Linq lambda 连接错误

Question

我一直在关注 Pro ASP.net MVC 2 Framework 这本书，我发现它非常出色。但这是一个真正的学习曲线，现在我被卡住了。

在本书中，您构建了如下所示的内容，它允许分页。

public ViewResult List([DefaultValue(0)] string cityzip, [DefaultValue(1)] int page)
{

var roomsToShow = roomsRepository.Rooms.Where(x => x.CountryID == cityzip);

var viewModel = new RoomsListViewModel
{

    Rooms = roomsToShow.Skip((page - 1) * PageSize).Take(PageSize).ToList(),
    PagingInfo = new PagingInfo
    {
        CurrentPage = page,
        ItemsPerPage = PageSize,
        TotalItems = roomsToShow.Count()
    }
};
return View(viewModel); // Passed to view as ViewData.Model (or simply Model) 

} 

我认为需要对此进行调整，以便我可以加入搜索

public ViewResult List([DefaultValue(0)] string cityzip, [DefaultValue(1)] int page)
{

var roomsToShow = roomsRepository.Rooms.Join(
                    roomCoordinatesRepository.RoomCoordinates,
                    room => room.RoomID,
                    roomCoordinate => roomCoordinate.RoomID,
                    (room, roomCoordinate) => new { RoomCoordinate = roomCoordinate, Room = room });

var viewModel = new RoomsListViewModel
{

    Rooms = roomsToShow.Skip((page - 1) * PageSize).Take(PageSize).ToList(),
    PagingInfo = new PagingInfo
    {
        CurrentPage = page,
        ItemsPerPage = PageSize,
        TotalItems = roomsToShow.Count()
    }
};
return View(viewModel); // Passed to view as ViewData.Model (or simply Model) 

} 

...但是我立即收到一个智能感知错误说- 无法将类型 'System.Collections.Generic.List<AnonymousType#1>' 隐式转换为 'System.Collections.Generic.IList<MeetingRoom.Domain.Entities.Room>'。存在显式转换（您是否缺少演员表？）

我显然对代码理解得不够透彻，无法找出问题所在。我也对这个 lamda linq 的东西感到有点不知所措

Room 是一个域对象，定义为：

   namespace MeetingRoom.Domain.Entities
    {
         [Table(Name = "Rooms")]
         public class Room
         {
            [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert)] 
            public int RoomID { get; set; }
            [Column] public string Name { get; set; }
            [Column] public string Description { get; set; }
            [Column] public decimal Price { get; set; }
            [Column] public string Category { get; set; }
            [Column] public string Pcode { get; set; }
            [Column] public int CountryID { get; set; }


            public MeetingRooms.Domain.entities.RoomCoordinate RoomCoordinate { get; set; }
      }
    }

并代表我的 Room 表。我是否需要某种父实体来表示 Room 和 Room-co-ordinates 表之间的连接？

坐标实体如下所示：

namespace MeetingRooms.Domain.entities
{
    [Table(Name = "RoomCoordinate")]
    public class RoomCoordinate
    {
        [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert, Name = "ID")]
        public int CoordID { get; set; }
        [Column]
        public int RoomID { get; set; }
        [Column]
        public string Coordinates { get; set; }
        [Column]
        public decimal Latitude { get; set; }  
        [Column]
        public decimal Longitude { get; set; }  
    }
}

RoomsListViewModel 如下所示： 命名空间 MeetingRoomsMVC.WebUI.Models { 公共类RoomsListViewModel { 公共 IList RoomsWithCoordinates { 得到；放; } 公共 PagingInfo PagingInfo { 获取；放; }
} }

Answer 1

这是我根据 OP 的进一步描述提出的建议：

1) 创建一个包含Room 和RoomCoordinates 信息的聚合类：

public class RoomWithCoordinates
{
    public Room Room { get; set; }
    public RoomCoordinates Coordinates { get; set; }
}

2) 修改你的控制器动作如下：

public ViewResult List([DefaultValue(0)] string cityzip, [DefaultValue(1)] int page)
{

var roomsToShow = roomsRepository.Rooms.Join(
                    roomCoordinatesRepository.RoomCoordinates,
                    room => room.RoomID,
                    roomCoordinate => roomCoordinate.RoomID,
                    (room, roomCoordinate) => new RoomWithCoordinates{ Coordinates = roomCoordinate, Room = room } );

var viewModel = new RoomsListViewModel
{

    RoomsWithCoordinates = roomsToShow.Skip((page - 1) * PageSize).Take(PageSize).ToList(),
    PagingInfo = new PagingInfo
    {
        CurrentPage = page,
        ItemsPerPage = PageSize,
        TotalItems = roomsToShow.Count()
    }
};
return View(viewModel); // Passed to view as ViewData.Model (or simply Model) 

} 

3) 修改您的 RoomsListViewModel 类和您的视图以反映这些更改。

Answer 2

问题是，在这段代码中

var roomsToShow = roomsRepository.Rooms.Join(
                roomCoordinatesRepository.RoomCoordinates,
                room => room.RoomID,
                roomCoordinate => roomCoordinate.RoomID,
                (room, roomCoordinate) => new { RoomCoordinate = roomCoordinate, Room = room });

您正在构建一个匿名类型对象的IEnumerable：(room, roomCoordinate) => new { RoomCoordinate = roomCoordinate, Room = room }

然后，在下一行中，您尝试将其分配给Room 的列表。

可以通过最初创建正确项目类型的IEnumerable 来解决问题：

var roomsToShow = roomsRepository.Rooms.Join(
                roomCoordinatesRepository.RoomCoordinates,
                room => room.RoomID,
                roomCoordinate => roomCoordinate.RoomID,
                (room, roomCoordinate) => new MeetingRoom.Domain.Entities.Room{ RoomCoordinate = roomCoordinate, Room = room });

（注意 lambda 中的类名）。