C＃：如何用从PostgreSQL表获取的数据填充ListView？

Question

I need some help with C# here... I'm trying to learn how to use Forms with Data taken from a PostgreSQL database. That's why my aim is to take some data from a table and insert this data into a ListView. My problem is: as soon I start the Form containing the ListView I get an exception, precisely System.InvalidOperationException. The connection is not Open System.InvalidOperationException. The connection is not Open . I have no idea why,

Here some code:

private void FormLoad(object sender, EventArgs e) {
            list.View = View.Details;
            list.GridLines = true;
            list.FullRowSelect = true;
            string connection = "Server=localhost;User Id=artemius;Password=prophet;" +
                "Database=Exams;";
            lista.Columns.Add("Course", 100);
            lista.Columns.Add("CFU", 70);
            lista.Columns.Add("Difficulty", 70);
            ListViewItem item;
            NpgsqlConnection conn;
            try {
                conn = new NpgsqlConnection(connection);
                conn.Open(); //Isn't it opened here?
                string sql = "select coursename, cfu, difficulty from courses where passed = false;";
                string[] courses= new string[3];
                NpgsqlCommand command = new NpgsqlCommand(sql);
                /*--On Row Below I get System.InvalidOperationException--*/
                NpgsqlDataReader dr = command.ExecuteReader();
                while (dr.Read()) {
                    courses[0] = dr[0].ToString();
                    courses[1] = dr[1].ToString();
                    courses[2] = dr[2].ToString();
                    item = new ListViewItem(corso);
                    lista.Items.Add(item);
                }
                conn.Close();
            }
            catch(NpgsqlException ecc) {
                Console.WriteLine(ecc.BaseMessage);
            }
        }

Do you guys have any idea how to solve the issue? Thank you

Answer 1

You haven't associated the connection and the command

Replace

NpgsqlCommand command = new NpgsqlCommand(sql);

by

NpgsqlCommand command = new NpgsqlCommand(sql,conn);