Combobox problem

[outlawbt]

Hello, i have a problem when filling a combobox by setting its datasource to a listArray that contains objects.

I make objects from a class like this:

class Teacher
    {
        public int id;
        public string firstName;
        public string lastName;

        public Teacher(int id, string firstName, string lastName)
        {
            this.id = id;
            this.firstName = firstName;
            this.lastName = lastName;
        }
    }

Then I have an other class where i have a function that returns me an arrayList with objects from mssql database.

class SqlCommands
    {
        public SqlConnection conn;

        public SqlCommands()
        {
            conn = new SqlConnection();
            conn.ConnectionString = @"Data Source=.\SQLExpress;Integrated Security=true; AttachDbFilename=|DataDirectory|\baza.mdf;User Instance=true";   
        }

        public ArrayList getAllTeachers()
        {
            conn.Open();

            SqlDataReader reader = null;
            SqlCommand command = new SqlCommand("SELECT id, firstName, lastName FROM teachers", conn);
            reader = command.ExecuteReader();

            ArrayList objArray = new ArrayList();

            while (reader.Read())
            {
                objArray.Add(new Teacher(Convert.toInt32(reader["id"]), reader["firstName"].ToString(), reader["lastName"].ToString()));
            }

            return objArray;
        }
    }

In the main class I tried to populate the combobox like this:

SqlCommands sql = new SqlCommands();

            ArrayList list = sql.getAllTeachers();

            cmbTeacher.DataSource = list;
            cmbTeacher.DisplayMember = "firstName";
            cmbTeacher.ValueMember = "id";

Then i get an error like in the attachment below.

Do anyone knows how to solve this?

Thanks in advance

[Magos]

Perhaps id has to be a string?