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 ?!
}

Answer 1

You have to actually create List instance

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

In the first line.