【C#】new Listの基本構文と初期化・追加・容量設定をやさしく解説

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // int型のListを作成し、初期値を追加
        List<int> numbers = new List<int> { 1, 2, 3 };
        numbers.Add(4); // 要素を追加
        // 要素の表示
        foreach (int num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3
4

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // 空のList<int>を作成
        List<int> numbers = new List<int>();
        // 要素を追加
        numbers.Add(10);
        numbers.Add(20);
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

10
20

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // 初期容量100のList<string>を作成
        List<string> fruits = new List<string>(100);
        fruits.Add("apple");
        fruits.Add("banana");
        Console.WriteLine($"Count: {fruits.Count}");
        Console.WriteLine($"Capacity: {fruits.Capacity}");
    }
}

Count: 2
Capacity: 100

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // 配列からListを作成
        int[] array = { 1, 2, 3, 4 };
        List<int> numbers = new List<int>(array);
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3
4

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // 型を明示的に指定
        List<string> fruits1 = new List<string>();
        // varを使い型推論
        var fruits2 = new List<string>();
        fruits2.Add("apple");
        fruits2.Add("banana");
        foreach (var fruit in fruits2)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
banana

List<int> numbers = new(); // List<int>型と推論される

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // コレクション初期化子を使ってList<int>を作成
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3
4
5

List<string> fruits = new List<string> { "apple", "banana", "cherry" };

List<Person> people = new List<Person>
{
    new Person { Name = "Alice", Age = 30 },
    new Person { Name = "Bob", Age = 25 }
};

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        int[] array = { 10, 20, 30, 40 };
        // 配列からList<int>を作成
        List<int> numbers = new List<int>(array);
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

10
20
30
40

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> original = new List<string> { "red", "green", "blue" };
        // 既存のListから新しいListを作成
        List<string> copy = new List<string>(original);
        copy.Add("yellow");
        Console.WriteLine("Original list:");
        foreach (var color in original)
        {
            Console.WriteLine(color);
        }
        Console.WriteLine("Copied list:");
        foreach (var color in copy)
        {
            Console.WriteLine(color);
        }
    }
}

Original list:
red
green
blue
Copied list:
red
green
blue
yellow

using System;
using System.Collections.Generic;
class Program
{
    static void PrintNumbers(params int[] numbers)
    {
        // 配列をListに変換
        List<int> numberList = new List<int>(numbers);
        foreach (var num in numberList)
        {
            Console.WriteLine(num);
        }
    }
    static void Main()
    {
        PrintNumbers(5, 10, 15, 20);
    }
}

5
10
15
20

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string>();
        fruits.Add("apple");  // 要素を追加
        fruits.Add("banana");
        fruits.Add("cherry");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
banana
cherry

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3 };
        int[] moreNumbers = { 4, 5, 6 };
        numbers.AddRange(moreNumbers);  // 配列の要素を一括追加
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3
4
5
6

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> colors = new List<string> { "red", "blue", "green" };
        colors.Insert(1, "yellow");  // インデックス1に"yellow"を挿入
        foreach (var color in colors)
        {
            Console.WriteLine(color);
        }
    }
}

red
yellow
blue
green

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> list = new List<int> { 1, 4, 5 };
        List<int> toInsert = new List<int> { 2, 3 };
        list.InsertRange(1, toInsert);  // インデックス1に複数要素を挿入
        foreach (var num in list)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3
4
5

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int>(2);  // 初期容量2
        Console.WriteLine($"Initial Capacity: {numbers.Capacity}");
        numbers.Add(1);
        numbers.Add(2);
        Console.WriteLine($"Capacity after adding 2 elements: {numbers.Capacity}");
        numbers.Add(3);  // ここで容量拡張が発生
        Console.WriteLine($"Capacity after adding 3rd element: {numbers.Capacity}");
    }
}

Initial Capacity: 2
Capacity after adding 2 elements: 2
Capacity after adding 3rd element: 4

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        // インデクサで要素を取得
        string firstFruit = fruits[0];
        Console.WriteLine($"1番目の果物: {firstFruit}");
        // インデクサで要素を更新
        fruits[1] = "blueberry";
        Console.WriteLine("更新後のリスト:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

