C# a having a list inside a list of objects. Will not allow .add

Question

I keep getting this error.

An unhandled exception of type 'System.NullReferenceException' occurred in CustomerExcelreader.exe

Additional information:

Object reference not set to an instance of an object.

Which I don't know why, since I have a global object, and I know the object works, since when I use non-list values of the object, everything works just fine.

This is the actual object in question.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CustomerExcelreader
{
public class customers
{
    public customers()
    {

    }
    /// <summary>
    /// These work just fine.
    /// </summary>
    public String Guest { get; set; }
    public String Contact { get; set; }
    public String ClubNumber { get; set; }//GuestNumber
    public String CasioNumber { get; set; }

    /// <summary>
    /// Lists of lists
    /// </summary>
    public List<String> Dates { get; set; }
    public List<String> Description { get; set; }
    public List<Double> moneyIN { get; set; }
    public List<Double> moneyOUT { get; set; }
    public List<Double> Balance { get; set; }
    public List<String> Remarks { get; set; }

}
}

and here is where it messes up with any of the List values, but normal variables are fine.

private void addButton_Click(object sender, EventArgs e)
{
    //customers add_customerHistory;
    if (listNames.SelectedIndex >= 0)// && listNames.SelectedIndex <= Pathlist.Capacity)//somehow SelectedIndex could be less then 0 by not picking something.
    {
        //add_customerHistory = CustomerHistoryList[listNames.SelectedIndex];
        textBox1.Text = CustomerHistoryList[listNames.SelectedIndex].ClubNumber;
        CustomerHistoryList[listNames.SelectedIndex].Dates.Add(DateBox.Text);
        CustomerHistoryList[listNames.SelectedIndex].Description.Add(DescriptionBox.Text);
        CustomerHistoryList[listNames.SelectedIndex].moneyIN.Add(Convert.ToDouble(InBox.Text));
        CustomerHistoryList[listNames.SelectedIndex].moneyOUT.Add(Convert.ToDouble(OutBox.Text));
        CustomerHistoryList[listNames.SelectedIndex].Balance.Add(Convert.ToDouble(BalanceBox.Text));
        CustomerHistoryList[listNames.SelectedIndex].Remarks.Add(RemarkBox.Text);
        //CustomerHistoryList[listNames.SelectedIndex].CasioNumber = "Its been changed";
        richTextBox1.Text = CustomerHistoryList[listNames.SelectedIndex].CasioNumber;

    }
    else
    {
        CustomerHistoryList[0].Dates.Add(DateBox.Text);
        CustomerHistoryList[0].Description.Add(DescriptionBox.Text);
        CustomerHistoryList[0].moneyIN.Add(Convert.ToDouble(InBox.Text));
        CustomerHistoryList[0].moneyOUT.Add(Convert.ToDouble(OutBox.Text));
        CustomerHistoryList[0].Balance.Add(Convert.ToDouble(BalanceBox.Text));
        CustomerHistoryList[0].Remarks.Add(RemarkBox.Text);
    }

}

It seems odd that it doesn't work here. I guess it has something to do with lists inside of objects? I heard those can act funky.

possible duplicate of [List of objects. Unable to add object to list — user2864740, Feb 01 '14 at 21:58

score 4 · Accepted Answer · answered Feb 01 '14 at 21:27

4

You need to initialise your list<String> properties in your class constuctor.

public class customers
{
    public customers()
    {
       Dates = new List<String>();
       Description = new List<String>();
       moneyIN  = new List<Double>();
       moneyOUT = new List<Double>();       
       Balance =  new List<Double>();
       Remarks = new List<String>();
    }


    // Other bits (not copied)



}

answered Feb 01 '14 at 21:27

Chris Hammond

2,064
5
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Did not know you needed to do that. – lockgar Feb 01 '14 at 21:33
`List` is an object type and, by default, will initialise to NULL when you instantiate your class. Same with any object type – Chris Hammond Feb 01 '14 at 21:37
Did not know that ether. – lockgar Feb 01 '14 at 21:52

score 1 · Answer 2 · answered Feb 01 '14 at 21:27

You have to create a new instance of a class (List in your case), before accesing its members (Add method in your case).

For example:

CustomerHistoryList[listNames.SelectedIndex].Dates = new List<string>();
CustomerHistoryList[listNames.SelectedIndex].Dates.Add(DateBox.Text);

score 0 · Answer 3 · edited May 23 '17 at 10:25

0

As Chris Hammond pointed out, just because you use a property setter/getter does not mean that it will automatically create property values for you. This only happens with non nullable value types.

Either put the logic in your constructor or change them to private fields.

public class customers
{
    public customers()
    {
        this.Dates = new List<String>();
        this.Description = new List<String>();
        ...
    }

This makes it tricky for inheritance, so you can utilize private fields

public class customers
{
    private List<String> _dates = new List<String>();
    private List<String> _description = new List<String>();

    public List<String> Dates
    { 
        { return _dates; }
        { _dates = value; }
    }

    public List<String> Description
    { 
        { return _description; }
        { _description = value; }
    }

    ....

See this other post about exposing Generic Lists as Properties

Why is it considered bad to expose List<T>?

edited May 23 '17 at 10:25

Community

1
1

answered Feb 01 '14 at 21:42

Steve

1,995
2
16
25

Just curious... why does it _make it tricky for inheritance_? – Chris Hammond Feb 01 '14 at 21:47
I could never wrap my head around this.X or this.Y. – lockgar Feb 01 '14 at 21:52
Have a look at [this](http://msdn.microsoft.com/en-us/library/dk1507sz.aspx) then :) – Chris Hammond Feb 01 '14 at 22:02
The "tricky inheritance" suggestion was for Code Analysis rule CA2214 which pertains to overriding virtual properties see: http://msdn.microsoft.com/en-us/library/ms182331.aspx [and] http://stackoverflow.com/questions/3963219/code-analysis-warning-2214-how-best-to-fix – Steve Feb 02 '14 at 19:35

C# a having a list inside a list of objects. Will not allow .add

3 Answers3