Adding to a list inside a class

Question

I have this class

public class User
{
   private string _name;
   public string UserName { get; set; }       
   public List<int> ControlNumber { get; set; }
   public User (string username)
   {
       _name = username;
   }
   public User()
   {
   }

}

And this function that fills the class.

User UserClass = new User(e.Key); //e.Key = user's name

public static void FillUserListClass(DataTable dt, ref UserClass)

{
    try
    {

        for (int ctr = 0; ctr < dt.Rows.Count; ctr++)
        {
            var row = dt.Rows[ctr];
            var userName = row["User"].ToString();

            if (UserList.ContainsKey(userName))
            {
                UserList[userName].Add(Convert.ToInt32(row["ControlNumber"]));
            }
            else
            {
                _user.UserName = row["User"].ToString();
                _user.ControlNumber.Add(Convert.ToInt32(row["ControlNumber"]));

            }
              //print out
            Console.WriteLine("UserList Class: ");
            Console.WriteLine(_user.UserName);
            Console.WriteLine(_user.ControlNumber);


        }
    }
    finally
    {
        //conn.Close();
    }

}

However, at the _user.ControlNumber.Add(Convert.ToInt32(row["ControlNumber"])); line I get the "Object reference not set to an instance of an object." error. Any help?

What i'm trying to do is create a class for each user that contains the user's control numbers. I then want all of these classes to be placed into a dictionary for reference.

Solution 1

You need to initialize your list in the constructor:

   public User (string username) : this()
   {
       _name = username;
   }
   public User()
   {
      this.ControlNumber = new List<int>();
   }

Otherwise, ControlNumber will have its default value of null.

Solution 2

Make sure you initialize your lists before you use them!

Corrected User-Class:

public class User
{
   private string _name;
   public string UserName { get; set; }       
   public List<int> ControlNumber { get; set; }
   public User (string username) : this()
   {
       _name = username;
   }
   public User()
   {
       ControlNumber = new List<int>();
   }

}

And also, before converting row["ControlNumber"] to an integer, you should make sure it is not null.

if (row["ControlNumber"] != null)
{
user.ControlNumber.Add(Convert.ToInt32(row["ControlNumber"]));
}

Solution 3

You have not initialised the ControlNumber, you can do this in the constructor like

public User (string username)
{
    _name = username;
    ControlNumber = new List<int>();
}

or just before you attempt to Add

_user.ControlNumber = new List<int>();
_user.ControlNumber.Add(Convert.ToInt32(row["ControlNumber"]));

I hope this helps.

Solution 4

I hate setting properties in constructor, so what I do is:

private List<int> _controlNumber;
public List<int> ControlNumber { 
    get { 
        if (_controlNumber == null)
            _controlNumber=new List<int>(); 

        return _controlNumber; 
    }

    set { _controlNumber = value } 
}

or simply

private List<int> _controlNumber = new List<int>();
public List<int> ControlNumber { 
    get { return _controlNumber; } 
    set { _controlNumber = value } 
}

Solution 5

By default, your ControlNumber property is set to null.

In your constructor, do the following to initialize it to an empty list:

this.ControlNumber = new List<int>();

View more solutions