AutoMapper strange IQueryable projection exception - asp.net-mvc

Question

To skip pain, go to EDIT below.
I'm using EF with AutoMapper IQueryableExtensions.
Two of my models are as follows:
public class Article
{
public int Key { get; set; }
public DateTimeOffset Created { get; set; }
public int? SemesterKey { get; set; }
public virtual Semester Semester { get; set; }
}
public class Semester
{
public int Key { get; set; }
public string Name { get; set; }
public virtual ICollection<Article> Articles { get; set; }
// Other relationships
public virtual ICollection<Subject> Subjects { get; set; }
public virtual ICollection<AppUser> Users { get; set; }
}
And I have the following DTOs:
public class ArticleDto
{
public int Key { get; set; }
public DateTimeOffset Created { get; set; }
// If I remove this (or ignore it), everything works.
public SemesterDto Semester { get; set; }
}
public class SemesterDto
{
public int Key { get; set; }
public string Name { get; set; }
}
Configuration:
Mapper.CreateMap<Semester, SemesterDto>().MaxDepth(1);
Mapper.CreateMap<Article, ArticleDto>().MaxDepth(1);
The query:
context.Articles.Include(a => a.Semester)
.OrderByDescending(a => a.Created)
.Take(5)
.ProjectTo<ArticleDto>()
.ToList();
Executing the query yields the following strange exception:
System.ArgumentException: Property 'NS.Models.Semester Semester' is not defined for type 'NS.Models.Semester'
This is some of the stack trace:
System.Linq.Expressions.Expression.Property(Expression expression, PropertyInfo property)
System.Linq.Expressions.Expression.MakeMemberAccess(Expression expression, MemberInfo member)
....
AutoMapper.MappingEngine.CreateMapExpression(ExpressionRequest request, Expression instanceParameter, IDictionary`2 typePairCount)
It seems as if AutoMapper generated an expression that is looking for a Semester property (in response to ArticleDto.Semester it seems) on the NS.Models.Semester type which of course doesn't exist.
I can get it working if I do the following:
Mapper.CreateMap<Article, ArticleDto>().MaxDepth(1)
.ForMember(a => a.Semester, c => c.MapFrom(a => a.Semester == null ? null : new SemesterDto() {
Key = a.Semester.Key,
Year = a.Semester.Year }));
But this is just the thing I'm trying to avoid writing by using AutoMapper!
It's probably something wrong on my side but I can't find anything suspicious.
EDIT:
I have the following ctors on SemesterDto:
public SemesterDto()
{
}
public SemesterDto(int key, string name)
{
Key = key;
Name = name;
}
When I remove the second one everything works. Seems this is it, this is really strange behavior though. I never thought the ctor would make a problem so I didn't include it for clarity, everything is possible isn't it. Sorry about that.
So, is this a bug from AutoMapper or is there something else I misunderstood?
EDIT 2:
Stripping this more, I tried mapping a Semester to SemesterDto directly and this is the result:
context.Semesters.ProjectTo<SemesterDto>().FirstOrDefault();
NotSupportedException: Only parameterless constructors and initializers are supported in LINQ to Entities.
This strengthens the idea that the ctor is causing strange behavior.

Answer 1

I can reproduce the issue in your second edit. Somehow (I don't know AutoMapper well enough to see why), the parametrized constructor always takes precedence over the parameterless one in AutoMapper. And EF doesn't support parametrized constructors in its expression tree.
Anyway, you can fix this issue by using ConstructProjectionUsing:
Mapper.CreateMap<Semester, SemesterDto>().MaxDepth(1)
.ConstructProjectionUsing(sem => new SemesterDto());

Answer 2

This has been confirmed as a bug, see this github issue. For now, I guess I'll just avoid ctors in my DTOs until the promised next release.
If you have v4.1.0 or above, then this should've been fixed.

Answer 3

Mapper.CreateMap<Article, Dto.Article>()
.ForMember(d => d.Semester, o => o.MapFrom(s => Mapper.Map<Dto.Semester>(s));
You need to tell it to use the Semester -> Dto.Semester mapping.