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

Question

I have the following entities and corresponding view models

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; }
}

I want to map an instance of type TopEntityViewModel to an instance of type TopEntity without overriding any value on BottomEntity.TopId that might already be there.

I've configured AutoMapper as follows:

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());
});

I've added .ForMember(dest => dest.TopId, opts => opts.Ignore()); to ignore BottomEntity.TopId during mapping thinking that it should help preserve the existing value.

I'm mapping the instances like so:

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);

When I access updatedTopEntity.Bottoms[<whatever>].TopId it's always set to 0. If I update my definition of the property BottomEntity.TopId on the class to have a default value like 1000, after mapping all my updatedTopEntity.Bottoms[<whatever>].TopId are set to that default value (1000 in the example).
I've checked if the references returned from mapper.Map map the original references and they do.

How do I prevent AutoMapper from removing any existing values on the existing instances ?

I'm using AutoMapper 10.1.1 with .NET Core 5. You can try a working example here.

Did you also check if `Bottoms` is the same collection instance? — mu88, Oct 18 '21 at 13:19

score 3 · Accepted Answer · answered Oct 18 '21 at 14:40

AutoMapper says this 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, according to the documentation, does:

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.

After installing AutoMapper.Collection I've updated the AutoMapper configuration as follows:

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);
  });

The line .EqualityComparison((src, dest) => src.Id == dest.Id); adds an equality comparison that AutoMapper will use as follows:

If Ids match, then AutoMapper will map BottomEntityViewModel to BottomEntity
If BottomEntityViewModel exists and BottomEntity doesn't, then AutoMapper will add a new BottomEntity mapped from BottomEntityViewModel to the collection
If BottomEntity exists and BottomEntityViewModel doesn't, then AutoMapper will remove BottomEntity from collection

This behavior of AutoMapper.Collection is described here
Working example: https://dotnetfiddle.net/tnaUjY

Thanks to Lucian Bargaoanu for pointing me to the correct direction

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

1 Answers1