Linq lambda联接错误

本文关键字：错误 lambda Linq | 更新日期: 2023-09-27 17:58:25

我一直在关注ProASP.net MVC 2 Framework这本书，我发现它非常出色。但这是一条真正的学习曲线，现在我陷入了困境。

在这本书中，您构建了类似下面的内容，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) 
}

我认为需要调整这个，这样我就可以加入搜索

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) 
}

但我立刻得到一个intelligense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（是否缺少强制转换？）

很明显，我对代码的理解还不够好，无法找出问题所在。我也觉得这个蹩脚的东西有点力不从心

房间是一个域对象，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; }
      }
    }

代表我的房间桌子。我是否需要某种父实体来表示房间和房间坐标表之间的连接？

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｛get；set；｝公共分页信息分页信息｛get；set；｝
}}

Linq lambda联接错误

问题是，在这个代码中，

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

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

然后，在下一行中，您将尝试将其附加到Room的列表中。

问题可以通过最初创建正确项目类型的IEnumerable来解决：

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）。

以下是我根据OP的进一步描述提出的建议：

1） 创建一个同时包含Room和RoomCoordinates信息的聚合类：

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

2） 修改您的控制器操作如下：

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RoomsListViewModel类和视图以反映这些更改。

相关文章：