1番目の果物: apple
更新後のリスト:
apple
blueberry
cherry

int index = fruits.IndexOf("cherry");
Console.WriteLine($"cherryのインデックス: {index}");

cherryのインデックス: 2

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 10, 15, 20, 25, 30 };
        // 20以上の最初の要素を取得
        int firstOver20 = numbers.Find(n => n >= 20);
        Console.WriteLine($"20以上の最初の数: {firstOver20}");
        // 20以上のすべての要素を取得
        List<int> allOver20 = numbers.FindAll(n => n >= 20);
        Console.WriteLine("20以上のすべての数:");
        foreach (var num in allOver20)
        {
            Console.WriteLine(num);
        }
        // 20以上の最初の要素のインデックス
        int index = numbers.FindIndex(n => n >= 20);
        Console.WriteLine($"20以上の最初の数のインデックス: {index}");
    }
}

20以上の最初の数: 20
20以上のすべての数:
20
25
30
20以上の最初の数のインデックス: 2

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 5, 10, 15, 20, 25, 30 };
        // 15以上の要素を抽出
        var filtered = numbers.Where(n => n >= 15).ToList();
        Console.WriteLine("15以上の数:");
        foreach (var num in filtered)
        {
            Console.WriteLine(num);
        }
    }
}

15以上の数:
15
20
25
30

// 最初の20以上の要素を取得(存在しなければ例外)
int first20 = numbers.First(n => n >= 20);
// 最初の20以上の要素を取得(存在しなければ0)
int firstOrDefault20 = numbers.FirstOrDefault(n => n >= 20);

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        // インデクサを使って2番目の要素を更新
        fruits[1] = "blueberry";
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
blueberry
cherry

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
        // 各要素を文字列に変換して新しいList<string>を作成
        List<string> stringNumbers = numbers.ConvertAll(n => $"Number: {n}");
        foreach (var str in stringNumbers)
        {
            Console.WriteLine(str);
        }
    }
}

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

for (int i = 0; i < numbers.Count; i++)
{
    numbers[i] = numbers[i] * 2;  // 各要素を2倍に更新
}

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry", "banana" };
        bool removed = fruits.Remove("banana");  // 最初の"banana"を削除
        Console.WriteLine($"削除成功: {removed}");
        Console.WriteLine("現在のリスト:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

削除成功: True
現在のリスト:
apple
cherry
banana

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 10, 20, 30, 40 };
        numbers.RemoveAt(2);  // インデックス2の要素(30)を削除
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

10
20
40

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> letters = new List<string> { "a", "b", "c", "d", "e", "f" };
        letters.RemoveRange(2, 3);  // インデックス2から3つの要素("c", "d", "e")を削除
        foreach (var letter in letters)
        {
            Console.WriteLine(letter);
        }
    }
}

a
b
f

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
        numbers.Clear();  // すべての要素を削除
        Console.WriteLine($"要素数: {numbers.Count}");
    }
}

要素数: 0

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int>(1000);  // 大きな容量で作成
        for (int i = 0; i < 10; i++)
        {
            numbers.Add(i);
        }
        Console.WriteLine($"容量(容量拡張後): {numbers.Capacity}");
        Console.WriteLine($"要素数: {numbers.Count}");
        numbers.TrimExcess();  // 容量を要素数に近づける
        Console.WriteLine($"TrimExcess後の容量: {numbers.Capacity}");
    }
}

容量(容量拡張後): 1000
要素数: 10
TrimExcess後の容量: 10

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
banana
cherry

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 10, 20, 30, 40 };
        for (int i = 0; i < numbers.Count; i++)
        {
            Console.WriteLine($"Index {i}: {numbers[i]}");
        }
    }
}

Index 0: 10
Index 1: 20
Index 2: 30
Index 3: 40

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        var enumerator = fruits.GetEnumerator();
        while (enumerator.MoveNext())
        {
            string fruit = enumerator.Current;
            Console.WriteLine(fruit);
        }
        enumerator.Dispose();
    }
}

apple
banana
cherry

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "banana", "apple", "cherry" };
        // 文字列の長さでソート(短い順)
        fruits.Sort((x, y) => x.Length.CompareTo(y.Length));
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
banana
cherry

using System;
using System.Collections.Generic;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}
class AgeComparer : IComparer<Person>
{
    public int Compare(Person x, Person y)
    {
        return x.Age.CompareTo(y.Age);
    }
}
class Program
{
    static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 25 },
            new Person { Name = "Charlie", Age = 35 }
        };
        people.Sort(new AgeComparer());
        foreach (var person in people)
        {
            Console.WriteLine($"{person.Name}: {person.Age}");
        }
    }
}

