Auto Mapper Unmapped members were found

Quick intro edit: as @mrTurkay answers below, this can be solved with the following configuration:

cfg.ValidateInlineMaps = false;

However, you should understand why the problem occours in the first place - so feel free to read on.

This problem occours when you're trying to map an object that you didn't create a mapping configuration for. What you need to keep in mind is, that it doesn't have to be the specific object you're trying to map; but one of it's navigation properties.

Say for instance you have a Car.cs that you want to map to a CarDTO.cs

The Car.cs looks like this:

public class Car
{
  public string Color { get; set; }

  public Engine Engine { get; set; }
}

And your DTO looks the same, but has a reference to the EngineDTO instead:

public class CarDTO
{
  public string Color { get; set; }

  public EngineDTO Engine { get; set; }
}

You configured the mapping like this:

    Mapper.CreateMap<DTO.CarDTO, Data.Model.Car>();
    Mapper.CreateMap<Data.Model.Car, DTO.CarDTO>();

    Mapper.CreateMap<DTO.EngineDTO, Data.Model.Engine>();
    Mapper.CreateMap<Data.Model.Engine, DTO.EngineDTO>();

All looks fine, right? However, in your EngineDTO, you probably have a navigation property like, lets say:

public class EngineDTO
{
public List<PartDTO> Parts { get; set; }
}

So while Automapper is Mapping from Engine to EngineDTO, it also tries to Map the PartDTO, but since you forgot to declare the mapping in the global.asax, you get the error:

AutoMapper.AutoMapperConfigurationException: Unmapped members were found. Review the types and members below. Add a custom mapping expression, ignore, add a custom resolver, or modify the source/destination type

If you don't want to map certain properties on a class, you can use Ignore:

Mapper.CreateMap<Engine, EngineDTO>()
    .ForMember(x => x.Parts, opt => opt.Ignore());

EDIT:

For a more robust AutoMapper configuration, I suggest that you use mapping profiles, instead of declaring the mapping directly in Global.asax. Here is an Example:

Profile:

public class CarProfile : Profile
{
    public CarProfile ()
    {
        CreateMap<Car, CarDTO>();
    }
}

Global.asax:

Mapper.Initialize(cfg =>
{
     cfg.AddProfile<CarProfile>();
}

it is about validation.

cfg.ValidateInlineMaps = false;

should be enough

In my case, I had forgotten to add Map Configuration to MapConfig.cs.