Automapper - Mapping from source child object to destination is including parent values

Question

I'm currently experiencing an issue when trying to map the entire destination object from a child property on the source object. Something similar as described here: Automapper - How to map from source child object to destination

I've made use of the .ConstructUsing method as described in the link above however I'm seeing some weird behaviour where the outputted, mapped object is getting values from the parent instead of the child.

I made a demo of the problem here: https://dotnetfiddle.net/OdaGUr

Is this a problem with my code, should I be using a different method to achieve what I'm trying to do or is this a fault with AutoMapper?

EDIT:

public static void Main()
{
    var config = new MapperConfiguration(cfg => {
        cfg.CreateMap<Child1, Child2>();
        cfg.CreateMap<Parent, Child2>().ConstructUsing((src, ctx) => ctx.Mapper.Map<Child2>(src.Child1));   
     });

    var mapper = config.CreateMapper();

    var parent = new Parent{
        Id = 1,
        Child1 = new Child1 {
            Id = 2
        }
    };

    var child2 = mapper.Map<Parent, Child2>(parent);
    Console.WriteLine(child2.Id); // Returns 1. Expect this to be 2 from Parent.Child1
}

public class Parent
{
    public int Id {get;set;}
    public Child1 Child1 {get;set;}
}

public class Child1
{
    public int Id {get;set;}
}

public class Child2
{
    public int Id {get;set;}
}

Answer 1

ConstructUsing() is used to create the destination object, where the value should be stored in. In your case you are returning a Child2 object with the Id value set to 2 (as returned by the ctx.Mapper.Map<Child1, Child2>(src.Child1) line).

However, after the object has been created, the default mapping will still be applied. This means that the Parent.Id value will be saved in the Child2.Id property, because the names of the property match ("Id"). So, the initial value of 2 will be replaced with the value 1 from the Parent object.

Depending on what you want to do, you might want to use ForMember() to configure special handling on how the property values should be mapped. An example would be:

.ForMember(dest => dest.Id, src => src.MapFrom(it => it.Child1.Id))