【C#/Blazor】実務Webアプリ開発編 (20)エンティティ・集約・集約ルートを実コードで理解する ～DDDの不変条件の守り方～

src/MentorApp.Domain/
  Models/
    Users/
      User.cs             ← 集約ルート
      Email.cs
      Role.cs
      IUserRepository.cs
    Mentorships/
      Mentorship.cs       ← 集約ルート
      MentorshipStatus.cs
      IMentorshipRepository.cs
    Topics/
      Topic.cs            ← 集約ルート
      Message.cs
      TopicStatus.cs
      ITopicRepository.cs
    Shared/
      ValidationError.cs
      ValidationExtensions.cs
      ValidationHelper.cs
      IUnitOfWork.cs
      IUnitOfWorkFactory.cs
  Services/
    MentorshipDuplicationCheckService.cs
    RoleChangeValidationService.cs

【User集約：シンプルな例】
外部のコード
    ↓
集約ルート（User）
    └─ 集約内の値（Email・Role）

【Topic集約：複合的な例（詳細は次回）】
外部のコード
    ↓
集約ルート（Topic）
    ├─ 集約内のエンティティ（Message）
    └─ 集約内の値（TopicStatus）

public Guid MentorUserId { get; private set; }
public Guid MenteeUserId { get; private set; }

public class User
{
    public const int ExternalIdMaxLength = 255;
    public const int DisplayNameMaxLength = 100;

    public Guid Id { get; private set; }
    public string ExternalId { get; private set; } = null!;
    public string DisplayName { get; private set; } = null!;
    public Email Email { get; private set; } = null!;
    public Role Role { get; private set; }
    public DateTimeOffset CreatedAt { get; private set; }

    // EF Core 用
    private User() { }

    public User(string? externalId, string? displayName, DateTimeOffset createdAt, string? email, Role role = Role.Mentee)
    {
        Validate(externalId, displayName, email).ThrowIfInvalid();

        Id = Guid.NewGuid();
        ExternalId = externalId!;
        DisplayName = displayName!;
        Email = new Email(email);
        Role = role;
        CreatedAt = createdAt;
    }
}

public static IEnumerable<ValidationError> Validate(
    string? externalId, string? displayName, string? email)
{
    return ValidateExternalId(externalId).ToValidationErrors(nameof(ExternalId))
        .Concat(ValidateDisplayName(displayName).ToValidationErrors(nameof(DisplayName)))
        .Concat(Email.Validate(email).ToValidationErrors(nameof(Email)));
}

public void UpdateDisplayName(string? displayName)
{
    ValidateDisplayName(displayName).ToValidationErrors(nameof(DisplayName)).ThrowIfInvalid();
    DisplayName = displayName!;
}

public void UpdateEmail(string email) => Email = new Email(email);

/// <remarks>認可チェックはアプリケーション層（UserService）で行うため、このメソッド自体は認可を持たない。</remarks>
public void ChangeRole(Role newRole) => Role = newRole;

// EF Core 用
private User() { }

public Mentorship(Guid mentorUserId, Guid menteeUserId, DateTimeOffset startedAt)
{
    Validate(mentorUserId, menteeUserId).ThrowIfInvalid();

    Id = Guid.NewGuid();
    MentorUserId = mentorUserId;
    MenteeUserId = menteeUserId;
    Status = MentorshipStatus.Active;
    StartedAt = startedAt;
}

public static IEnumerable<ValidationError> Validate(Guid mentorUserId, Guid menteeUserId)
{
    return ValidateMentorUserId(mentorUserId).ToValidationErrors(nameof(MentorUserId))
        .Concat(ValidateMenteeUserId(menteeUserId).ToValidationErrors(nameof(MenteeUserId)))
        .Concat(ValidateDifferentUsers(mentorUserId, menteeUserId).ToValidationErrors(nameof(MenteeUserId)));
}

public void Complete(DateTimeOffset endedAt)
{
    if (Status != MentorshipStatus.Active)
        throw new InvalidOperationException("Active 状態の Mentorship のみ完了にできます。");

    Status = MentorshipStatus.Completed;
    EndedAt = endedAt;
}

public void Cancel(DateTimeOffset endedAt)
{
    if (Status != MentorshipStatus.Active)
        throw new InvalidOperationException("Active 状態の Mentorship のみキャンセルできます。");

    Status = MentorshipStatus.Cancelled;
    EndedAt = endedAt;
}

public Guid MentorUserId { get; private set; }
public User MentorUser { get; private set; } = null!;

public Guid MenteeUserId { get; private set; }
public User MenteeUser { get; private set; } = null!;