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:

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Answer 1

I just did some benchmarks using the new C# 6 null conditional operator ?.

Consider the following scenario: class A has a child class B, which has a child C, whose Name property we want to flatten into a DTO. I tested two variants:

// using mapfrom
CreateMap()
    .ForMember(dto => dto.Name, o => o.MapFrom(a => a.B.C.Name));

// using resolveusing with elvis
CreateMap()
    .ForMember(dto => dto.Name, o => o.ResolveUsing(x => x.Y?.Z?.Name));

I called _mapper.Map(x); or _mapper.Map(a); for 1000 different ResolveUsingX x and MapFromA a and took the time using a System.Diagnostics.StopWatch. Here are my results:

Distinct elements per batch: 1000; # batches for average: 25

A->B->C.Name, C is never null.
MapForm - average time taken for 1000x: 5527,84 ticks = 1,44 ms.
ResolveUsing - average time taken for 1000x: 5479,76 ticks =  1,4 ms.

A->B->C.Name, C is null 1/3 of the time.
MapForm - average time taken for 1000x: 72924,4 ticks = 27,44 ms.
ResolveUsing - average time taken for 1000x: 5351,2 ticks =  1,48 ms.

A->B->C.Name, C is null 1/2 of the time.
MapForm - average time taken for 1000x: 107016,92 ticks = 40,52 ms.
ResolveUsing - average time taken for 1000x: 5835,32 ticks =  1,56 ms.

A->B->C.Name, C is null 2/3 of the time.
MapForm - average time taken for 1000x: 141437,96 ticks = 53,64 ms.
ResolveUsing - average time taken for 1000x: 5789,72 ticks =  1,56 ms.

MapFrom has to catch NullReferenceExceptions, which is slower than ResolveUsing with the elvis operator ?.