自动映射用null覆盖嵌套字段

我有以下代码:

1. 将实体转换为dto
2. 更新dto中的属性
3. 将数据转换回实体
4. 使用更新的属性更新原始实体

这几乎可以工作,但在最终更新期间-其中一个字段被设置为null,但不应触摸它。

主要内容:

Entity e = new Entity("Id")
{
    Additional = new EntityAdditional()
    {
        Editable = "Change Me!",
        NotEditable = "Don't Change Me!"
    }
};

Dto dto = Mapper.Map<Dto>(e);

dto.Additional.Editable = "Changed!";

Mapper.Map<Dto, Entity>(dto, e); // e NotEditable is null!

映射:

config.CreateMap<Entity, Dto>()
    .ForMember(d => d.Additional,
    input => input.MapFrom(i => new DtoAdditional{
        Editable = i.Additional.Editable
    }));

config.CreateMap<Dto, Entity>()
    .ForMember(d => d.Additional,
    input => input.MapFrom(i => new EntityAdditional
    {
        Editable = i.Additional.Editable
    }));

实体:

public class Entity
{
    public string Id { get; set; }
    public EntityAdditional Additional { get; set; }

    public Entity(string id) {
        Id = id;
        Additional = new EntityAdditional()
        {
            Editable = "Editable",
            NotEditable = "UnEditable"
        };
    }
}

public class EntityAdditional
{
    public string Editable { get; set; }
    public string NotEditable { get; set; }
}

Dto:

public class Dto
{
    public DtoAdditional Additional { get; set; }
}

public class DtoAdditional
{
    public string Editable { get; set; }
}