使用数据库实体与业务逻辑实体时，如何避免重复定义属性？

针对实体类与业务类属性重复的问题，提供三种可行方案，你可以根据项目架构需求选择：

方案1：业务类继承实体类 #

让RoomBL直接继承Room实体，复用所有数据属性，仅添加业务专属的属性与方法：

public class RoomBL : Room
{
    public IRoomUserRepository UserRepository { get; }
    public RoomExtraData ExtraData { get; }

    public RoomBL(Room room, IRoomUserRepository userRepository, RoomExtraData extraData)
    {
        // 复制基类实体属性
        Id = room.Id;
        Name = room.Name;
        Layout = room.Layout;
        OwnerId = room.OwnerId;
        MaxUsersAllowed = room.MaxUsersAllowed;
        Description = room.Description;
        IsMuted = room.IsMuted;
        Settings = room.Settings;
        PaintSettings = room.PaintSettings;
        PlayerRights = room.PlayerRights;
        Tags = room.Tags;
        Likes = room.Likes;
        FurnitureItems = room.FurnitureItems;
        // 初始化业务专属内存状态
        ChatMessages = [];

        UserRepository = userRepository;
        ExtraData = extraData;
    }

    public async Task RunPeriodicCheckAsync()
    {
        var users = UserRepository.GetAll();
        
        foreach (var roomUser in users)
        {
            await roomUser.RunPeriodicCheckAsync();
        }

        var statusWriter = new RoomUserStatusWriter(users).GetAllBytes();
        var dataWriter = new RoomUserDataWriter(users).GetAllBytes();

        await UserRepository.BroadcastDataAsync(statusWriter);
        await UserRepository.BroadcastDataAsync(dataWriter);
    }

    public async ValueTask DisposeAsync()
    {
        await UserRepository.DisposeAsync();
    }
}

优点：完全消除属性重复，代码简洁；业务类可直接使用实体的所有属性。
缺点：实体与业务类耦合度高，实体结构变更时业务类需同步调整；实体上的数据库特性会被业务类继承，不符合单一职责。

方案2：实体类添加非持久化属性 #

在Room实体中用[NotMapped]标记不需要存入数据库的属性，同时将业务方法整合到实体类中：

public class Room
{
    public int Id { get; set; }
    public string Name { get; set; }
    public RoomLayout Layout { get; }
    public int OwnerId { get; set; }
    public int MaxUsersAllowed { get; set; }
    public string Description { get; set; }
    public bool IsMuted { get; set; }
    public RoomSettings Settings { get; set; }
    public RoomPaintSettings PaintSettings { get; set; }
    public List<RoomPlayerRight> PlayerRights { get; set; }
    public List<RoomChatMessage> ChatMessages { get; set; } = [];
    public List<RoomTag> Tags { get; set; }
    public List<PlayerRoomLike> Likes { get; set; }
    public List<RoomFurnitureItem> FurnitureItems { get; set; }

    // 非持久化属性，标记NotMapped避免数据库映射
    [NotMapped]
    public IRoomUserRepository UserRepository { get; set; }
    [NotMapped]
    public RoomExtraData ExtraData { get; set; }

    public async Task RunPeriodicCheckAsync()
    {
        var users = UserRepository.GetAll();
        
        foreach (var roomUser in users)
        {
            await roomUser.RunPeriodicCheckAsync();
        }

        var statusWriter = new RoomUserStatusWriter(users).GetAllBytes();
        var dataWriter = new RoomUserDataWriter(users).GetAllBytes();

        await UserRepository.BroadcastDataAsync(statusWriter);
        await UserRepository.BroadcastDataAsync(dataWriter);
    }

    public async ValueTask DisposeAsync()
    {
        await UserRepository.DisposeAsync();
    }
}

优点：无需额外业务类，所有逻辑集中在一个类，调用便捷。
缺点：违反单一职责原则，实体类同时承担数据持久化与业务逻辑职责，后期代码会臃肿难维护；依赖注入放到实体类中，会增加实体创建的复杂度。

方案3：组合模式（推荐） #

让RoomBL持有Room实体的实例，通过包装属性复用数据，彻底分离业务逻辑与数据模型：

public class RoomBL
{
    private readonly Room _room;
    public IRoomUserRepository UserRepository { get; }
    public RoomExtraData ExtraData { get; }
    public List<RoomChatMessage> ChatMessages { get; } = [];

    public RoomBL(Room room, IRoomUserRepository userRepository, RoomExtraData extraData)
    {
        _room = room;
        UserRepository = userRepository;
        ExtraData = extraData;
    }

    // 包装实体属性，对外暴露
    public int Id => _room.Id;
    public string Name
    {
        get => _room.Name;
        set => _room.Name = value;
    }
    public RoomLayout Layout => _room.Layout;
    public int OwnerId => _room.OwnerId;
    public int MaxUsersAllowed
    {
        get => _room.MaxUsersAllowed;
        set => _room.MaxUsersAllowed = value;
    }
    public string Description
    {
        get => _room.Description;
        set => _room.Description = value;
    }
    public bool IsMuted => _room.IsMuted;
    public RoomSettings Settings => _room.Settings;
    public RoomPaintSettings PaintSettings => _room.PaintSettings;
    public List<RoomPlayerRight> PlayerRights => _room.PlayerRights;
    public List<RoomTag> Tags => _room.Tags;
    public List<PlayerRoomLike> Likes => _room.Likes;
    public List<RoomFurnitureItem> FurnitureItems => _room.FurnitureItems;

    public async Task RunPeriodicCheckAsync()
    {
        var users = UserRepository.GetAll();
        
        foreach (var roomUser in users)
        {
            await roomUser.RunPeriodicCheckAsync();
        }

        var statusWriter = new RoomUserStatusWriter(users).GetAllBytes();
        var dataWriter = new RoomUserDataWriter(users).GetAllBytes();

        await UserRepository.BroadcastDataAsync(statusWriter);
        await UserRepository.BroadcastDataAsync(dataWriter);
    }

    public async ValueTask DisposeAsync()
    {
        await UserRepository.DisposeAsync();
    }
}

优点：严格遵循单一职责，Room仅负责数据持久化，RoomBL仅负责业务逻辑；两者耦合度低，修改其中一个不会影响另一个；依赖注入逻辑更清晰。
缺点：需要手动包装实体属性，初期代码量稍大，可通过代码生成工具（如T4模板、Roslyn源码生成）自动生成包装代码，减少重复劳动。

内容的提问来源于stack exchange，提问作者user5405648