Peter Peter - 1 month ago 28
C# Question

AutoMapper throwing StackOverflowException when calling ProjectTo<T>() on IQueryable

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?

Answer

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.