AutoMapper overwrites recursive type

Question

I have a Dto that looks somewhat like this:

class TypeDto
{    
    int Id {get; set;}
    string Name {get; set;}
    string DisplayName {get; set;}
    IEnumerable<TypeDto> Children {get; set;}
}

Now I need to map to it from two different sources. That's because one of them contains Name and the other contains DisplayName. So the types:

class Type1
{
    int Id {get; set;}
    string Name {get; set;}
    IEnumerable<Type1> Children {get; set;}
}

class Type2
{
    int Id {get; set;}
    string DisplayName {get; set;}
    IEnumerable<Type2> Nested {get; set;}
}

Notice the name difference in the Children/Nested enumerable.

Now for the map I would do:

config.CreateMap<Type1, TypeDto>();

config.CreateMap<Type2, TypeDto>()
    .ForMember(dest => dest.Children, opts => opts.MapFrom(src => src.Nested));

var dto = _mapper.Map<TypeDto>(type1Instance);

_mapper.Map(type2Instance, dto);

The first map works as expected, mapping the children recursively, filling the Id and Name fields and leaving DisplayName equal to null everywhere. The second map, however, fills in the DisplayName for the root object properly, but then in its children, it nullifies the Name field. So for example:

var type1Instance = new Type1 
{ 
    Id = 1, 
    Name = "root", 
    Children = new[] { new Type1 
        {
            Id = 2,
            Name = "child"
        }}
};

var type2Instance = new Type2 
{ 
    Id = 1, 
    DisplayName = "Root", 
    Children = new[] { new Type2
        {
            Id = 2,
            DisplayName = "Child"
        }}
};

After mapping the following instances the resultant has its fields set to:

Id = 1,
Name = "root",
DisplayName = "Root",
Children = { TypeDto { Id = 2, Name = null, DisplayName = "Child", Children = null } }

so the child's Name is nullified, and that's not what I want. I'd like it to be "child", obviously. How should I configure the mapper to get the wanted behavior?

I cannot change the Type1 or Type2 classes, they're from an external API.

AutoMapper's version is 6.2.1, .NET Framework 4.5.1.

Answer 1

Taken from Lucian Bargaoanu's comment.

The AutoMapper.Collection package solves my problem. All that was needed is adding this statement to configuration:

config.AddCollectionMappers();

and then defining EqualityComparison on both my maps:

config.CreateMap<Type1, TypeDto>()
    .EqualityComparison((src, dest) => src.Id == dest.Id);

config.CreateMap<Type2, TypeDto>()
    .EqualityComparison((src, dest) => src.Id == dest.Id)
    .ForMember(dest => dest.Children, opts => opts.MapFrom(src => src.Nested));

After that the collections are properly updated. Citing the docs from here, the second mapping, i.e.

_mapper.Map(type2Instance, dto);

will now map recursively on any collection members with matching Id, add to the collection and map any items from type2Instance.Nested which do not occur in dto.Children collection and remove any items that dto.Children contains, but type2Instance.Nested does not.