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.