无法使用 linq lambda 表达式强制转换类型为“System.Data.Entity.Infrastructure.DbQuery`1[]”的对象

Question

我是 mvc 的新手并试图了解我做错了什么我创建了一个 Dbcontaxt 类

 public class DataBaseContext : DbContext
{
    public DataBaseContext()
        : base("DefaultConnection")
    {
    }
    public DbSet<Membership> Membership { get; set; }
    public DbSet<OAuthMembership> OAuthMembership { get; set; }
    public DbSet<Role> Roles { get; set; }
    public DbSet<UserProfile> UserProfiles { get; set; }
    public DbSet<Category> Categorys { get; set; }
    public DbSet<SubCategory> SubCategorys { get; set; }
    public DbSet<Product> Products { get; set; }
    public DbSet<Color> Colors { get; set; }
    public DbSet<Size> Sizes { get; set; }
    public DbSet<Company> Companys { get; set; }
    public DbSet<UsersInRoles> UsersInRoles { get; set; }
  }
}

我创建了一个模型类来创建一个强类型视图

    [Bind(Exclude = "AddUserToRoleID")]
    public class AddUserToRole
{
    [ScaffoldColumn(false)]
    public int AddUserToRoleID { get; set; }
    [Required]
    [Display(Name = "User name")]
    public string UserName { get; set; }
    [Required]
    [Display(Name = "Role name")]
    public string RoleName { get; set; }

}
}

在控制器中，我尝试通过添加视图来创建详细信息视图并选择AddUserToRole 作为强类型视图的模型

    public ActionResult Details(int id = 0)
    {

        var UserInRole = db.UserProfiles
        .Join(db.UsersInRoles,
              u => u.UserId,
              uir => uir.UserId,
              (u, uir) => new {u = u,uir = uir})
        .Join(db.Roles,
              temp0 => temp0.uir.RoleId,
              r => r.RoleId,
              (temp0, r) => new { temp0 = temp0,r = r })
        .Where(temp1 => (temp1.temp0.u.UserId == id))
        .Select(temp1 => new AddUserToRole {
            AddUserToRoleID = temp1.temp0.u.UserId,
            UserName = temp1.temp0.u.UserName,
            RoleName = temp1.r.RoleName
        });            

        return View(UserInRole);
    }

它给了我这个错误

The model item passed into the dictionary is of type
'System.Data.Entity.Infrastructure.DbQuery`1[SeniorProject.Models.AddUserToRole]', but
this dictionary requires a model item of type 'SeniorProject.Models.AddUserToRole'.

当我投射 return View((UsersInRoles)UserInRole); 时，它给了我这个错误

Unable to cast object of type
'System.Data.Entity.Infrastructure.DbQuery`1[SeniorProject.Models.AddUserToRole]' 
 to type'SeniorProject.Models.UsersInRoles'.

和视图

@model SeniorProject.Models.AddUserToRole

@{
ViewBag.Title = "Details";
}

<h2>Details</h2>

<fieldset>
    <legend>AddUserToRole</legend>

<div class="display-label">
     @Html.DisplayNameFor(model => model.UserName)
</div>
<div class="display-field">
    @Html.DisplayFor(model => model.UserName)
</div>

<div class="display-label">
     @Html.DisplayNameFor(model => model.RoleName)
</div>
<div class="display-field">
    @Html.DisplayFor(model => model.RoleName)
</div>
</fieldset>
<p>
@Html.ActionLink("Edit", "Edit", new { id=Model.AddUserToRoleID }) |
@Html.ActionLink("Back to List", "Index")
</p>

在这种情况下我该怎么办？

Answer 1

你需要实现它。将 FirstOrDefault() 放在查询的末尾

var materializedUser = UserInRole.SingleOrDefault();
return View(materializedUser);

编辑：以下 pjotr 评论将 FirstOrDefault() 替换为 SingleOrDefault()