How do you map a Dto to an existing object instance with nested objects using AutoMapper?

Question

I have the following Dto and entity with a nested sub entity.

public class Dto
{
    public string Property { get; set; }
    public string SubProperty { get; set; }
}

public class Entity
{
    public string Property { get; set; }
    public SubEntity Sub { get; set; }
}

public class SubEntity
{
    public string SubProperty { get; set; }
}

How can I set up a mapping with AutoMapper that will allow me to update an existing instance of Entity with the values from a Dto.

I'm using Mapper.Map(dto, entity) to update an existing entity but when I try to map Dto.SubProperty to Entity.Sub.SubProperty I get an exception for "must resolve to top-level member. Parameter name: lambdaExpression".

If I create a mapping from Dto to SubEntity using FromMember then Entity.Sub gets replaced with a new instance of SubEntity but that's not what I want. I just want it to update the properties of the existing instance of SubEntity on the Sub property of Entity.

How can I achieve this?

Answer 1

I solved it by using a combination of the ResolveUsing<T>() method and implementing IValueResolver and the ConvertUsing<T>() method and implementing ITypeConverter<TSource,TDestination>.

Some of my mapping scenarios are more complicated than normal including bidirectional mapping and nested classes and nested collections. The above helped me to solve them.

---

EDIT

As requested, I've included an example solution. This example is much simpler than the actual types I was dealing with.

using System;
using AutoMapper;

namespace TestAutoMapperComplex
{
    public class Dto
    {
        public string Property { get; set; }
        public string SubProperty { get; set; }
    }

    public class Entity
    {
        public string Property { get; set; }
        public SubEntity Sub { get; set; }
    }

    public class SubEntity
    {
        public string SubProperty { get; set; }
    }

    static class MapperConfig
    {
        public static void Initialize()
        {
            Mapper.CreateMap<Dto, Entity>()
                .ForMember(entity => entity.Sub, memberOptions =>
                    memberOptions.MapFrom(dto => dto));
            Mapper.CreateMap<Dto, SubEntity>();
        }
    }

    static class MapperConfig2
    {
        private class MyResolver : IValueResolver
        {

            public ResolutionResult Resolve(ResolutionResult source)
            {
                var destinationSubEntity = ((Entity)source.Context.DestinationValue).Sub;

                Mapper.Map((Dto)source.Value, destinationSubEntity);

                return source.New(destinationSubEntity, typeof(SubEntity));
            }
        }

        public static void Initialize()
        {
            Mapper.CreateMap<Dto, Entity>()
                .ForMember(entity => entity.Sub, memberOptions =>
                    memberOptions.ResolveUsing<MyResolver>());
            Mapper.CreateMap<Dto, SubEntity>();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MapperConfig.Initialize();

            var dto = new Dto {Property = "Hello", SubProperty = "World"};
            var subEntity = new SubEntity {SubProperty = "Universe"};
            var entity = new Entity {Property = "Good bye", Sub = subEntity};

            Mapper.Map(dto, entity);

            Console.WriteLine(string.Format("entity.Property == {0}, entity.Sub.SubProperty == {1}",
                entity.Property, entity.Sub.SubProperty));
            Console.WriteLine(string.Format("entity.Sub == subEntity: {0}", 
                entity.Sub == subEntity));

        }
    }
}

If you run the example, which is using MapperConfig, you'll get the following output:

entity.Property == Hello, entity.Sub.SubProperty == World
entity.Sub == subEntity: False

The string properties all get updated as one would want them to, but entity.Sub gets replaced with a new instance of SubEntity which is no good for when you are wanting to update entities for an ORM that will be persisted to a database.

If you modify Main so that MapperConfig2 is used instead, you'll still have the string properties updated as before, but, entity.sub still has the same instance of SubEntity that it had before. Running the example with MapperConfig2 gives this output:

entity.Property == Hello, entity.Sub.SubProperty == World
entity.Sub == subEntity: True

The key difference in MapperConfig2 is that ResolveUsing is used along with MyResolver to preserve the value of entity.Sub.