[C#] プロパティ応用編 — init・required・抽象プロパティ・インデクサまで

public class Person
{
    public string Name { get; init; }
    public int Age { get; init; }
}

// オブジェクト初期化子で設定 — OK
var p = new Person { Name = "Alice", Age = 30 };

// 初期化後の変更 — コンパイルエラー
// p.Name = "Bob";

public class Product
{
    private string _sku = "";

    public string Sku
    {
        get => _sku;
        init
        {
            if (string.IsNullOrWhiteSpace(value))
                throw new ArgumentException("SKU は空にできません。");
            _sku = value;
        }
    }
}

var product = new Product { Sku = "ABC-123" }; // OK
// var bad = new Product { Sku = "" };          // ArgumentException

public class User
{
    public required string Email { get; set; }
    public required string DisplayName { get; set; }
    public string? Bio { get; set; } // こちらは任意
}

// Email と DisplayName は必ず設定する必要がある
var user = new User
{
    Email = "alice@example.com",
    DisplayName = "Alice"
};

// Email を省略するとコンパイルエラー
// var bad = new User { DisplayName = "Bob" };

public class Order
{
    public required string OrderId { get; init; }
    public required DateTime CreatedAt { get; init; }
    public string? Note { get; set; }
}

var order = new Order
{
    OrderId = "ORD-001",
    CreatedAt = DateTime.UtcNow,
    Note = "急ぎ"
};

// order.OrderId = "ORD-002"; // コンパイルエラー（init のため変更不可）

using System.Diagnostics.CodeAnalysis;

public class User
{
    public required string Email { get; set; }
    public required string DisplayName { get; set; }

    public User() { }

    [SetsRequiredMembers]
    public User(string email, string displayName)
    {
        Email = email;
        DisplayName = displayName;
    }
}

// コンストラクタ経由 — required の警告なし
var user = new User("alice@example.com", "Alice");

public abstract class Shape
{
    public abstract double Area { get; }
    public abstract string Name { get; }
}

public class Circle : Shape
{
    public double Radius { get; }

    public Circle(double radius) => Radius = radius;

    public override double Area => Math.PI * Radius * Radius;
    public override string Name => "Circle";
}

public class Square : Shape
{
    public double Side { get; }

    public Square(double side) => Side = side;

    public override double Area => Side * Side;
    public override string Name => "Square";
}

public class Animal
{
    public virtual string Sound => "...";
}

public class Dog : Animal
{
    public override string Sound => "Woof!";
}

public class Cat : Animal
{
    public override string Sound => "Meow!";
}

Animal animal = new Dog();
Console.WriteLine(animal.Sound); // Woof!

public interface IIdentifiable
{
    string Id { get; }
}

public interface ITimestamped
{
    DateTime CreatedAt { get; }
    DateTime? UpdatedAt { get; set; }
}

public class Article : IIdentifiable, ITimestamped
{
    public string Id { get; } = Guid.NewGuid().ToString();
    public DateTime CreatedAt { get; } = DateTime.UtcNow;
    public DateTime? UpdatedAt { get; set; }
}

public interface IVersioned
{
    int Version => 1; // デフォルト実装
}

public class Document : IVersioned
{
    // Version を実装しなくてもコンパイル可能
    // IVersioned 経由でアクセスするとデフォルト値 1 が返る
}

IVersioned doc = new Document();
Console.WriteLine(doc.Version); // 1

public class Sentence
{
    private string[] _words;

    public Sentence(string text)
    {
        _words = text.Split(' ');
    }

    // インデクサ
    public string this[int index]
    {
        get => _words[index];
        set => _words[index] = value;
    }

    public int Length => _words.Length;

    public override string ToString() => string.Join(" ", _words);
}

var sentence = new Sentence("Hello World Today");
Console.WriteLine(sentence[0]); // Hello
sentence[1] = "C#";
Console.WriteLine(sentence);    // Hello C# Today

public class HttpHeaders
{
    private readonly Dictionary<string, string> _headers = new(StringComparer.OrdinalIgnoreCase);

    public string? this[string name]
    {
        get => _headers.TryGetValue(name, out var value) ? value : null;
        set
        {
            if (value is null)
                _headers.Remove(name);
            else
                _headers[name] = value;
        }
    }
}

var headers = new HttpHeaders();
headers["Content-Type"] = "application/json";
headers["Authorization"] = "Bearer token123";
Console.WriteLine(headers["content-type"]); // application/json（大文字小文字を無視）

public class Matrix
{
    private readonly double[,] _data;

    public Matrix(int rows, int cols)
    {
        _data = new double[rows, cols];
    }

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

var m = new Matrix(2, 2);
m[0, 0] = 1.0;
m[0, 1] = 2.0;
m[1, 0] = 3.0;
m[1, 1] = 4.0;
Console.WriteLine(m[1, 1]); // 4