An exception of type 'AutoMapper.AutoMapperMappingException' occurred in AutoMapper.dll but was not handled in user code

Question

Somehow my code doesn't work any more (it did work before with the exact same code). This is the problem:

The code

I'm trying to map some objects to ViewModels with this code:

Configuration:

Mapper.CreateMap<BookcaseItem, FoundBookcaseItemViewModel>()
    .ForMember(x => x.Title, opt => opt.MapFrom(src => src.Book.Title))
    .ForMember(x => x.Authors, opt => opt.MapFrom(src => src.Book.Authors.Select(x => x.Name).Aggregate((i, j) => i + ", " + j)))
    .ForMember(x => x.Identifiers, opt => opt.MapFrom(src => (!string.IsNullOrEmpty(src.Book.Isbn10) ? ("ISBN10: " + src.Book.Isbn10 + "\r\n") : string.Empty) +
                                                                (!string.IsNullOrEmpty(src.Book.Isbn13) ? ("ISBN13: " + src.Book.Isbn13) : string.Empty)))
    .ForMember(x => x.Pages, opt => opt.MapFrom(src => src.Book.Pages))
    .ForMember(x => x.ImageUri, opt => opt.MapFrom(src => src.Book.ThumbnailUriSmall));

The usage:

public ActionResult Index()
{
    string facebookId = _accountService.GetLoggedInUserFacebookId();

    IEnumerable<BookcaseItem> items = _bookcaseItemService.GetBookcaseItemsForUser(facebookId);
    IEnumerable<FoundBookcaseItemViewModel> viewModels = items.Select(Mapper.Map<BookcaseItem, FoundBookcaseItemViewModel>);

    return PartialView(viewModels);
}

The error

This results in the following error:

An exception of type 'AutoMapper.AutoMapperMappingException' occurred in AutoMapper.dll but was not handled in user code

The debug data

First of all I ensure that there are no configuration errors by calling:

Mapper.AssertConfigurationIsValid();

I've set breakpoints all over my code and try to debug it, but I can't make sense of it. The 'items' collection is filled with data (proxy classes generated by Entity Framework), but the the 'viewModels' collection is filled with strange data. It has a 'message' that says this:

Mapping types: BookcaseItem_B9B52593B2659AC05C47AB2A6E0F7AEA9989CC34D3527DF5B6AA988ED57166FB -> String System.Data.Entity.DynamicProxies.BookcaseItem_B9B52593B2659AC05C47AB2A6E0F7AEA9989CC34D3527DF5B6AA988ED57166FB -> System.String

Destination path: FoundBookcaseItemViewModel.Authors

Source value: System.Data.Entity.DynamicProxies.BookcaseItem_B9B52593B2659AC05C47AB2A6E0F7AEA9989CC34D3527DF5B6AA988ED57166FB

And then there's a stacktrace property that says:

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()

at System.Linq.SystemCore_EnumerableDebugView`1.get_Items()

Oh and finally there's another property called 'context' with the following data:

Can anyone explain what's going on here and why my code isn't working any longer? I did a couple of changes to my solution recently, but I've rolled them back by Git, so they shouldn't have any effect on the code.

My setup

• Visual Studio 12 RC
• ASP.NET MVC 4
• .NET Framework 4.0 (I had 4.5 but that caused too many errors, so I rolled back with Git to version 4.0)
• Entity Framework 5.0 RC
• AutoMapper 2.1.267

The entity and ViewModel

I don't know if it's relevant, but here is the source class for the mapping:

public class BookcaseItem : Entity
{
    public Guid Id { get; set; }
    public bool IsRenting { get; set; }
    public bool IsSwapping { get; set; }
    public bool IsSelling { get; set; }
    public decimal RentingPrice { get; set; }
    public decimal SellingPrice { get; set; }
    public string Currency { get; set; }
    public bool IsAvailable { get; set; }
    public virtual Guid BookId { get; set; }
    public virtual Guid UserId { get; set; }

    public virtual Book Book { get; set; }
    public virtual User User { get; set; }

    public BookcaseItem()
    {
        IsAvailable = true;
        Currency = "USD";
    }
}

And this is the destination class for the mapping:

public class FoundBookcaseItemViewModel
{
    public Guid Id { get; set; }
    public bool IsRenting { get; set; }
    public bool IsSwapping { get; set; }
    public bool IsSelling { get; set; }
    public decimal RentingPrice { get; set; }
    public decimal SellingPrice { get; set; }
    public string Title { get; set; }
    public string Authors { get; set; }
    public string Identifiers { get; set; }
    public int Pages { get; set; }
    public string ImageUri { get; set; }
}

Solution

It seems like there is a problem with mapping Authors property. This Aggregate call will throw an exception if the Authors sequence is null or empty.

.ForMember(x => x.Authors, 
           opt => opt.MapFrom(src => src.Book.Authors.Select(x => x.Name).Aggregate((i, j) => i + ", " + j)))

Comments

[Leon Cullens]

Looks like you've found the root of the problem good sir. I've added a null check in the MapFrom() method and now the exception is gone :)

[Ufuk Hacıoğulları]

@LeonCullens You should add an empty check too.

[Leon Cullens]

Well, 'Authors' is a collection of strings, so I just check if the collection contains more than 0 items (the strings in the collection can't be null or empty, that's handled elsewhere) :-)

[Brendan]

Why does Mapper.AssertConfigurationIsValid() pass as true despite of it being invalid at runtime?

[Ufuk Hacıoğulları]

@Brendan AutoMapper validation checks if the property names and types match on source and destination types. The exception is thrown inside Select method that is called while mapping.