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>();

Answer 2

I also tried Olivier's example and got the same StackOverflow errors. I also tried subkamran's solution but not luck there as I am not using a base class from the entity model code generation. Automapper still blows up. Until I find a better solution, I just set the Context to not create Proxies when I create a Context object.

model.Configuration.ProxyCreationEnabled = false; 
model.Configuration.LazyLoadingEnabled = true; 

I would also like to see an answer to the problem perhaps using something build into Automapper...

UPDATE: The Pre-release of Automapper corrects this issue and allows for the mapping to cover a DynamicProxy with no extra configuration.

The release this works in is 2.2.1

Answer 3

Building on Olivier's response, I could not get his to work in my context... it kept going in an infinite loop and threw a StackOverflowException.

In this example, AbstractClass is my base class and AbstractViewModel is my base view model (not marked as abstract mind you).

However, I did get it to work using this hackish looking converter:

    public class ProxyConverter<TSource, TDestination> : ITypeConverter<TSource, TDestination>
        where TSource : class
        where TDestination : class
    {
        public TDestination Convert(ResolutionContext context)
        {
            // Get dynamic proxy base type
            var baseType = context.SourceValue.GetType().BaseType;

            // Return regular map if base type == Abstract base type
            if (baseType == typeof(TSource))
                baseType = context.SourceValue.GetType();

            // Look up map for base type
            var destType = (from maps in Mapper.GetAllTypeMaps()
                           where maps.SourceType == baseType
                           select maps).FirstOrDefault().DestinationType;

            return Mapper.DynamicMap(context.SourceValue, baseType, destType) as TDestination;
        }
    }

    // Usage

    Mapper.CreateMap<AbstractClass, AbstractViewModel>()
        .ConvertUsing(new ProxyConverter<AbstractClass, AbstractViewModel>());

So, a DerivedClassA will map normally, but a DynamicProxy_xxx will also map properly as this code inspects its base type (DerivedClassA).

Please, please, please show me that I don't have to do this crazy lookup crap. I don't know enough AutoMapper to fix Olivier's answer properly.