How to add a class to another class that has a one-to-many relationship

Question

I have a system where users can log in and create boards. For this, there is the two separate classes 'User' and 'Board'.

public class User
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string Username { get; set; }
    public string Password { get; set; }
    public string UserType { get; set; }
    public DateTime LastLoginDate { get; set; }
    public virtual ICollection<Board> Boards { get; set; }
    public virtual ICollection<Post> Posts { get; set; }

public class Board
{
    public int ID { get; set; }
    public string Name { get; set; }
    public DateTime DateCreated { get; set; }
    public virtual ICollection<Post> Posts { get; set; }

I have a webform to create a board and I'm getting a NullReferenceException whenever I try to run the site.

public partial class AddBoard : System.Web.UI.Page
{
    Board board = new Board();
    User user = new User();
    Utility utility = new Utility();

    static User loggedInUser;
    static Board boardToAdd;

    protected void CreateButton_Click(object sender, EventArgs e)
    {
        string name = NameTextBox.Text;
        loggedInUser = (User)Session["loggedInUser"];

        string checkName = utility.CheckBoardName(name);
        if (checkName == "OK")
        {
            boardToAdd.Name = name;
            boardToAdd.DateCreated = DateTime.Now;
            user.AddBoard(boardToAdd, loggedInUser);


        }
        else
        {
            CreateLabel.Text = checkName;
        }
    }
}

The exception occurs on this line:

boardToAdd.Name = name;

I am trying to call this method to add the board to the User class:

public User AddBoard(Board board, User user)
    {
        BulletinContext _context = new BulletinContext();
        User _user = _context.Users.Find(user.ID);

        _context.Boards.Add(board);
        _context.Users.Add(user);
            return null;

    }

Edit: That error has been solved, but I'm still unable to create a board that is attached to the user. For clarification, each board has a 'User_ID' that should be the ID of the user who created the board.

Edit 2:

This is the layout of the table for Board

It consists of an ID, a name, the date it was created and the ID of the user who created the board.

Answer 1

That occurs because of you didn't initialized boardToAdd object, it's initial value is null. Try creating instance of board.

static Board boardToAdd = new Board();

Answer 2

It looks like you haven't defined foreign keys. So you'll need to do something like.

public User AddBoard(Board board, User user)
{
    BulletinContext _context = new BulletinContext();
    board.User_ID = user.ID;
    _context.Boards.Add(board);
        return null;

}

EDIT: I can't see a _context.SaveChanges() in your code. So you may need to add that if it isn't there.

If you have foreign keys defined:

Change the User Class to create a new HashSet of Boards something like:

public User() 
{
     Boards = new HashSet<Board>();
}
public User AddBoard(Board board, User user)
{
    BulletinContext _context = new BulletinContext();
    User _user = _context.Users.Find(user.ID);
    _user.Boards.Add(board);
    return null;

}