List's select method question

Question

Hi all

Currently, I am using the select at this way to assign values to my OrderDetailDTO object.

 
List ordereportlist = orderdetailBO.GetOrderDetailbyOrderId(OrderId).Select(p => new OrderDetailDTO { ID = p.ID, OrderId = p.OrderId, FileName = p.FileName, Remark = p.Remark }).ToList();

But I think this way is better, and I get an error from complier:

 List ordereportlist = orderdetailBO.GetOrderDetailbyOrderId(OrderId).Select(p => { OrderDetailDTO.Mapping(p); }).ToList();

List<OrderDetailDTO> ordereportlist = orderdetailBO.GetOrderDetailbyOrderId(OrderId).Select(p => { OrderDetailDTO.Mapping(p); }).ToList();

OrderDetailDTO.Mapping return a new OrderDetailDTO. Have I used the select wrong? Could you help me point out way p?