Error when adding element to list of objects

Question

I wish to dynamically create and save objects to a list. When program reachs the add method to add objects in the list I get an error:

Object reference not set to an instance of an object.

What have I done wrong here?

List<Category> categoryList;

public Main(string firstname, string lastname, string status)
{
    InitializeComponent();
    label1.Text = (firstname + lastname + status).Trim();

    string connection = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\Trgovina.mdf;Integrated Security=True";
    SqlConnection cn = new SqlConnection(connection);

    try
    {
        cn.Open();
    }
    catch (Exception) { MessageBox.Show("Error occurred during database communication!"); }

    string sqlQuery = "SELECT * FROM Kategorije_art";
    SqlCommand categoryCommand = new SqlCommand(sqlQuery, cn);
    SqlDataReader categoryDataRead = categoryCommand.ExecuteReader();

    categoryList.Add(new Category(1, "a")); //ERROR ?!
}

Well where do you initialize `categoryList`? I can't see anything assigning it a value. — Jon Skeet, Feb 05 '13 at 16:52
See http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net/4660186#4660186 — JG in SD, Feb 05 '13 at 16:57

score 7 · Accepted Answer · answered Feb 05 '13 at 16:52

7

You have to actually create List instance

List<Category> categoryList= new List<Category>();

In the first line.

answered Feb 05 '13 at 16:52

Anri

5,947
3
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score 4 · Answer 2 · answered Feb 05 '13 at 16:54

Your member, categoryList, is not initialised.

Try:

List<Category> categoryList = new List<Category>();

public Main(string firstname, string lastname, string status)
{
    InitializeComponent();
    label1.Text = (firstname+lastname+status).Trim();

    string connection = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\Trgovina.mdf;Integrated Security=True";
    SqlConnection cn = new SqlConnection(connection);

    try 
    {
        cn.Open();
    }
    catch (Exception) { MessageBox.Show("Error occurred during database communication!"); }

    string sqlQuery = "SELECT * FROM Kategorije_art";
    SqlCommand categoryCommand = new SqlCommand(sqlQuery, cn);
    SqlDataReader categoryDataRead = categoryCommand.ExecuteReader();

    categoryList.Add(new Category(1, "a")); //ERROR ?!
}

Also note you should probably have your SqlConnection in a using block, or at least call Close/Dispose on it so you aren't leaking connections.

I'd be interested to know who voted this down, and why...seems legit to me! — Matthew Layton, Feb 05 '13 at 16:56

score 2 · Answer 3 · answered Feb 05 '13 at 16:52

2

Your categoryList is not initialized. Change your first line to:

List<Category> categoryList = new List<Category>();

The null reference exception is coming when you try to access the .Add(...) method on a null value.

answered Feb 05 '13 at 16:52

Aren

50,600
8
63
100

score 1 · Answer 4 · answered Feb 05 '13 at 16:53

1

Your variable named categoryList is not initialized. So it is null.

You can't call any method on a unitialized variable.

Try :

List<Category> categoryList= new List<Category>();

answered Feb 05 '13 at 16:53

xlecoustillier

15,451
14
56
79

score 1 · Answer 5 · answered Feb 05 '13 at 16:53

1

do you create categoryList = new List<Category>()?

answered Feb 05 '13 at 16:53

Aniket Inge

24,028
4
42
74

score 1 · Answer 6 · answered Feb 05 '13 at 17:13

1

Your error line should look like this:

categoryList = new List<Category>() { new Category(1,"a")};

so You create list and add one created element and store reference to this list in categoryList

answered Feb 05 '13 at 17:13

rumburak

1,051
9
19

Error when adding element to list of objects

6 Answers6