c# lambda 表达式错误...此字典需要类型的模型项

Question

当我更改查询时出现错误。你能帮忙吗？

    public ActionResult Index(int? id)
    {
        Models.MyProjectEntities entity = new Models.MyProjectEntities();

         // NORMAL QUERY, NO PROBLEM
         //var Messages = entity.Message.Where(x => x.Active);

         // JOINED QUERY, GENERATES ERROR
         var Messages = entity.Message.Join(entity.Categories, 
                            m => m.CategoriID,
                            k => k.CategoriID,
                            (m, k) => new { Message = m, Categories = k })
                            .Where(x => x.Message.Active);


        return View(Messages);
    }

这是aspx文件的第一行

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<IEnumerable<MyProject.Models.Message>>" %>

这是错误

The model item passed into the dictionary is of type 'System.Data.Objects.ObjectQuery`1[<>f__AnonymousType7`2[MyProject.Models.Message,MyProject.Models.Categories]]', but this dictionary requires a model item of type 'System.Collections.Generic.IEnumerable`1[MyProject.Models.Message]'.

Answer 1

您的视图被声明为采用一组Messages。
您正在尝试向其传递一组匿名类型。

正如错误明确指出的那样，它不适合。

相反，您应该创建一个类来保存来自连接的数据，然后将视图声明为采用该类的集合。
（视图不能轻易使用匿名类型的模型）

Answer 2

因为你加入了你创建了一个匿名类型，你需要这样做

var Messages = entity.Message.Join(entity.Categories, 
    m => m.CategoriID,
    k => k.CategoriID,
    (m, k) => new { Message = m, Categories = k })
    .Where(x => x.Message.Active)
    .Select(x => new Message { ... .. ..  } );

Answer 3

public ActionResult Index(int? id)
{
    Models.MyProjectEntities entity = new Models.MyProjectEntities();

     // NORMAL QUERY, NO PROBLEM
     //var Messages = entity.Message.Where(x => x.Active);

     // JOINED QUERY, GENERATES ERROR
     var Messages = entity.Message.Join(entity.Categories, 
                        m => m.CategoriID,
                        k => k.CategoriID,
                        (m, k) => new { Message = m, Categories = k })
                        .Where(x => x.Message.Active)
                        .Select(x => x.Message);


    return View(Messages);
}

或者如果您也需要类别，您应该更改视图模型类型

编辑：

public ActionResult Index(int? id)
{
    Models.MyProjectEntities entity = new Models.MyProjectEntities();

     // NORMAL QUERY, NO PROBLEM
     //var Messages = entity.Message.Where(x => x.Active);

     // JOINED QUERY, GENERATES ERROR
     var Messages = entity.Message.Join(entity.Categories, 
                        m => m.CategoriID,
                        k => k.CategoriID,
                        (m, k) => new MessageWithCategories { Message = m, Categories = k })
                        .Where(x => x.Message.Active);


    return View(Messages);
}

且模型类型应为 MessageWithCategories（必须创建此类）