【C#】structとclassの違い！5分で理解する使い分けのコツ

struct Point {
  public int X { get; set; }
  public int Y { get; set; }
  public Point(int x, int y) {
    X = x;
    Y = y;
  }
}

var point1 = new Point(1, 2);  // struct
var point2 = point1;
point2.X = 3;

Console.WriteLine(point1.X); // 1
Console.WriteLine(point2.X); // 3

class Person
{
  public string Name { get; set; }

  public Person(string name)
  {
    Name = name;
  }
}

var person1 = new Person("Alice");  // class
var person2 = person1;
person2.Name = "Bob";

Console.WriteLine(person1.Name); // Bob
Console.WriteLine(person2.Name); // Bob

public struct Point
{
    public int X { get; set; }
    public int Y { get; set; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }

    public double DistanceTo(Point other)
    {
        var dx = X - other.X;
        var dy = Y - other.Y;
        return Math.Sqrt(dx * dx + dy * dy);
    }
}

// 使用例
var point1 = new Point(0, 0);
var point2 = new Point(3, 4);
var distance = point1.DistanceTo(point2);
Console.WriteLine(distance); // 5

public class Animal
{
    public virtual void MakeSound() { }
}

public class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("ワン！");
    }
}

public class Cat : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("ニャー！");
    }
}

public class User
{
    public string Name { get; set; }
    public string Email { get; set; }
    public DateTime DateOfBirth { get; set; }
    private List<string> _roles = new List<string>();

    public User(string name, string email, DateTime dateOfBirth)
    {
        Name = name;
        Email = email;
        DateOfBirth = dateOfBirth;
    }

    public void AddRole(string role)
    {
        _roles.Add(role);
    }

    public bool HasRole(string role)
    {
        return _roles.Contains(role);
    }

    public int GetAge()
    {
        return DateTime.Now.Year - DateOfBirth.Year;
    }
}

// 使用例
var user = new User("田中太郎", "tanaka@example.com", new DateTime(1990, 1, 1));
user.AddRole("管理者");
var isAdmin = user.HasRole("管理者");
var age = user.GetAge();

Console.WriteLine(isAdmin); // true
Console.WriteLine(age); // 34

public struct Point3D
{
    public float X, Y, Z;
}

// structを使用した場合
var points1 = new Point3D[1000000];

// classを使用した場合
public class Point3DClass
{
    public float X, Y, Z;
}
var points2 = new Point3DClass[1000000];

using System;
using System.Diagnostics;

public struct Point
{
    public int X { get; set; }
    public int Y { get; set; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }

    public double DistanceTo(Point other)
    {
        var dx = X - other.X;
        var dy = Y - other.Y;
        return Math.Sqrt(dx * dx + dy * dy);
    }
}

public class PointClass
{
    public int X { get; set; }
    public int Y { get; set; }

    public PointClass(int x, int y)
    {
        X = x;
        Y = y;
    }

    public double DistanceTo(PointClass other)
    {
        var dx = X - other.X;
        var dy = Y - other.Y;
        return Math.Sqrt(dx * dx + dy * dy);
    }
}

class Program
{
    static void Main()
    {
        const int count = 10000000;
        var structPoints = new Point[count];
        var classPoints = new PointClass[count];

        var structTime = MeasureTime(() => {
            for (int i = 0; i < count; i++)
            {
                structPoints[i] = new Point(i, i);
            }
            var totalDistance = 0.0;
            for (int i = 1; i < count; i++)
            {
                totalDistance += structPoints[i].DistanceTo(structPoints[i - 1]);
            }
        });

        var classTime = MeasureTime(() => {
            for (int i = 0; i < count; i++)
            {
                classPoints[i] = new PointClass(i, i);
            }
            var totalDistance = 0.0;
            for (int i = 1; i < count; i++)
            {
                totalDistance += classPoints[i].DistanceTo(classPoints[i - 1]);
            }
        });

        Console.WriteLine($"Struct time: {structTime}ms");
        Console.WriteLine($"Class time: {classTime}ms");
    }

    static long MeasureTime(Action action)
    {
        var stopwatch = new Stopwatch();
        stopwatch.Start();
        action();
        stopwatch.Stop();
        return stopwatch.ElapsedMilliseconds;
    }
}