AutoMapper: What is the difference between ForMember() and ForPath()?

Question

I am reading AutoMapper's ReverseMap() and I can not understand the difference between ForMember() and ForPath(). Implementations was described here. In my experience I achieved with ForMember().

See the following code where I have configured reverse mapping:

public class Customer
{
    public string Surname { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
}
public class CustomerDto
{
    public string CustomerName { get; set; }
    public int Age { get; set; }
}

static void Main(string[] args)
{
    Mapper.Initialize(cfg =>
    {
        cfg.CreateMap<Customer, CustomerDto>()
           .ForMember(dist => dist.CustomerName, opt => opt.MapFrom(src => $"{src.Surname} {src.Name}"))
            .ReverseMap()
            .ForMember(dist => dist.Surname, opt => opt.MapFrom(src => src.CustomerName.Split(' ')[0]))
            .ForMember(dist => dist.Name, opt => opt.MapFrom(src => src.CustomerName.Split(' ')[1]));
    });

    // mapping Customer -> CustomerDto            
    //... 
    //

    // mapping CustomerDto -> Customer
    var customerDto = new CustomerDto
    {
        CustomerName = "Shakhabov Adam",
        Age = 31
    };
    var newCustomer = Mapper.Map<CustomerDto, Customer>(customerDto);
}

It is working.

---

Question

Do ForMember and ForPath the same things or when should I use ForPath() over ForMember()?

Answer 1

In this case, to avoid inconsistencies, ForPath is translated internally to ForMember. Although what @IvanStoev says makes sense, another way to look at it is that ForPath is a subset of ForMember. Because you can do more things in ForMember. So when you have a member, use ForMember and when you have a path, use ForPath :)

Answer 2

ForMember() can only access root-properties of the mapped object. ForPath() can also access its sub-properties.

In my experience they do the same thing with the one exception. If you have something like this:

a.CreateMap<Source, Destination>()
  .ForMember(
    a => a.RootProp.SubProp, 
    b => b.MapFrom(c => c.RootProp.Get()))

it's going to fail producing the following error.

System.ArgumentException: 'Expression 'a => a.RootProp.SubProp' must resolve to top-level member and not any child object's properties. You can use ForPath, a custom resolver on the child type or the AfterMap option instead. (Parameter 'lambdaExpression')'

Because of that you need to replace ForMember() with ForPath() like so.

a.CreateMap<Source, Destination>()
  .ForPath(
    a => a.RootProp.SubProp, 
    b => b.MapFrom(c => c.RootProp.Get()))

I have limited experience using AutoMapper but it tells me that ForPath() has the same funcionallity as ForMember() (although with some extras). In usual cases we use ForMember() instead of ForPath(). I am not sure why, perhaps due to performance issues.