Bob: 25
Alice: 30
Charlie: 35

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 10, 20, 30, 40, 50 };
        numbers.Sort();  // 事前にソートが必要
        int index = numbers.BinarySearch(30);
        Console.WriteLine($"30のインデックス: {index}");
        int notFoundIndex = numbers.BinarySearch(35);
        Console.WriteLine($"35の検索結果: {notFoundIndex}");
    }
}

30のインデックス: 2
35の検索結果: -4

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        fruits.Reverse();
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

cherry
banana
apple

fruits.Reverse(0, 2);  // インデックス0から2つの要素を反転

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int>(10);  // 初期容量10
        Console.WriteLine($"初期 Capacity: {numbers.Capacity}");  // 10
        Console.WriteLine($"初期 Count: {numbers.Count}");        // 0
        numbers.Add(1);
        numbers.Add(2);
        Console.WriteLine($"追加後 Capacity: {numbers.Capacity}"); // 10
        Console.WriteLine($"追加後 Count: {numbers.Count}");       // 2
    }
}

初期 Capacity: 10
初期 Count: 0
追加後 Capacity: 10
追加後 Count: 2

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int>();
        numbers.Capacity = 1000;  // 容量を明示的に設定
        for (int i = 0; i < 1000; i++)
        {
            numbers.Add(i);
        }
        Console.WriteLine($"Capacity: {numbers.Capacity}");
        Console.WriteLine($"Count: {numbers.Count}");
    }
}

Capacity: 1000
Count: 1000

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int>();
        Console.WriteLine($"初期 Capacity: {numbers.Capacity}");
        numbers.EnsureCapacity(500);
        Console.WriteLine($"EnsureCapacity(500)後 Capacity: {numbers.Capacity}");
    }
}

初期 Capacity: 0
EnsureCapacity(500)後 Capacity: 500

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int>(1000);
        for (int i = 0; i < 10; i++)
        {
            numbers.Add(i);
        }
        Console.WriteLine($"容量(追加後): {numbers.Capacity}");
        Console.WriteLine($"要素数: {numbers.Count}");
        numbers.TrimExcess();
        Console.WriteLine($"TrimExcess後の容量: {numbers.Capacity}");
    }
}

容量(追加後): 1000
要素数: 10
TrimExcess後の容量: 10

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        string[] fruitArray = fruits.ToArray();
        foreach (var fruit in fruitArray)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
banana
cherry

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3 };
        var readOnlyNumbers = numbers.AsReadOnly();
        Console.WriteLine("読み取り専用リストの要素:");
        foreach (var num in readOnlyNumbers)
        {
            Console.WriteLine(num);
        }
        // readOnlyNumbers.Add(4); // コンパイルエラー: 変更不可
        numbers.Add(4);  // 元のリストは変更可能
        Console.WriteLine("元のリスト変更後の読み取り専用リスト:");
        foreach (var num in readOnlyNumbers)
        {
            Console.WriteLine(num);
        }
    }
}

読み取り専用リストの要素:
1
2
3
元のリスト変更後の読み取り専用リスト:
1
2
3
4

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<string> letters = new List<string> { "a", "b", "c", "d", "e" };
        List<string> subList = letters.GetRange(1, 3);  // インデックス1から3つの要素を取得
        foreach (var letter in subList)
        {
            Console.WriteLine(letter);
        }
    }
}

b
c
d

using System;
using System.Collections.Generic;
using System.Linq;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}
class Program
{
    static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 25 }
        };
        // 名前だけのリストを作成
        List<string> names = people.Select(p => p.Name).ToList();
        foreach (var name in names)
        {
            Console.WriteLine(name);
        }
    }
}

Alice
Bob

private readonly object _lock = new object();
private List<int> _list = new List<int>();
public void AddItem(int item)
{
    lock (_lock)
    {
        _list.Add(item);
    }
}
public int GetItem(int index)
{
    lock (_lock)
    {
        return _list[index];
    }
}

