private void buildDropDownList()
    {
        Service buyAkolWS = new Service();
        DataTable dt_typesOFProducts = buyAkolWS.getTypeOfProducts();
        ddl_typesOfProducts.DataSource = dt_typesOFProducts;
        ddl_typesOfProducts.DataBind();
    }

    protected void ddl_typesOfProducts_SelectedIndexChanged(object sender, EventArgs e)
    {
     פה אני לא יודע מה לכתוב.
       
    }