mapconfig_original.cs

// What we originally had
namespace Example
{
    public class UserMapProfile : Profile
    {
        public UserMapProfile()
        {
            CreateMap<ExternalUser, InternalUser>()
                .ForMember(dest => dest.Name, opt => opt.MapFrom(src => src.Profile.DisplayName))
        }
    }
}

mapconfig_updated.cs

// What we wound up doing
namespace Example
{
    public class UserMapProfile : Profile
    {
        public UserMapProfile()
        {
            CreateMap<ExternalUser, InternalUser>()
                .ForMember(dest => dest.Name, opt => opt.MapFrom(src => src.Profile.DisplayName))
                .AfterMap((src, dest) => dest.Name = src.Profile.Name);
        }
    }
}

models.cs

namespace Example
{
  // This is a model representing something in our system
  public class InternalUser {
    public string Name {get; set;}
  }

  // This models part of a datamodel from a third party api
  public class ExternalUserProfile {
    public string Name {get; set;} // This field is always set
  }

  // This models a datamodel from a third party api
  public class ExternalUser {
    // This field is only sometimes set - we cannot control nor influence nor predict this
    // We also find it confusing and frustrating that this particular API returns names in 2 places
    // and seemingly only one of them is reliably provided each time, but is out of our control.
    public string Name {get;set;}
    public ExternalUserProfile Profile {get;set;}
  }
}

userservice.cs

namespace Example
{
  public class UserService() 
  {
    public IEnumerable<InternalUser> GetUsersFromThirdParty() {
        var 3pusers = LoadUsersFromThirdParty()
        // mapper.Map is going to see that ExternalUser and InternalUser both have a Name attribute
        // It is not going to actually use the override mapping in the UserMapProfile. It appears to find
        // the 1:1 mapping between fields, and ignore what we set in mapconfig_original.
        // In order to use the behavior we want, we must rely on AfterMap, as done in mapconfig_updated
        return mapper.Map<IEnumerable<ExternalUser>, IEnumerable<InternalUser>>(3pUsers)
    }
    
    public IEnumerable<ExternalUser> LoadUsersFromThirdParty() {
      // imagine that this hits a rest api, gets back json, and hydrates it into a list of ExternalUsers
      // I've cut all that code out for the sake of brevity as it doesn't impact how automapper behaves.
    }
  }
}