AutoMapper objects with different property types

Question

I want to map my Entity Framework entities (generated from a legacy database) to custom DTO objects (which should be nice and clean).

My legacy DB has entities looking a bit like this:

internal class Order {
    int id;
    string Shipping_date;
    string quantity;
}

And I want to map it to a nicer DTO object:

public class OrderDto {
    int id;
    DateTime? ShippingDate;
    int Quantity;
}

I have written an "entity container" to provide dependency injection, which returns values this way:

public IEnumerable<OrderDto> GetPaginatedOrders(int page, int pageSize)
{
    return this.db.Orders
               .OrderByDescending(c => c.id)
               .Paginate(page, pageSize)
               .Project()
               .To<OrderDto>()
               .AsEnumerable();
}

So: change of types, and change of property names.

Were it only change of property names, it would be easy-but-tedious:

Mapper.CreateMap<Order, OrderDto>()
  .ForMember(dest => dest.Quantity, opt => opt.MapFrom(src => src.quantity))
  .ForMember(dest => dest.ShippingDate, opt => opt.MapFrom(src => src.Shipping_date));

This is not enough with type changes. I tried a whole bunch of stuff:

Parsing the properties at the mapping declaration, like src => int.Parse(src.quantity) but Linq doesn't like it.
Extending the EF entities with custom properties like QuantityInt { get { return int.Parse(this.quantity) } } and using these in the mapping, but AutoMapper doesn't like it, and explicitly don't support them.
Mapping system types one to another like Mapper.CreateMap<string, int>().ConvertUsing(Convert.ToInt32) but I still get Unable to create a map expression from System.String to System.Int32 errors.
Using custom converters for my class, but I always get empty values from ResolutionContext.SourceValues at run-time from my entities (I'm guessing that they are disposed before AutoMapper gets them or something like this).

I'm realizing that AutoMapper is convention-based, so maybe I should use another tool, but which one exist?

Thanks for your help!

I guess you're mapping inside db query (linq), but not the final object itself (after calling ToList, First, Single etc). So I guess you should query your entity first and then try to map. — Aurimas Neverauskas
Out of curiousoty, are you using Mapper.Map<T>(...) to map your queries or are you doing .Project().To<T>() from the Queryable Extensions namespace under Automapper.QueryableExtensions? — Scott Chamberlain
I have added a usage sample to the question ; I'm using Project().To() on the query. — thomasb

severin severin · Accepted Answer · 2014-10-20T16:26:43

.Project() uses Linq to entities, which generates SQL and naturally only understands a very limited set of functions.

If you use

Mapper.Map<IEnumerable<Order>, IEnumerable<OrderDto>>(src)

your conversions will work fine.

AutoMapper objects with different property types

1 Answers