Auto Mapper Unmapped members were found

We are using Automapper for a project, and seem to get the following error randomly:

> 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

The code hasn't been changed in months. I get that error, refresh and the error is gone and page works fine. I'm using

Mapper.AssertConfigurationIsValid();

not sure why it complains the mappings are not good and then a refresh and it's fine again, has anyone run into this? Debugging doesn't help as it's random, sometimes no errors and then other days it will popup somewhere on the site, come back to it and it's fine. The error also comes up on random pages, not the same page, not the same mapping.

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.