C#のプロパティの基本 { get; set; } の使い方をやさしく解説

// 手動実装プロパティの例
public class Person
{
    // バッキングフィールド(backing field)
    private string _name = "Unknown";

    // プロパティ
    public string Name
    {
        get
        {
            // 取得時にそのまま返す例
            return _name;
        }
        set
        {
            // 代入時にnullや空文字を許さない
            if (string.IsNullOrWhiteSpace(value))
            {
                throw new ArgumentException("Nameは空にできません。");
            }
            _name = value.Trim();
        }
    }
}

using System;

public class Person
{
    // 自動実装プロパティ(バッキングフィールドはコンパイラが自動生成)
    public string Name { get; set; }
    public int Age { get; set; }
}

public class Program
{
    public static void Main()
    {
        var p = new Person();
        p.Name = "Taro";
        p.Age = 20;

        Console.WriteLine($"Name: {p.Name}, Age: {p.Age}");
    }
}

public class Account
{
    public string Owner { get; set; } = "Guest";  // 既定値
    public decimal Balance { get; set; } = 0m;    // 既定値
}

using System;

public class Program
{
    public static void Main()
    {
        var a = new Account();
        Console.WriteLine($"Owner: {a.Owner}, Balance: {a.Balance}");

        var b = new Account { Owner = "Hanako", Balance = 1500m };
        Console.WriteLine($"Owner: {b.Owner}, Balance: {b.Balance}");
    }
}

using System;

public class Rectangle
{
    public double Width  { get; set; }
    public double Height { get; set; }

    // 計算プロパティ(読み取り専用)
    public double Area
    {
        get
        {
            return Width * Height; // 面積を毎回計算して返す
        }
    }
}

public class Program
{
    public static void Main()
    {
        var r = new Rectangle { Width = 3, Height = 4 };
        Console.WriteLine($"Area: {r.Area}");
        r.Width = 10;
        Console.WriteLine($"Area after resize: {r.Area}");
    }
}

using System;

public class Product
{
    private string _name = "Unnamed";
    private decimal _price;

    public string Name
    {
        get => _name;
        set
        {
            if (string.IsNullOrWhiteSpace(value))
                throw new ArgumentException("Nameは空白不可です。");
            _name = value.Trim(); // 余分な空白を除去
        }
    }

    public decimal Price
    {
        get => _price;
        set
        {
            if (value < 0m)
                throw new ArgumentOutOfRangeException(nameof(value), "Priceは0以上にしてください。");
            _price = value;
        }
    }
}

public class Program
{
    public static void Main()
    {
        var p = new Product();
        p.Name = "  Coffee  "; // setでTrim
        p.Price = 250m;
        Console.WriteLine($"{p.Name} - {p.Price}円");
    }
}

public class Member
{
    private int _age;
    public int Age
    {
        get => _age;
        set
        {
            // 不正値を受け取らない
            if (value < 0) throw new ArgumentOutOfRangeException(nameof(value), "Ageは0以上です。");
            _age = value;
        }
    }
}

using System;

public class Order
{
    public string Number { get; }
    public DateTime CreatedAt { get; } = DateTime.UtcNow;

    // 外部からは読み取り専用、内部では更新可能
    public DateTime LastUpdated { get; private set; } = DateTime.UtcNow;

    public Order(string number)
    {
        Number = number ?? throw new ArgumentNullException(nameof(number));
    }

    public void UpdateSomething()
    {
        // 何らかの更新処理...
        LastUpdated = DateTime.UtcNow; // 内部でのみ更新
    }
}

public class Program
{
    public static void Main()
    {
        var o = new Order("A-001");
        Console.WriteLine($"CreatedAt: {o.CreatedAt:O}");
        Console.WriteLine($"LastUpdated: {o.LastUpdated:O}");

        o.UpdateSomething();
        Console.WriteLine($"LastUpdated(after): {o.LastUpdated:O}");
    }
}

using System;

public class User
{
    // 必須項目はgetのみ(読み取り専用)にし、コンストラクタで設定
    public string Id { get; }
    public string Name { get; private set; }

    // 任意項目は既定値を設定しておく
    public string Email { get; set; } = "unset@example.com";

    public User(string id, string name)
    {
        // 簡単な引数チェック
        if (string.IsNullOrWhiteSpace(id)) throw new ArgumentException("Idは必須です。");
        if (string.IsNullOrWhiteSpace(name)) throw new ArgumentException("Nameは必須です。");

        Id = id.Trim();
        Name = name.Trim();
    }

    public void Rename(string newName)
    {
        if (string.IsNullOrWhiteSpace(newName)) throw new ArgumentException("Nameは空にできません。");
        Name = newName.Trim();
    }
}

public class Program
{
    public static void Main()
    {
        var u = new User("u-100", "Sakura");
        Console.WriteLine($"{u.Id}: {u.Name}, {u.Email}");
        u.Rename("Sakura Tanaka");
        Console.WriteLine($"{u.Id}: {u.Name}, {u.Email}");
    }
}

using System;
using System.Collections.Generic;

public class Book
{
    public string Title  { get; set; } = "Untitled";
    public string Author { get; set; } = "Unknown";
    public int    Year   { get; set; } = 0;
}

public class Program
{
    public static void Main()
    {
        // オブジェクト初期化子で一括設定
        var b1 = new Book { Title = "C#入門", Author = "Yamada", Year = 2025 };
        var b2 = new Book { Title = "やさしいOOP", Author = "Sato" }; // Yearは既定値

        // コレクション初期化子と併用する例
        var shelf = new List<Book>
        {
            new Book { Title = "LINQ実践", Author = "Kato", Year = 2024 },
            b1,
            b2
        };

        foreach (var b in shelf)
        {
            Console.WriteLine($"{b.Title} by {b.Author} ({b.Year})");
        }
    }
}