add ตัวเลือกใน Combobox สามารถกำหนด displaymember และ value member ให้มันเป็นค่าว่าง

            DataTable dt = new DataTable();
            dt.Columns.Add("CategoryID");
            dt.Columns.Add("CategoryName");
            DataRow dr = null;
            dr = dt.NewRow();
            dr["CategoryID"] = "";
            dr["CategoryName"] = "ไม่มีประเภทสินค้า";
            dt.Rows.Add(dr);

            conn = new SqlConnection(strConnect);
            conn.Open();

            SqlDataAdapter da1 = new SqlDataAdapter("SELECT * FROM Categories", conn);
            da1.Fill(dt);
            cboCategoryID.DataSource = dt;//กำหนด DataSet
            cboCategoryID.DisplayMember = "CategoryName";
            cboCategoryID.ValueMember = "CategoryID";
            conn.Close();

                string ProductName = txtProductName.Text.Trim();
                string CategoryID = Convert.ToString(cboCategoryID.SelectedValue);
                //string SupplierID = Convert.ToString(cboSuppliers.SelectedValue);
                string QuantityPerUnit = txtQuantity.Text.Trim();
                string UnitPrice = txtUnitPrice.Text.Trim();
                string UnitsInStock = txtUnitsInStock.Text.Trim();
                string UnitsOnOrder = txtUnitsOnOrder.Text.Trim();
                string ReOrderLevel = txtReOrderLevel.Text.Trim();
                string sqlInsertProducts = "Insert into Products(ProductName,CategoryID,Discontinued)"  
                                           + " values(@ProductName,@CategoryID,@Discontinued)";
                SqlCommand comm = new SqlCommand(sqlInsertProducts, conn);
                comm.Parameters.Add("@ProductName", SqlDbType.VarChar).Value = ProductName;
                comm.Parameters.AddWithValue("@CategoryID", SqlDbType.VarChar).Value = CategoryID;
                //comm.Parameters.Add("@SupplierID", SqlDbType.VarChar).Value = SupplierID;
                comm.Parameters.Add("@Discontinued", SqlDbType.VarChar).Value = discontinued;
                comm.ExecuteNonQuery();
                MessageBox.Show("เพิ่มข้อมูลแล้ว");
                ShowData("Select * From Products");
                ClearForm();