将项添加到实体的集合属性中

本文关键字：集合 属性 实体 添加 | 更新日期: 2023-09-27 18:07:03

我正在Visual Studio Ultimate 2013中的MVC 5项目上工作。

我需要处理一个学生ApplicationUser在注册到系统后应该加入学习小组的情况，如果他填写了一个注册码(仅用于加入学习小组)，他已经提供了。

问题是，当我试图添加一个项目到StudyGroup属性ICollection<ApplicationUser> Members我得到

系统。NullReferenceException:对象引用没有设置为对象的实例。

注册行动: 

public async Task<ActionResult> Register(RegisterViewModel model)
    {
        UserManager.UserValidator = new UserValidator<ApplicationUser>(UserManager) { AllowOnlyAlphanumericUserNames = false };
        if (ModelState.IsValid)
        {
            var user = new ApplicationUser()
            { 
                UserName = model.UserName,
                Name = model.Name,
                Surname = model.Surname
            };
            var result = await UserManager.CreateAsync(user, model.Password);
            if (result.Succeeded)
            {
                await SignInAsync(user, isPersistent: false);
                UserManager.AddToRole(user.Id, "Student");
                var group =
                    (from g in db.StudyGroups
                     where g.RegistrationCode == model.GroupCode
                     select g).FirstOrDefault();
                if (group != null)
                {
                    user.StudyGroup.Add(group);
                    group.Members.Add(user);
                }
                return RedirectToAction("Index", "Home");
            }
            else
            {
                AddErrors(result);
            }
        }
        // If we got this far, something failed, redisplay form
        return View(model);
    }

ApplicationUser: 

public class ApplicationUser : IdentityUser
{
    public string Name { get; set; }
    public string Surname { get; set; }
    public virtual ICollection<StudyGroup> StudyGroup { get; set; }
    public virtual ICollection<Test> Tests { get; set; }
}

StudyGroup: 

public class StudyGroup
{
    [Key]
    public int StudyGroupId { get; set; }
    [Required(ErrorMessage = "Group Name is required.")]
    [Display(Name = "Group Name")]
    public string Name { get; set; }
    [Display(Name = "Registration Code", Description = "The code which students will use to join this study group.")]
    public string RegistrationCode { get; set; }
    public virtual ICollection<ApplicationUser> Members { get; set; }
    public virtual ICollection<Template> Templates { get; set; }
}

最后是我的Model Builder for StudyGroup:

modelBuilder.Entity<StudyGroup>()
            .HasMany(s => s.Members)
            .WithMany(m => m.StudyGroup);

将这些项目添加到实体属性的正确方法应该是什么，为什么我的方法不起作用，请问?很多谢谢!

将项添加到实体的集合属性中

看起来你没有在你的用户上初始化ICollection<StudyGroup> StudyGroup属性?

尝试添加一个初始化属性的默认构造函数:

public class ApplicationUser : IdentityUser
{
    public ApplicationUser() 
    {
        this.StudyGroup = new List<StudyGroup>();
        this.Tests = new List<Test>();
    }
    public string Name { get; set; }
    public string Surname { get; set; }
    public virtual ICollection<StudyGroup> StudyGroup { get; set; }
    public virtual ICollection<Test> Tests { get; set; }
}

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