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
class Program
{
    static void Main()
    {
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        // Listを読み取り専用コレクションに変換
        ReadOnlyCollection<string> readOnlyFruits = new ReadOnlyCollection<string>(fruits);
        Console.WriteLine("読み取り専用コレクションの要素:");
        foreach (var fruit in readOnlyFruits)
        {
            Console.WriteLine(fruit);
        }
        // readOnlyFruits.Add("date"); // コンパイルエラー: 変更不可
        // 元のリストは変更可能
        fruits.Add("date");
        Console.WriteLine("元のリスト変更後の読み取り専用コレクション:");
        foreach (var fruit in readOnlyFruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

読み取り専用コレクションの要素:
apple
banana
cherry
元のリスト変更後の読み取り専用コレクション:
apple
banana
cherry
date

using System;
using System.Collections.Immutable;
class Program
{
    static void Main()
    {
        // 空のイミュータブルリストを作成
        ImmutableList<string> fruits = ImmutableList<string>.Empty;
        // 要素を追加すると新しいリストが返る
        fruits = fruits.Add("apple");
        fruits = fruits.Add("banana");
        fruits = fruits.Add("cherry");
        Console.WriteLine("イミュータブルリストの要素:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
        // 元のリストは変更されない
        var newFruits = fruits.Add("date");
        Console.WriteLine("元のリスト:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
        Console.WriteLine("新しいリスト:");
        foreach (var fruit in newFruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

イミュータブルリストの要素:
apple
banana
cherry
元のリスト:
apple
banana
cherry
新しいリスト:
apple
banana
cherry
date

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // 3x3の2次元リストを作成
        List<List<int>> matrix = new List<List<int>>();
        for (int i = 0; i < 3; i++)
        {
            List<int> row = new List<int>();
            for (int j = 0; j < 3; j++)
            {
                row.Add(i * 3 + j);
            }
            matrix.Add(row);
        }
        // 2次元リストの要素を表示
        for (int i = 0; i < matrix.Count; i++)
        {
            for (int j = 0; j < matrix[i].Count; j++)
            {
                Console.Write(matrix[i][j] + " ");
            }
            Console.WriteLine();
        }
    }
}

0 1 2
3 4 5
6 7 8

// 例: ジャグ配列のように行ごとに異なる長さのリストを作成
List<List<int>> jaggedList = new List<List<int>>
{
    new List<int> { 1, 2, 3 },
    new List<int> { 4, 5 },
    new List<int> { 6, 7, 8, 9 }
};

using System;
using System.Collections.Generic;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public override bool Equals(object obj)
    {
        if (obj is Person other)
        {
            return Name == other.Name && Age == other.Age;
        }
        return false;
    }
    public override int GetHashCode()
    {
        return HashCode.Combine(Name, Age);
    }
}
class Program
{
    static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 25 }
        };
        var target = new Person { Name = "Alice", Age = 30 };
        Console.WriteLine(people.Contains(target));  // true
        people.Remove(target);
        Console.WriteLine(people.Count);              // 1
    }
}

True
1

using System;
using System.Collections.Generic;
class Person : IComparable<Person>
{
    public string Name { get; set; }
    public int Age { get; set; }
    public int CompareTo(Person other)
    {
        if (other == null) return 1;
        return Age.CompareTo(other.Age);  // 年齢で昇順ソート
    }
}
class Program
{
    static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person { Name = "Charlie", Age = 35 },
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 25 }
        };
        people.Sort();
        foreach (var person in people)
        {
            Console.WriteLine($"{person.Name}: {person.Age}");
        }
    }
}

Bob: 25
Alice: 30
Charlie: 35

using System;
using System.Collections.Generic;
record Person(string Name, int Age);
class Program
{
    static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person("Alice", 30),
            new Person("Bob", 25)
        };
        var target = new Person("Alice", 30);
        Console.WriteLine(people.Contains(target));  // true
        people.Remove(target);
        Console.WriteLine(people.Count);              // 1
    }
}

True
1

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
        try
        {
            foreach (var num in numbers)
            {
                if (num % 2 == 0)
                {
                    numbers.Remove(num);  // 例外発生
                }
            }
        }
        catch (InvalidOperationException ex)
        {
            Console.WriteLine("例外発生: " + ex.Message);
        }
    }
}

例外発生: Collection was modified; enumeration operation may not execute.

for (int i = numbers.Count - 1; i >= 0; i--)
{
    if (numbers[i] % 2 == 0)
    {
        numbers.RemoveAt(i);
    }
}

numbers.RemoveAll(n => n % 2 == 0);

