How do you add a table list to a ViewModel in .net core

Question

I have a User model.

public class User
{
    public int Id { get; set; }
    [Required]
    public string FirstName { get; set; }
    [Required]
    public string LastName { get; set; }
    [Required]
    public int UserGroupId { get; set; }
    public UserGroup UserGroup { get; set; }
}

This is is my UserGroup model that would contain user groups like Admin, System Admin, etc.

public class UserGroup
{
    public int Id {get; set;}
    public bool Admin {get; set;}
    public bool SystemAdmin {get; set}
    public List<User> Users {get; set;}
}

I just started .net core so I hope my relationship is correct. What I would like is to have a User Create View that had a html selection that list all the groups. Something like this but being dynamic.

<select class="form-control m-b" asp-for="Role" required>
    <option value=""></option> 
    <option value="Admin">Admin</option>
    <option value="SystemAdmin">System Admin</option>
</select>

This is the UserGroupViewModel that I'm send to the User Create Form.

public class UserCreateViewModel
{
    [Required]
    public string FirstName { get; set; }
    [Required]
    public string LastName { get; set; }
    [Required]
    public List<UserGroup> UserGroups {get; set;}
}

Finally on my UserController I'm going to be looping the List of UserGroups that I got from my database and I'll add it to the UserCreateViewModel.

public IActionResult Create()
    {
        UserCreateViewModel userCreateViewModel = new UserCreateViewModel();

        var userGroups = db.UserGroups.ToList();

        foreach(var group in userGroups)
        {
            userCreateViewModel.UserGroups.Add(group);
        }


        return View(userCreateViewModel);
    }

The problem is that I'm getting an error that I'm not sure how to deal with it.

What does this error means and How can I solve it. I'm also open for a different approach from experience developers.

Possible duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Tseng, Aug 18 '17 at 19:37

score 2 · Answer 1 · answered Aug 18 '17 at 18:45

You are storing groups in a local variable but you need to set the UserGroups property on the instance of UserCreateViewModel which you created at start, so just change the following:

var userGroups = db.UserGroups.ToList();

to :

userCreateViewModel.UserGroups = db.UserGroups.ToList();

and you can skip the loop that you are doing down there, because we have already set the UserGroups collection with the result which was needed.

Hope it helps!

Thanks I love the new solution is alot cleaner. – Millenial2020 Aug 18 '17 at 19:10 — Millenial2020, Aug 18 '17 at 19:10

score 2 · Accepted Answer · answered Aug 18 '17 at 18:55

Your code is crashing with a null reference exception because you are trying to call the Add method on NULL . When you create UserCreateViewModel object, you are not initializing the UserGroups property to an empty list and you do not have a constructor which does the same. So basically UseGroups property is null.

The solution is to initialize the UserGroups property before calling Add method on that.

var userCreateViewModel = new UserCreateViewModel();
userCreateViewModel.UserGroups =new List<UserGroup>(); // Initialize to empty list

var userGroups = db.UserGroups.ToList();

foreach(var group in userGroups)
{
      userCreateViewModel.UserGroups.Add(group);
}

Or yo can directly set the collection to that property.

var userCreateViewModel = new UserCreateViewModel 
{
   UserGroups= db.UserGroups.ToList()
};

Thanks that makes sense. – Millenial2020 Aug 18 '17 at 19:10 — Millenial2020, Aug 18 '17 at 19:10

DOMZE · Answer 3 · 2017-08-18T19:50:11.483

0

userCreateViewModel.UserGroups = userGroups;

public IActionResult Create()
{
    UserCreateViewModel userCreateViewModel = new UserCreateViewModel();

    var userGroups = db.UserGroups.ToList();

    userCreateViewModel.UserGroups = userGroups;

    return View(userCreateViewModel);
}

edited Aug 18 '17 at 19:50

answered Aug 18 '17 at 18:46

DOMZE

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indeed! was too quick. Fixed! ;-) – DOMZE Aug 18 '17 at 19:50

How do you add a table list to a ViewModel in .net core

3 Answers3