Combo box depends on another combo box

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C#

combobox

I am trying to make a combo box dependant on a selection from another combobox.
Each are connected to a mysql server which is three tables.
what i want a is when you choose type it fills brand depending on the type. and once you choose brand do the same and fill the model. this is the code i got online but i implemented to work with me but it dosent . Can it be the tables?

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
             MySqlConnection con = new MySqlConnection("datasource=localhost;port=3306;username=root;password=Deprisa!79849;");
            //---comboBox2,3,and 9--- 
            
            comboBox3.SelectedIndex = -1;
            //comboBox3.Items.Clear();
            if (con.State == ConnectionState.Closed)
            {
                con.Open();
            }
            if (comboBox2.SelectedValue != null)
            {
                DataView dv = comboBox2.DataSource as DataView;
                dv.RowFilter = "brand = " + comboBox2.SelectedValue.ToString();

                MySqlCommand cmd = new MySqlCommand("Select * from brand where idtype='" + comboBox2.SelectedItem + "'", con);
                MySqlDataReader rd = cmd.ExecuteReader();
                while (rd.Read())
                {
                    comboBox2.Items.Add(rd[0]);
                }
                rd.Close();
                cmd.Dispose();
            }
        }

        private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
        {
            MySqlConnection con = new MySqlConnection("datasource=localhost;port=3306;username=root;password=Deprisa!79849;");
            comboBox9.SelectedIndex = -1;
            //comboBox9.Items.Clear();

            if (con.State == ConnectionState.Closed)
            {
                con.Open();
            }

            MySqlCommand cmd = new MySqlCommand("Select distinct(idbrand) from model where idtype='" + comboBox2.SelectedItem + "'", con);

            MySqlDataReader rd = cmd.ExecuteReader();

            while (rd.Read())
            {
                comboBox9.Items.Add(rd[0]);
            }
            rd.Close();
            cmd.Dispose();
        }




</code>