WriteLine(userDictionary[user1]); // ユーザー情報1
    }
}

ユーザー情報1

using System;
using System.Collections.Generic;
class CustomKey
{
    public string KeyPart1 { get; set; }
    public string KeyPart2 { get; set; }
    public override int GetHashCode()
    {
        // カスタムハッシュ関数
        return (KeyPart1 + KeyPart2).GetHashCode();
    }
}
class Program
{
    static void Main()
    {
        Dictionary<CustomKey, string> customDictionary = new Dictionary<CustomKey, string>();
        CustomKey key = new CustomKey { KeyPart1 = "A", KeyPart2 = "B" };
        customDictionary[key] = "カスタムデータ";
        Console.WriteLine(customDictionary[key]); // カスタムデータ
    }
}

カスタムデータ

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        Dictionary<string, List<int>> grades = new Dictionary<string, List<int>>();
        // 成績の追加
        grades["数学"] = new List<int> { 80, 90 };
        grades["英語"] = new List<int> { 85, 95 };
        // 成績の表示
        foreach (var subject in grades)
        {
            Console.WriteLine($"{subject.Key}: {string.Join(", ", subject.Value)}");
        }
    }
}

数学: 80, 90
英語: 85, 95

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> items = new List<string> { "apple", "banana", "apple", "orange", "banana", "banana" };
        Dictionary<string, int> countDictionary = new Dictionary<string, int>();
        foreach (var item in items)
        {
            if (countDictionary.ContainsKey(item))
            {
                countDictionary[item]++;
            }
            else
            {
                countDictionary[item] = 1;
            }
        }
        // カウント結果の表示
        foreach (var entry in countDictionary)
        {
            Console.WriteLine($"{entry.Key}: {entry.Value}");
        }
    }
}

apple: 2
banana: 3
orange: 1

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        Dictionary<string, Dictionary<string, string>> userAttributes = new Dictionary<string, Dictionary<string, string>>();
        // ユーザー属性の追加
        userAttributes["user1"] = new Dictionary<string, string>
        {
            { "名前", "山田" },
            { "年齢", "30" }
        };
        userAttributes["user2"] = new Dictionary<string, string>
        {
            { "名前", "佐藤" },
            { "年齢", "25" }
        };
        // ユーザー属性の表示
        foreach (var user in userAttributes)
        {
            Console.WriteLine($"ユーザーID: {user.Key}");
            foreach (var attribute in user.Value)
            {
                Console.WriteLine($"{attribute.Key}: {attribute.Value}");
            }
        }
    }
}

ユーザーID: user1
名前: 山田
年齢: 30
ユーザーID: user2
名前: 佐藤
年齢: 25