MVC CodeFirst 计算属性出现错误 "ObjectContext instance disposed"

Question

我有一个 ASP.NET MVC Code First 站点。我有一个模型 Teacher，它有一个 Student 的列表，每个列表都有一个计算的属性 CompletedTests。尝试从包含 Students 列表的 ViewModel 中访问计算的属性 CompletedTests 时出现以下错误：

An exception of type 'System.ObjectDisposedException' occurred in EntityFramework.dll but was not handled in user code

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

老师：

public class Teacher
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Student> Students{ get; set; }
}

学生：

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int TeacherId { get; set; }

    public virtual Teacher Teacher { get; set; }

    public int TotalTests
    {
        get
        {
            return Tests.Count;
        }
    }

    public int CompletedTests
    {
        get
        {
            return Tests.Count(p => p.IsCompleted);
        }
    }

    public virtual ICollection<Test> Tests{ get; set; }
}

测试：

public class Test
{
    public int Id { get; set; }

    public int StudentId { get; set; }

    public int QuestionsTotal
    {
        get
        {
            return Questions.Count;
        }
    }

    public bool IsCompleted
    {
        get
        {
            return Questions.Count(q => q.Completed) >= QuestionsTotal;
        }
    }

    public virtual Student Student { get; set; }
    public virtual ICollection<Question> Questions{ get; set; }
}

问题：

public class Question
{
    public int Id { get; set; }
    public int TestId { get; set; }

    public virtual Question Question { get; set; }
}

HomeViewModel:

public class HomeViewModel
{
    public string TeacherName { get; set; }
    public int StudentCount { get; set; }
    public IEnumerable<Student> Students { get; set; }
}

家庭控制器：

public ActionResult Index()
{
    var model = new HomeViewModel();
    var userId = User.Identity.GetUserId();
    using (var context = new ApplicationDbContext())
    {
        var teacher = context.Teachers.FirstOrDefault(c => c.IdentityId == userId);
        if (teacher != null)
        {
            model.TeacherName = teacher.Name;
            model.Students = teacher.Students.ToList();
        }
        return View(model);
    }
}

Index.cshtml 节：

        <tbody>
            @foreach (var student in Model.Students)
            {
                <tr>
                    <td>
                        @student.Name
                    </td>
                    <td>
                        @(student.CompletedTests + "/" + student.TotalTests)
                    </td>
                </tr>
            }
        </tbody>

有人可以指出为什么我在 student.CompletedTests 部分收到处置实例错误吗？

Answer 1

在加载 Student 实体时使用 Include 来包含 Tests 实体以避免延迟加载：

model.Students = teacher.Students.Include(t => t.Tests).ToList();

这称为预先加载：

https://msdn.microsoft.com/en-us/data/jj574232.aspx

您收到的错误是因为 ObjectContext (ApplicationDbContext) 在您的视图中不再可用。它已经在控制器的方法中进行了处理。

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