Automapper - Mapping a nested collection to a list

Question

I'm using Automapper to do some mapping from XSD-generated serialization object to more sane POCO's.

I'm having an issue with a particular type of mapping.

public class SourceOuterObject
{
  public SourceSet SourceSet { get; set; }
}

public class SourceSet
{
  public List<SourceObject> SourceList{ get; set; }
}

I want to map this to:

public class TargetOuterObject
{
  public List<TargetObject> TargetList{ get; set; }
}

I have tried a wide variety of mapping configurations, but the only thing that I've been able to make work is this:

Mapper.CreateMap<SourceOuterObject, TargetOuterObject>();
Mapper.CreateMap<SourceObject, TargetObject>();
Mapper.CreateMap<SourceSet, List<TargetObject>>()
    .ConvertUsing(ss => ss.SourceList.Select(bs => Mapper.Map<SourceObject, TargetObject>(bs)).ToList());

This works but seems to be way more complicated than it should be - I feel dirty calling a mapping within a mapping config.

Is this the best I can hope for or is there a better way?

Answer 1

When dealing with collections, the trick is always to set up a mapping of the items in the collections and never (well, I've never seen a case that needed) a mapping from one collection to another. That's the point of the generic.

The trick I always use is to work backwards, starting with a mapping between the types inside the collections and then going up a level. If one object needs to go up 2 levels, it's time to use ForMember to get to a nested property.

What you want to do in this case is to set up the following mappings:

Mapper.CreateMap<SourceObject, TargetObject>();
Mapper.CreateMap<SourceOuterObject, TargetOuterObject>()
    .ForMember(dest => dest.TargetList, opt => opt.MapFrom(src => src.SourceSet.SourceList);