[SerializableAttribute()] 
public class RoundingCollection : System.Collections.ObjectModel.Collection<Rounding> 

class MyPage : System.Web.UI.Page
{
    public Dynamicweb.eCommerce.Prices.RoundingCollection GetRoundings(string RoundingID)
    {
        string SQL = "SELECT * FROM EcomRoundings WHERE RoundingID = '" + RoundingID + "'";
        Dynamicweb.eCommerce.Prices.Rounding objRounding = default(Dynamicweb.eCommerce.Prices.Rounding);
        IDataReader objDataReader = Database.CreateDataReader(SQL, "Ecom.mdb");
        Dynamicweb.eCommerce.Prices.RoundingCollection col = new Dynamicweb.eCommerce.Prices.RoundingCollection();

        while (objDataReader.Read())
        {
            objRounding = new Dynamicweb.eCommerce.Prices.Rounding(objDataReader);
            col.Add(objRounding);
        }
 
        objDataReader.Close();
        objDataReader.Dispose();

        return col;
    }
}