急切加载的数据错误说上下文已经处理

Question

编辑 Driver class 根据要求：

public class Driver
{
    public int Id { get; set; }
    public ApplicationUser User { get; set; }
    public string DriverDirectoryTitle { get; set; }
    public string FullNames { get; set; }
    public string AlternatePhoneNumber { get; set; }
    public string IdNumber { get; set; }
    public DateTime DateOfBirth { get; set; }
    public DateTime RegistrationDate { get; set; }
    public Gender Gender { get; set; }
    public Ethnicity Ethnicity { get; set; }
    public bool Disability { get; set; }
    public string Address { get; set; }
    public City City { get; set; }
    public string Nationality { get; set; }
    public bool OwnVehicle { get; set; }
    public bool DriversLicense { get; set; }
    public Experience Experience { get; set; }
    public string Availability { get; set; }
    public Vehicle Vehicle { get; set; }
    public bool IsPremium { get; set; }
    public bool Deleted { get; set; }
}

问题

我的应用有点像 Euro Truck Simulator 的东西，它列出了工作的司机。

我正在 return 将 DTO 添加到我的页面（顺便说一句，这是 ASPX Web 窗体），它与 Driver 数据模型密切相关：

public class DriverViewModel
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public string FullNames { get; set; }
    public string DriverDirectoryTitle { get; set; }
    public string RegistrationDate { get; set; }
    public string AlternatePhoneNumber { get; set; }
    public string IdNumber { get; set; }
    public DateTime DateOfBirth { get; set; }
    public Ethnicity Ethnicity { get; set; }
    public Experience Experience { get; set; }
    public City City { get; set; }
    public Gender Gender { get; set; }
    public List<DriverLicensesViewModel> LicenseCodes { get; set; }
    public Vehicle Vehicle { get; set; }
    public bool Disability { get; set; }
    public string Address { get; set; }
    public string Nationality { get; set; }
    public bool OwnVehicle { get; set; }
    public bool DriversLicense { get; set; }
    public string Availability { get; set; }
    public bool IsPremium { get; set; }
    public bool ReferenceChecked { get; set; }
    public bool PoliceClearance { get; set; }
}

请注意，none 的关系属性定义为 virtual - 因此 none 的数据被延迟加载。

在 return 页面上的视图模型之前，我正在做一些工作，但特别是，只有一些关系属性的映射导致以下错误。

这是映射代码：

using (var db = new ApplicationDbContext())
{
    var drivers = db.Drivers
        .Include(x => x.User).Include(x => x.City)
        .Include(x => x.Ethnicity).Include(x => x.Experience)
        .Include(x => x.Gender).Include(x => x.Vehicle)
        .OrderBy(x => x.FullNames).ToList();

    var driverModels = new List<DriverViewModel>();
    foreach (var driver in drivers)
    {
        var driverModel = new DriverViewModel
        {
            Id = driver.Id,
            FullNames = driver.FullNames,
            DriverDirectoryTitle = driver.DriverDirectoryTitle,
            RegistrationDate = driver.RegistrationDate.ToString("ddd, dd MMM yyy"),
            AlternatePhoneNumber = driver.AlternatePhoneNumber,
            IdNumber = driver.IdNumber,
            DateOfBirth = driver.DateOfBirth,
            Ethnicity = driver.Ethnicity,
            Experience = driver.Experience,
            Gender = driver.Gender,
            Vehicle = driver.Vehicle,
            Disability = driver.Disability,
            Address = driver.Address,
            Nationality = driver.Nationality,
            OwnVehicle = driver.OwnVehicle,
            DriversLicense = driver.DriversLicense,
            Availability = driver.Availability,
            IsPremium = driver.IsPremium,
            LicenseCodes = new List<DriverLicensesViewModel>()
        };
    }
}

此处显示的代码工作正常，数据完美加载到页面上，但是，如果我将城市属性的映射添加到映射中，则会发生错误：

The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.

我已经检查了其他一些问题，例如下面链接的问题，但似乎都涉及延迟加载数据，而我显然没有这样做：

那么，为什么我在此处添加的城市关系会导致问题，而其他关系的 none 会导致问题，我该如何解决？

Answer 1

如果树中的任何实体包含 virtual 属性，它们将被 Entity Framework 延迟加载，而不仅仅是根实体。

似乎 City 确实包含 virtual 属性，因此一旦首次访问它们（即在您的映射中），它将导致对数据库的另一个 SQL 查询必要的数据。

如果你的上下文被释放，这将失败，所以解决方案是删除 virtual 关键字，或者使用 Select:

预先加载

db.Drivers
    .Include(x => x.City.Select(c => c.LazyProperty));

为了完整起见，对于 EF Core，这将是：

db.Drivers
    .Include(x => x.City)
        .ThenInclude(c => c.LazyProperty);

急切加载的数据错误说上下文已经处理

Eager loaded data errors saying context already disposed

c#

entity-framework-6