Best way to create a map between two entities with a third one from another context

Hi I'd like to create a map between two entities (source: User, target: UserInfosDto) while one member of the target DTO (UserItemPreference) needs info from a third entity inside another context.

public class UserInfosDto
{
    //public int UserId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }

    public UserItemPreferencesDto UserItemPreferences { get; set; }
}
public class UserItemPreferencesDto
{
    public bool SeeActuality { get; set; }
    public bool IsInEditorMode { get; set; }
}
public class User
{
    public string IdentityId { get; set; }
    //...
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
}

public class UserIdentity
{
    public string IdentityId { get; set; }
    //...  
    public bool SeeActuality { get; set; }
    public bool IsInEditorMode { get; set; }  
}

User and UserIdentity come from different databases but have a common property IdentityId. I thought about using ITypeConverter in which I would inject the UserIdentity dbContext. Problem is that I can't find a way to use ITypeConverter on one member only.

Use an IValueResolver instead, which allows to resolve separate members instead of full types.

For your case above it will look like

public class UserItemPreferencesResolver
    : IValueResolver<User, UserInfosDto, UserItemPreferencesDto>
{
    private readonly UserEntityDbContext _dbContext;

    public UserItemPreferencesResolver(UserEntityDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public UserItemPreferencesDto Resolve(
        User source,
        UserInfosDto destination,
        UserItemPreferencesDto destinationMember,
        ResolutionContext context
        )
    {
        UserItemPreferencesDto preferences = /* resolve from _dbContext (and transform) */
        return preferences;
    }
}

Your create the mapping via

CreateMap<User, UserInfosDto>()
    .ForMember(
        dest => dest.UserItemPreferences, 
        opt => opt.MapFrom<UserItemPreferencesResolver>()
    );