使用 @Jasen 评论我得到了它的工作.主要问题是我的映射方向相反.官方文档中的这句话解决了问题:

Using @Jasen comments I got it working. The main problem was that I am mapping in a reversed direction. This sentence in official documentation solves the problem:

Unflattening 仅针对 ReverseMap 配置.如果你想要unflattening，你必须配置Entity -> Dto 然后调用ReverseMap 从Dto -> Entity.

Unflattening is only configured for ReverseMap. If you want unflattening, you must configure Entity -> Dto then call ReverseMap to create an unflattening type map configuration from the Dto -> Entity.

这是链接:

https://github.com/AutoMapper/AutoMapper/blob/master/docs/Reverse-Mapping-and-Unflattening.md

换句话说，为了不讨人喜欢地工作，我必须(或必须)朝这个方向映射:

In other words, to get unflattering to work, I have (or must) to map in this direction:

CreateMap<HierarchicalObject, FlattenedObject>()
    .ReverseMap();