在实体框架中为多对多关系插入新项目时出现 System.NullReferenceException

Question

为多对多关系添加项目时，我收到此错误

gEchoLu.dll 中出现“System.NullReferenceException”类型的异常，但未在用户代码中处理

没有更多细节。

下面是抛出异常的代码：

protected void btn_AddMembers_Click(object sender, EventArgs e)
{
        gEchoLuDBContext _db = new gEchoLuDBContext();
        int wallId = Convert.ToInt32(grd_Walls.SelectedDataKey.Value);

        DiscussionWall selectedWall = _db.DiscussionWalls.Single(dw => dw.WallId == wallId);

        foreach (ListItem item in cbList_AllMembers.Items)
        {
            if (item.Selected)//if the checkbox item (i.e., user) is selected
            {
                Person selectedPerson = _db.People.Single(p => p.Id == item.Value.ToString());
//then retrieve the user from db

                selectedWall.Members.Add(selectedPerson);//**this line throws the error**
//add user to the selected wall
            }
        }

        _db.SaveChanges();

        BindWallMembers();
}

在上面的代码中，当我调试它时，我看到没有一个实例（selectedWall 和selectedPerson）为空。以下是相关类的相关部分以及它们之间的关系：

public class DiscussionWall
{
    [Key]
    public int WallId { get; set; }

    public List<Person> Members { get; set; }
}

[Table("AspNetUsers")]
public class Person : IdentityUser
{
    public List<Course> RegisteredCourses { get; set; }
    public List<DiscussionWall> AttendedDiscussionWalls { get; set; }
}

流利的api代码：

modelBuilder.Entity<DiscussionWall>()
            .HasMany(dw => dw.Members)
            .WithMany(p => p.AttendedDiscussionWalls)
            .Map(m => m.MapLeftKey("WallId").MapRightKey("Id")
            .ToTable("EnrollmentsToDiscussionWalls"));

有人知道我的逻辑和代码有什么问题吗？

Answer 1

看起来DiscussionWall.Members 为空。它没有被您的代码初始化为任何东西。试试：

private List<Person> _members;
public List<Person> Members
{
  get { return _members ?? (_members = new List<Person>()); }
  set { _members = value; }
}

另请参阅：Why is my Entity Framework Code First proxy collection null and why can't I set it?