public DetailCollection Details {get; set;}

class MyPage : System.Web.UI.Page
{
    private Product objProduct;
    private Product objProductNew;
    private void SaveDetails()
    {
         foreach (Dynamicweb.eCommerce.Products.Detail det in objProduct.Details)
         {
             // create new detail and add to product
             det.Save("", objProductNew);
         }
    }
}