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!

Answer 1

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.

Answer 2

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.