List<int> numbers = new List<int>();
for (int i = 0; i < 10000; i++)
{
    numbers.Add(i);  // 容量不足で何度も拡張が発生
}

List<int> numbers = new List<int>(10000);

numbers.EnsureCapacity(10000);

using System;
using System.Collections.Generic;
class Person
{
    public string Name { get; set; }
}
class Program
{
    static void Main()
    {
        Person p = new Person { Name = "Alice" };
        List<Person> list1 = new List<Person> { p };
        List<Person> list2 = new List<Person> { p };
        list1[0].Name = "Bob";
        Console.WriteLine(list2[0].Name);  // "Bob"と表示される
    }
}

Bob

using System;
using System.Collections;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3 };
        // 非ジェネリックIListにキャストするとボックス化が発生
        IList list = numbers;
        list.Add(4);  // ここでintがobjectにボックス化される
        foreach (var num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3
4

using System;
class Program
{
    static void Main()
    {
        int[] array = { 1, 2, 3, 4, 5 };
        Span<int> span = new Span<int>(array, 1, 3);  // 配列の一部を参照
        for (int i = 0; i < span.Length; i++)
        {
            span[i] *= 2;  // 値を2倍に変更
        }
        foreach (var num in array)
        {
            Console.WriteLine(num);
        }
    }
}

1
4
6
8
5

using System;
using System.Buffers;
class Program
{
    static void Main()
    {
        var pool = ArrayPool<int>.Shared;
        int[] buffer = pool.Rent(1000);  // 1000要素以上の配列を借りる
        // バッファを使用
        for (int i = 0; i < 1000; i++)
        {
            buffer[i] = i;
        }
        // 使用後は返却
        pool.Return(buffer);
        Console.WriteLine("ArrayPoolを使ったバッファ処理完了");
    }
}

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // C# 2.0以前の初期化方法
        List<int> numbers1 = new List<int>();
        numbers1.Add(1);
        numbers1.Add(2);
        numbers1.Add(3);
        // C# 3.0以降のコレクション初期化子
        List<int> numbers2 = new List<int> { 1, 2, 3 };
        foreach (var num in numbers2)
        {
            Console.WriteLine(num);
        }
    }
}

1
2
3

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // 従来の書き方
        List<string> fruits1 = new List<string>();
        // C# 9.0以降のtarget-typed new
        List<string> fruits2 = new();
        fruits2.Add("apple");
        fruits2.Add("banana");
        foreach (var fruit in fruits2)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
banana

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // Listの作成と初期化
        List<string> fruits = new List<string> { "apple", "banana", "cherry" };
        // 要素の追加
        fruits.Add("date");
        // 要素の取得と表示
        Console.WriteLine("現在のフルーツリスト:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
        // 要素の更新
        fruits[1] = "blueberry";
        // 要素の削除
        fruits.Remove("cherry");
        // 並べ替え
        fruits.Sort();
        Console.WriteLine("\n更新後のフルーツリスト:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

現在のフルーツリスト:
apple
banana
cherry
date
更新後のフルーツリスト:
apple
blueberry
date

using System;
using System.Collections.Generic;
using System.Linq;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}
class Program
{
    static void Main()
    {
        // Personオブジェクトのリストを作成
        List<Person> people = new List<Person>
        {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 25 },
            new Person { Name = "Charlie", Age = 35 },
            new Person { Name = "Diana", Age = 28 }
        };
        // 30歳以上の人を抽出
        var adults = people.Where(p => p.Age >= 30).ToList();
        // 年齢順に並べ替え
        adults.Sort((x, y) => x.Age.CompareTo(y.Age));
        Console.WriteLine("30歳以上の人(年齢順):");
        foreach (var person in adults)
        {
            Console.WriteLine($"{person.Name} ({person.Age}歳)");
        }
        // 新しい人を追加
        people.Add(new Person { Name = "Eve", Age = 22 });
        // 名前で検索
        var searchName = "Bob";
        var foundPerson = people.Find(p => p.Name == searchName);
        Console.WriteLine($"\n名前が {searchName} の人:");
        if (foundPerson != null)
        {
            Console.WriteLine($"{foundPerson.Name} は {foundPerson.Age} 歳です。");
        }
        else
        {
            Console.WriteLine("見つかりませんでした。");
        }
    }
}