I have a project and user Model that inherits the default identity class.

These two share a many to many relationship.

    public class Project
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }

        public ICollection<AppUser> users { get; set; }
    }

    public class AppUser : IdentityUser
    {
        public string DisplayName { get; set; }
        public ICollection<Project> projects { get; set; }  
    }

In my projects controller I display all the projects that belong to the current user; (I check for the user with the user id)

        [Authorize]
        public IActionResult Index()
        {
            var userId = this.User.FindFirstValue(ClaimTypes.NameIdentifier);

            IEnumerable<Project> objProjectList = _unitOfWork.Project.Where(userId);
            return View(objProjectList);
        }

and on the post method I'm trying to add the current user to the newly created project object to establish the relation between these two i.e. (this user is a member of this project) I've used dependency injection to inject usermanager that uses my custom user class (Appuser, which inherits the default scaffolded identity)

        [HttpPost]
        [ValidateAntiForgeryToken]
        [Authorize]
        public async Task<IActionResult> CreateAsync(Project obj)
        {
            if (ModelState.IsValid)
            {
                AppUser user = await _userManager.GetUserAsync(User);
                obj.users.Add(user);
                _unitOfWork.Project.Add(obj);
                _unitOfWork.Project.Save();
                return RedirectToAction("Index");
            }
            else
            {
                return View(obj);
            }

        }

However I'm getting the following error;

System.NullReferenceException: 'Object reference not set to an instance of an object.'

What exactly am I doing wrong? Should I be adding the user some other way?

I'd appreciate any input, thanks.

CodePudding user response：

In your model design:

public ICollection<AppUser> users { get; set; }

ICollection is an interface, you can't use it to add something direcetly, Or you will get NullReferenceException. You can create an instance, and then use it to add what you want.

AppUser user = await _userManager.GetUserAsync(User);

//create an instance.
List<AppUser> users = new List<AppUser>();

users.Add(user);

obj.users = users;
_unitOfWork.Project.Add(obj);
_unitOfWork.Project.Save();
return RedirectToAction("Index");