//krijg alle producten uit DB
 public List<Product> GetAll()
 {
     List<Product> producten = new List<Product>();
     using (OracleConnection conn = Database.Instance.GetConnection())
     {
         conn.Open();
         string query = string.Format("Select * from product");
         using (OracleCommand cmd = new OracleCommand(query, conn))
         {
             using (OracleDataReader reader = cmd.ExecuteReader())
             {
                 while (reader.Read())
                 {
                     Product product = new Product(Convert.ToInt32(reader["ID"]),
                         reader["productnaam"].ToString(), Convert.ToDecimal(reader["prijs"]),
                         reader["sale"].ToString()
                         , Convert.ToInt32(reader["categorie_Id"]), Convert.ToInt32(reader["voorraad"]),
                         Convert.ToInt32(reader["Specificaties_ID"]));
                     producten.Add(product);
                 }
             }
         }
     }
     return producten;
 }
 public Retourneeritem(Product product, int factuurnummerId)
 {
     Product = product;
     Factuurnummer_ID = factuurnummerId;
 }