AutoMapper throwing StackOverflowException when calling ProjectTo() on IQueryable

Question

I have created classes using EF Code First that have collections of each other. Entities:

public class Field
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual List<AppUser> Teachers { get; set; }
    public Field()
    {
        Teachers = new List<AppUser>();
    }
}

public class AppUser
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string UserName => Email;
    public virtual List<Field> Fields { get; set; }
    public AppUser()
    {
        Fields = new List<FieldDTO>();
    }
}

DTOs:

public class FieldDTO
{ 
    public int Id { get; set; }
    public string Name { get; set; }
    public List<AppUserDTO> Teachers { get; set; }
    public FieldDTO()
    {
        Teachers = new List<AppUserDTO>();
    }
}

 public class AppUserDTO
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string UserName => Email;
    public List<FieldDTO> Fields { get; set; }
    public AppUserDTO()
    {
        Fields = new List<FieldDTO>();
    }
}

Mappings:

Mapper.CreateMap<Field, FieldDTO>();
Mapper.CreateMap<FieldDTO, Field>();
Mapper.CreateMap<AppUserDTO, AppUser>();
Mapper.CreateMap<AppUser, AppUserDTO>();

And I am getting StackOverflowException when calling this code (Context is my dbContext):

protected override IQueryable<FieldDTO> GetQueryable()
{
    IQueryable<Field> query = Context.Fields;
    return query.ProjectTo<FieldDTO>();//exception thrown here
}

I guess this happens because it loops in Lists calling each other endlessly. But I do not understand why this happens. Are my mappings wrong?

You're right. The problem is an infinite loop when calling mapper on lists. Your mappings are right. You can try to empty lists before convert entities. — erikscandola, May 16 '16 at 10:35

score 36 · Accepted Answer · edited Jul 19 '17 at 18:27

36

You have self-referencing entities AND self-referencing DTOs. Generally speaking self-referencing DTOs are a bad idea. Especially when doing a projection - EF does not know how to join together and join together and join together a hierarchy of items.

You have two choices.

First, you can force a specific depth of hierarchy by explicitly modeling your DTOs with a hierarchy in mind:

public class FieldDTO
{ 
    public int Id { get; set; }
    public string Name { get; set; }
    public List<TeacherDTO> Teachers { get; set; }
    public FieldDTO()
    {
        Teachers = new List<TeacherDTO>();
    }
}

public class TeacherDTO 
{
    public int Id { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string UserName => Email;
}

public class AppUserDTO : TeacherDTO
{
    public List<FieldDTO> Fields { get; set; }
    public AppUserDTO()
    {
         Fields = new List<FieldDTO>();
    }
}

This is the preferred way, as it's the most obvious and explicit.

The less obvious, less explicit way is to configure AutoMapper to have a maximum depth it will go to traverse hierarchical relationships:

CreateMap<AppUser, AppUserDTO>().MaxDepth(3);

I prefer to go #1 because it's the most easily understood, but #2 works as well.

edited Jul 19 '17 at 18:27

Mazdak Shojaie

1,620
1
25
32

answered May 16 '16 at 15:52

Jimmy Bogard

26,045
5
74
69

Thank you Jimmy. You really helped me out. I think I will go for first choice. It will take some time to refactor whole code but it will be worth it. – Peter May 16 '16 at 16:01
@Jimmy Bogard: in the first approach, is the mapping order important? – Mazdak Shojaie Jul 25 '17 at 07:53
I find that .MaxDepth(n) is very useful for making the map complete during testing so that I can then dig in to find out where the recursion is coming from for complex graphs. But I always take it out afterwards, so that if the model changes, and recursion is re-introduced by accident, we'll get an error again. – Nich Overend Jan 16 '18 at 19:25
Great feature. awesome. – Prasad Kanaparthi May 18 '18 at 07:25
2

`MaxDepth` doesn't seem to work for me - version `7.0.1.0` – JobaDiniz Apr 28 '20 at 17:19
1

we were using `MaxDepth` + `PreserveReferences` and oddly using both was the problem. We removed `PreserveReferences` and left only `MaxDepth` and it worked in our case – JobaDiniz Apr 28 '20 at 17:47

score 12 · Answer 2 · answered Oct 08 '16 at 17:21

12

Other option is using PreserveReferences() method.

CreateMap<AppUser, AppUserDTO>().PreserveReferences();

answered Oct 08 '16 at 17:21

glanes

149
1
2

If I'm not mistaking, MaxDepth automaticaly enables it – cdie Mar 21 '17 at 10:57
1

PreserveReferences doesn't apply to ProjectTo. – Lucian Bargaoanu Aug 19 '17 at 17:38

score 0 · Answer 3 · answered May 27 '17 at 08:28

I use this generic method:

        public static TTarget Convert<TSource, TTarget>(TSource sourceItem)
    {
        if (null == sourceItem)
        {
            return default(TTarget);
        }

        var deserializeSettings = new JsonSerializerSettings { ObjectCreationHandling = ObjectCreationHandling.Replace, ReferenceLoopHandling = ReferenceLoopHandling.Ignore };

        var serializedObject = JsonConvert.SerializeObject(sourceItem, deserializeSettings);

        return JsonConvert.DeserializeObject<TTarget>(serializedObject);
    }

score -1 · Answer 4 · answered Sep 03 '19 at 17:00

-1

...
MapperConfiguration(cfg =>
{
    cfg.ForAllMaps((map, exp) => exp.MaxDepth(1));
...

answered Sep 03 '19 at 17:00

user2665492

11

score -1 · Answer 5 · answered Feb 21 '21 at 10:29

When you giving 1 navigation_property to 2nd entity and visa-versa it go in an infinite loop state. So, the compiler automatically throws a Stackoverflow exception.

So, to avoid that, you just need to remove one navigation_property from any of the entities.

AutoMapper throwing StackOverflowException when calling ProjectTo() on IQueryable

5 Answers5

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