ComboBox อยากให้โชว์ คําว่าเลือก ก่อนเป็นรายการแรกครับ

store_cat_cbx.DataSource = dt;
                store_cat_cbx.DisplayMember = "name_th";
                store_cat_cbx.ValueMember = "id_store_cat";
                store_cat_cbx.SelectedItem = null; // ใช้ตัวนี้ครับ

store_cat_cbx.SelectedItem == null

private void store_create_Load(object sender, EventArgs e)
        {
            adap = new MySqlDataAdapter("SELECT * FROM mts_store_cat", con);
            DataTable dt = new DataTable();
            adap.Fill(dt);

            if (dt.Rows.Count > 0)
            {
                store_cat_cbx.DataSource = dt;
                store_cat_cbx.DisplayMember = "name_th";
                store_cat_cbx.ValueMember = "id_store_cat";
       store_cat_cbx.item.add("เลือก");
            }
        }

adap = new MySqlDataAdapter("SELECT * FROM mts_store WHERE id_store = '" + id + "'", con);
            DataTable dt = new DataTable();
            adap.Fill(dt);
            adap = new MySqlDataAdapter("SELECT * FROM mts_store_cat", con);
            DataTable cbx = new DataTable();
            adap.Fill(cbx);

            

            if (dt.Rows.Count > 0)
            {
                id_store.Text = dt.Rows[0]["id_store"].ToString();
                store_code.Text = dt.Rows[0]["store_code"].ToString();
                store_cat_cbx.DataSource = dt;
                store_cat_cbx.DisplayMember = "name_th";
                store_cat_cbx.ValueMember = "id_store_cat";
                store_cat_cbx.Items.Insert(0, new ListItem("--Select--", "0"));
                name_th.Text = dt.Rows[0]["name_th"].ToString();
                name_en.Text = dt.Rows[0]["name_en"].ToString();
                comment.Text = dt.Rows[0]["comment"].ToString();
                if (dt.Rows[0]["status"].ToString() == "1")
                {
                    yes.Checked = true;
                }
                else
                {
                    no.Checked = true;
                }
            }