如何防止 AutoMapper 覆盖目标对象上的现有值？

Question

我有以下实体和相应的视图模型

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

  public ICollection<BottomEntity> Bottoms { get; set; }
}

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

  public int TopId { get; set; }
}

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

  public ICollection<BottomEntityViewModel> Bottoms { get; set; }
}

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

我想将类型 TopEntityViewModel 的实例映射到类型 TopEntity 的实例，而不覆盖 BottomEntity.TopId 上可能已经存在的任何值。

我按如下方式配置了 AutoMapper：

var mapperConfig = new MapperConfiguration(exp =>
{
  exp.CreateMap<TopEntity, TopEntityViewModel>();

  exp.CreateMap<BottomEntity, BottomEntityViewModel>();

  exp.CreateMap<TopEntityViewModel, TopEntity>();

  exp.CreateMap<BottomEntityViewModel, BottomEntity>()
    .ForMember(dest => dest.TopId, opts => opts.Ignore());
});

我添加了 .ForMember(dest => dest.TopId, opts => opts.Ignore()); 以在映射期间忽略 BottomEntity.TopId，认为它应该有助于保留现有值。

我正在像这样映射实例：

mapperConfig.AssertConfigurationIsValid();
            
var mapper = mapperConfig.CreateMapper();

var topEntity = new TopEntity
{
  Id = 45,
  Bottoms = new List<BottomEntity>
  {
    new BottomEntity
    {
      Id = 56,
      TopId = 45
    },
    new BottomEntity
    {
      Id = 57,
      TopId = 45
    }
  }
};

var topEntityVm = new TopEntityViewModel
{
  Id = 45,
  Bottoms = new List<BottomEntityViewModel>
  {
    new BottomEntityViewModel
    {
      Id = 56,
    },
    new BottomEntityViewModel
    {
      Id = 57
    }
  }
};

            
var updatedTopEntity = mapper.Map(topEntityVm, topEntity);

当我访问 updatedTopEntity.Bottoms[<whatever>].TopId 时，它总是设置为 0。如果我更新 class 上属性 BottomEntity.TopId 的定义以具有类似 1000 的默认值，映射后我的所有 updatedTopEntity.Bottoms[<whatever>].TopId 都设置为该默认值值（示例中的 1000）。
我检查了从 mapper.Map 返回的引用是否映射了原始引用，它们确实映射了。

如何防止 AutoMapper 删除现有实例上的任何现有值？

我在 .NET Core 5 中使用 AutoMapper 10.1.1。You can try a working example here。

Answer 1

AutoMapper 这样说 in the documentation:

When mapping to an existing collection, the destination collection is cleared first. If this is not what you want, take a look at AutoMapper.Collection.

AutoMapper.Collection，根据文档，确实：

Adds ability to map collections to existing collections without re-creating the collection object.
Will Add/Update/Delete items from a preexisting collection object based on user defined equivalency between the collection's generic item type from the source collection and the destination collection.

安装后 AutoMapper.Collection 我更新了 AutoMapper 配置如下：

var mapperConfig = new MapperConfiguration(exp =>
  {
    exp.AddCollectionMappers();
                
    exp.CreateMap<TopEntity, TopEntityViewModel>();

    exp.CreateMap<BottomEntity, BottomEntityViewModel>();

    exp.CreateMap<TopEntityViewModel, TopEntity>();

    exp.CreateMap<BottomEntityViewModel, BottomEntity>()
      .ForMember(dest => dest.TopId, opts => opts.Ignore())
      .EqualityComparison((src, dest) => src.Id == dest.Id);
  });

行 .EqualityComparison((src, dest) => src.Id == dest.Id); 添加了 AutoMapper 将使用的相等比较，如下所示：

如果 ID 匹配，则 AutoMapper 会将 BottomEntityViewModel 映射到 BottomEntity
如果 BottomEntityViewModel 存在而 BottomEntity 不存在，则 AutoMapper 将从 BottomEntityViewModel 映射的新 BottomEntity 添加到集合中
如果 BottomEntity 存在而 BottomEntityViewModel 不存在，则 AutoMapper 将从集合中移除 BottomEntity

AutoMapper.Collectionis described here
的这种行为工作示例：https://dotnetfiddle.net/tnaUjY

感谢Lucian Bargaoanu给我指明了正确的方向

如何防止 AutoMapper 覆盖目标对象上的现有值？

How to prevent AutoMapper from overwriting existing values on destination object?

c#

automapper

.net-core

automapper-10