无法将类型 'int?' 隐式转换为 'System.Collections.Generic.List<Model>

Question

我是 MVC 和 LINQ 的新手。

我正在尝试使用 LINQ 为视图提供模型。此模型由另一个模型的列表组成。

这是第一个模型。

public class MasterCategoryModel
{
    [Key]
    public int CategoryId { get; set; }

    [Required(ErrorMessage = "Category name is mandatory")]
    [StringLength(255, ErrorMessage = "Category name cannot exceed 255 characters")]
    [Display(Name = "Category name ")]
    public string Name {get;set;}

    [StringLength(1000, ErrorMessage = "Category description cannot exceed 1000 characters")]
    [Display(Name = "Category description")]
    public string Description{get;set;}
}

这是问题较多的模型：

public class CategoryModel
{
    [Key]
    public int CategoryId { get; set; }

    [Required]
    [StringLength(255, ErrorMessage = "Category name cannot exceed 255 characters")]
    [Display(Name = "Category nam")]
    public string Name { get; set; }

    [StringLength(1000, ErrorMessage = "Category description cannot exceed 1000 characters")]
    [Display(Name = "Category description")]
    public string Description { get; set; }

    [Required]
    public List<MasterCategoryModel> MasterCategory { get; set; }
}

在我的控制器中，我目前有以下内容：

[Authorize(Users = "Administrator")]
[HttpGet]
public PartialViewResult _SubCategory()
{
    using (var db = new TSCEntities())
    {
        var query = from p in db.Categories
                    where p.ParentCategoryId != null
                    select new CategoryModel() 
                    { 
                       CategoryId = p.CategoryId, 
                       Name = p.Name, 
                       Description = p.Description, 
                       MasterCategory = p.ParentCategoryId 
                    };

        return PartialView(query.ToList());
    }
}

MasterCategory = p.ParentCategoryId 失败。

任何人都可以帮助提供一个 LINQ 表达式，它会以某种方式给出一个列表吗？

数据库table代码：

CREATE TABLE [product].[Categories](
    [CategoryId] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](255) NOT NULL,
    [Description] [nvarchar](1000) NULL,
    [ParentCategoryId] [int] NULL,
PRIMARY KEY CLUSTERED 
(
    [CategoryId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [product].[Categories]  WITH CHECK ADD  CONSTRAINT [FK_Category_ParentCategory] FOREIGN KEY([ParentCategoryId])
REFERENCES [product].[Categories] ([CategoryId])
GO

ALTER TABLE [product].[Categories] CHECK CONSTRAINT [FK_Category_ParentCategory]
GO

这是由 EF 生成的 class。

public partial class Category
    {
        public Category()
        {
            this.Categories1 = new HashSet<Category>();
        }

        public int CategoryId { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public Nullable<int> ParentCategoryId { get; set; }

        public virtual ICollection<Category> Categories1 { get; set; }
        public virtual Category Category1 { get; set; }


    }

Answer 1

根据您的代码，MasterCategory 是一个 List，您需要从数据库中通过 id 获取它。 List 可以使用 constructor with IEnumerable 作为参数来创建：

var query = from p in db.Categories
    where p.ParentCategoryId != null
    select new CategoryModel() 
    { 
       CategoryId = p.CategoryId, 
       Name = p.Name, 
       Description = p.Description, 
       MasterCategory = new List<MasterCategoryModel>((from c in db.Categories where c.CategoryId == p.ParentCategoryId select c).AsEnumerable())
    };

更新

我认为由于缺少 using 指令，并非所有的 Linq 扩展方法都可用：

using System.Linq;

Answer 2

您必须像这样加入表格：

var query = from p in db.Categories
            join c in db.Categories on p.ParentCategoryId equals c.CategoryId 
            where p.ParentCategoryId != null
            select new CategoryModel() 
            { 
             CategoryId = p.CategoryId, 
             Name = p.Name, 
             Description = p.Description, 
             MasterCategory = c.ToList().Select(x=> 
                                      new MasterCategoryModel 
                                      {
                                         CategoryId= x.CategoryId,
                                         Name = x.Name,
                                         Description = x.Description
                                      }).ToList()
            };

Answer 3

阅读后有答案this post .

这一点有效。

var query = from p in db.Categories
                            where p.ParentCategoryId != null
                            select new CategoryModel()
                            {
                                CategoryId = p.CategoryId,
                                Name = p.Name,
                                Description = p.Description,
                                MasterCategory = new List<MasterCategoryModel>((from c in db.Categories where c.CategoryId == p.ParentCategoryId select new MasterCategoryModel { CategoryId = c.CategoryId, Name = c.Name, Description = c.Description}).AsEnumerable().ToList())
                            };

非常感谢大家，特别是@Ehsan Sajjad 和@VMAtm。

无法将类型 'int?' 隐式转换为 'System.Collections.Generic.List<Model>

Cannot implicitly convert type 'int?' to 'System.Collections.Generic.List<Model>

linq

asp.net-mvc