.net - Automapper : mapping issue with inheritance and abstract base class on collections with Entity Framework 4 Proxy Pocos

Question

I am having an issue using AutoMapper (which is an excellent technology) to map a business object to a DTO where I have inheritance off of an abstract base class within a collection.

Here are my objects:

abstract class Payment
class CashPayment : Payment
class CreditCardPayment : Payment

I also have an invoice object which contains a collection of payments like so:

    public class Invoice
    {
       ... properties...

       public ICollection<Payment> Payments { get; set; }
    }

I also have corresponding DTO versions of each of these objects.

The DtoInvoice object is defined as:

[DataContract]
public class DtoInvoice
{
   ...properties...

   [DataMember]
   public List<DtoPayment> Payments { get; set; }
}

This is what my Mapper definitions look like:

Mapper.CreateMap<Invoice, DtoInvoice>();

Mapper.CreateMap<Payment, DtoPayment>()
  .Include<CashPayment, DtoCashPayment>()
  .Include<CreditCardPayment, DtoCreditCardPayment>();

Mapper.CreateMap<CashPayment, DtoCashPayment>();
Mapper.CreateMap<CreditCardPayment, DtoCreditCardPayment>();

The code to perform the mapping looks like this:

var invoice = repo.GetInvoice(invoiceId);

var dtoInvoice = Mapper.Map<Invoice, DtoInvoice>(invoice);

So for example if my invoice object contains a collection of specific payments (say 1 cash and 1 credit card) when mapper tries to map them I get an error that the abstract class Payment cannot be created. If I remove the abstract keyword from the Payment object then the code works but I only get a collection of Payment object, I do not get their specific objects (Cash & Credit Card payments).

So the question is: How can I get AutoMapper to map the specific payment types and not the base class?

---

Update

I did some more digging and think I see a problem but am not sure how I can solve this with AutoMapper. I think this is more of an EF thing and not AutoMapper's fault. :-)

In my code I am using Entity Framework 4 Proxy POCOs with lazy loading.

So when I try to map an entity returned from EF that is a proxy POCO it gets that funny looking type like:

System.Data.Entity.DynamicProxies.CashPayment_86783D165755C316A2F58A4343EEC4842907C5539AF24F0E64AEF498B15105C2

So my theory is that when AutoMapper tries to map CashPayment to DtoCashPayment and the payment passed in is of the proxy type AutoMapper sees it as a "non match" and then maps the generic Payment type. But since Payment is an abstract class AutoMapper bombs with a "System.InvalidOperationException: Instances of abstract classes cannot be created." exception.

So the question is: Is there a way for me to use AutoMapper to map EF POCO proxy objects to Dtos.

Answer 1

This answer comes 'a bit' late as I've just faced the same issue with EF4 POCO proxies.

I solved it using a custom converter that calls Mapper.DynamicMap<TDestination>(object source) to invoke the runtime type conversion, rather than the .Include<TOtherSource, TOtherDestinatio>().

It works fine for me.

In your case you would define the following converter:

class PaymentConverter : ITypeConverter<Payment, DtoPayment> {
    public DtoPayment Convert( ResolutionContext context ) {
        return Mapper.DynamicMap<DtoPayment>( context.SourceValue );
    }
}

And then:

Mapper.CreateMap<Payment, DtoPayment>().ConvertUsing<PaymentConverter>();
Mapper.CreateMap<CashPayment, DtoCashPayment>();
Mapper.CreateMap<CreditCardPayment, DtoCreditCardPayment>();