How to ignore property of property in AutoMapper mapping?

Question

Image a Person and a Group class with a many-to-many relationship. A person has a list of groups and a group has a list of people.

When mapping Person to PersonDTO I have a stack overflow exception because AutoMapper can't handle the Person>Groups>Members>Groups>Members>...

So here's the example code:

public class Person
{
    public string Name { get; set; }
    public List<Group> Groups { get; set; }
}

public class Group
{
    public string Name { get; set; }
    public List<Person> Members { get; set; }
}

public class PersonDTO
{
    public string Name { get; set; }
    public List<GroupDTO> Groups { get; set; }
}

public class GroupDTO
{
    public string Name { get; set; }
    public List<PersonDTO> Members { get; set; }
}

When I use .ForMember in creating a mapper, the first mapper that gets executed throws a null reference exception.

Here's the code for the mapper:

CreateMap<Person, PersonDTO>()
    .ForMember(x => x.Groups.Select(y => y.Members), opt => opt.Ignore())
    .ReverseMap();

CreateMap<Group, GroupDTO>()
    .ForMember(x => x.Members.Select(y => y.Groups), opt => opt.Ignore())
    .ReverseMap();

So what am I missing or doing wrong? When I remove the .ForMember methods, the null reference exception is not thrown anymore.

UPDATE: I really want to emphasize the main point of my question is how to ignore a property of a property. This code is just a rather simple example.

UPDATE 2: This is how I fixed it, big thanks to Lucian-Bargaoanu

CreateMap<Person, PersonDTO>()
    .ForMember(x => x.Groups.Select(y => y.Members), opt => opt.Ignore())
    .PreserveReferences() // This is the solution!
    .ReverseMap();

CreateMap<Group, GroupDTO>()
    .ForMember(x => x.Members.Select(y => y.Groups), opt => opt.Ignore())
    .PreserveReferences() // This is the solution!
    .ReverseMap();

Thanks to .PreserveReferences() the circular references get fixed!

Thank you @Esperadoce, but my code is a bit less simple than the example. I really want to ignore a **property of a property** in AutoMapper. — Mason, Jul 25 '17 at 08:43
Yes @KirillShlenskiy, they are indeed fields, I just wanted to keep it very simple. I'll update my question. — Mason, Jul 25 '17 at 08:46
Why don't you just use `.ForMember(x => x.members, o => o.Ignore())` ? — kayess, Jul 25 '17 at 08:46
@kayess, it's not `x.Members` I want to ignore, it's the `Groups` property of the `x.members`, to withstand stack overflow exception. — Mason, Jul 25 '17 at 08:49
your problem is loop reference So check this https://stackoverflow.com/a/11505745/2954082 — Hicham Bouchilkhi, Jul 25 '17 at 08:49
@kayess no the `members.Groups` has a property referring back to `members`, that's what's creating the stack overflow — Mason, Jul 25 '17 at 08:59
@Esperadoce Doesn't work for my code... Because I work with `IList` properties — Mason, Jul 25 '17 at 09:10
Sorry @kayess I forgot to update that part of the code. I updated it because someone was addressing properties != fields — Mason, Jul 25 '17 at 09:40

score 2 · Accepted Answer · answered Jul 25 '17 at 10:24

2

This should just work. See https://github.com/AutoMapper/AutoMapper/wiki/5.0-Upgrade-Guide#circular-references. There is also a PR pending https://github.com/AutoMapper/AutoMapper/pull/2233.

answered Jul 25 '17 at 10:24

Lucian Bargaoanu

3,336
3
14
19

Yes! Oh my gawd that fixed it! Thank you so much! I hoped I would not have to use a lot of ugly code, but wow just adding `.PreserveReferences()` fixed it! Thank you again. – Mason Jul 25 '17 at 11:57
2

A link to a potential solution is always welcome, but please [add context around the link](//meta.stackoverflow.com/a/8259) so your fellow users will have some idea what it is and why it’s there. **Always quote the most relevant part of an important link, in case the target site is unreachable or goes permanently offline.** Take into account that being _barely more than a link to an external site_ is a possible reason as to [Why and how are some answers deleted?](//stackoverflow.com/help/deleted-answers). – kayess Jul 25 '17 at 12:01

mr100 · Answer 2 · 2017-07-25T10:02:52.357

I think the problem you are experiencing comes from wrong assumption that Groups in PersonDTO.Groups are the same as GroupDTO - it cannot be so without the infinite dependency loop. The following code should work for you:

CreateMap<Person, PersonDTO>()
    .ForMember(x => x.Groups, opt => opt.Ignore())
    .ReverseMap()
    .AfterMap((src, dest) => 
    {
        dest.Groups = src.Groups.Select(g => new GroupDTO { Name = g.Name }).ToList()
    });

CreateMap<Group, GroupDTO>()
    .ForMember(x => x.Members, opt => opt.Ignore())
    .ReverseMap()
    .AfterMap((src, dest) => 
    {
        dest.Members = src.Members.Select(p => new PersonDTO { Name = p.Name }).ToList()
    });

You basically need to teach AutoMapper that in case of PersonDTO.Groups property it should map GroupDTO objects differently.

But I think that your problem is more like architectural issue than code one. PersonDTO.Groups should not be of type GroupDTO - you are here only interested in groups particular user belongs to and not other members of his groups. You should have some simpler type like:

public class PersonGroupDTO
{
    public string Name { get; set; }
}

(the name is up to you of course) to only identify the group without passing additionally members.

Thank you for your answer. That was I way I tried it, but it just didn't look so clean having to make a seperate DTO just to fix that issue. I updated my question with a very simple solution to fix circular references with automapper. — Mason, Jul 25 '17 at 12:02
Only take into account that using PreserveReferences slows down automapper to some degree :) Except of that you are fine. — mr100, Jul 25 '17 at 13:16

How to ignore property of property in AutoMapper mapping?

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