gistfile1.cs

public Cart GetShoppingCart(int orderGroupId)
{
    var parameters = new OrderSearchParameters
    {
        SqlWhereClause = "OrderGroupId = " + orderGroupId
    };
    var options = new OrderSearchOptions
    {
        Classes = new StringCollection { "ShoppingCart" },
        RecordsToRetrieve = 1
    };

    var findCarts = OrderContext.Current.FindCarts(parameters, options);
    Cart cart = findCarts.FirstOrDefault();

    if (cart == null)
        throw new Exception("Can't find order! Something has clearly gone wrong");

    return cart;
}