AutoMapper: What is the difference between MapFrom and ResolveUsing?

Question

Ignoring the ResolveUsing overloads that take an IValueResolver, and looking only at these 2 methods:

void ResolveUsing(Func<TSource, object> resolver);
void MapFrom<TMember>(Expression<Func<TSource, TMember>> sourceMember);

The main difference between these 2 seems to be that ResolveUsing takes a Func<TSource, object>, whereas MapFrom takes an Expression<Func<TSource, TMember>>.

However in client code that actually uses one of these methods with a lambda expression, they seem to be interchangeable:

Mapper.CreateMap<SourceType, DestType>() // uses ResolveUsing
   .ForMember(d => d.DestPropX, o => o.ResolveUsing(s => s.SourcePropY));

Mapper.CreateMap<SourceType, DestType>() // uses MapFrom
   .ForMember(d => d.DestPropX, o => o.MapFrom(s => s.SourcePropY));

So what ultimately is the difference between the above 2 choices? Is one faster than the other? Is one a better choice than the other and if so, when / why?

Answer 1

In the past I had a long email exchange on the mailing list with the author of Automapper. MapFrom will do null checks all the way trough the expression:

So you can do opt => opt.MapFrom(src => src.SomeProp.Way.Down.Here.Somewhere) and each level will get checked for nulls (as it already does for flattening).