entity-framework - 延迟加载异常( Entity Framework )

Question

我在我的项目中使用 Entity Framework 。这个问题是众所周知的，但假设的解决方案(例如 this 和 this )对我来说不起作用。

/// <summary>
/// Returns complete list of lecturers from DB.
/// </summary>
public IEnumerable<Lecturer> GetAllLecturers()
{
    IList<Lecturer> query;
    using (var dbb = new AcademicTimetableDbContext())
    {
        query = (from b in dbb.Lecturers select b).ToList();
    }
    Debug.WriteLine(query[0].AcademicDegree); // Exception (***)
    return query;
}

异常(***):

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

public class Lecturer
{
    public Lecturer()
    {
        this.Timetables = new List<Timetable>();
        this.Courses = new List<Course>();
    }

    public int Id_Lecturer { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public string Phone_Number { get; set; }
    public Nullable<int> Academic_Degree_Id { get; set; }
    public virtual AcademicDegree AcademicDegree { get; set; }
    public virtual ICollection<Timetable> Timetables { get; set; }
    public virtual ICollection<Course> Courses { get; set; }
}

出了什么问题？

Answer 1

延迟加载将一直有效，直到您的 DbContext 存活为止。

通过 using，您可以处置 DbContext，因此当您尝试访问 using block 外部的导航属性时，EF 将引发异常。

您可以通过将 Debug.WriteLine 移动到 using block 内来测试这一点，这样它就不会抛出异常:

using (var dbb = new AcademicTimetableDbContext())
{
    query = (from b in dbb.Lecturers select b).ToList();
    Debug.WriteLine(query[0].AcademicDegree);
}

解决方案是告诉 EF eagerly load the navigation properties使用 using Inlude 方法:

using (var dbb = new AcademicTimetableDbContext())
{
    query = (from b in dbb.Lecturers.Include(l => l.AcademicDegree) select b)
      .ToList();

}
Debug.WriteLine(query[0].AcademicDegree);