AutoMapper：MapFrom和ResolveUsing有什麼區別？

Question

忽略ResolveUsing重載採取IValueResolver，並且僅在這2種方法尋找：

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

這些2之間的主要差別似乎是ResolveUsing需要Func<TSource, object>，而MapFrom需要一個Expression<Func<TSource, TMember>>。

然而，在實際使用的這些方法lambda表達式一個客戶端代碼，他們似乎是可互換的：

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)); 

那麼最終是上述2個選擇之間的區別？一個比另一個快嗎？一個是比另一個更好的選擇，如果是的話，何時/爲什麼？

Answer 1

在過去，我有一個long email exchange on the mailing list與作者Automapper。 MapFrom會做null檢查一路低谷表達：

所以，你可以做opt => opt.MapFrom(src => src.SomeProp.Way.Down.Here.Somewhere)和每個級別將得到空值檢查 （因爲它已經這樣做了扁平化）。

Answer 2

MapFrom有a few extra smarts。例如（從mailing list）：

在MapFrom，我自作聰明約在深入瞭解孩子的屬性（很像正常的扁平一樣）。 MapFrom試圖模仿扁平化，並增加了一些允許重定向的功能。 ResolveUsing沒有這種行爲。

我不知道這是否完全是documented任何地方（除了在source code）。

Answer 3

根據源代碼，ResolveUsing比較複雜。源值可以是任何對象;因此，您可以使用任何想要填充目標成員的值，例如通過「解析」給定對象獲得的int或bool。然而，MapFrom只使用成員來映射。

/// <summary> 
/// Resolve destination member using a custom value resolver callback. Used instead of MapFrom when not simply redirecting a source member 
/// This method cannot be used in conjunction with LINQ query projection 
/// </summary> 
/// <param name="resolver">Callback function to resolve against source type</param> 
void ResolveUsing(Func<TSource, object> resolver); 

/// <summary> 
/// Specify the source member to map from. Can only reference a member on the <typeparamref name="TSource"/> type 
/// This method can be used in mapping to LINQ query projections, while ResolveUsing cannot. 
/// Any null reference exceptions in this expression will be ignored (similar to flattening behavior) 
/// </summary> 
/// <typeparam name="TMember">Member type of the source member to use</typeparam> 
/// <param name="sourceMember">Expression referencing the source member to map against</param> 
void MapFrom<TMember>(Expression<Func<TSource, TMember>> sourceMember); 

Answer 4

我只是用新的C＃6 null conditional operator?.

考慮以下情形做了一些基準測試：類A有一個子類B，其中有一個孩子C，我們要壓扁成其Name財產一個DTO。我測試了兩個變種：

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

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

我叫_mapper.Map<ResolveUsingXDto>(x);或_mapper.Map<MapFromADto>(a); 1000不同ResolveUsingX x和MapFromA a和使用System.Diagnostics.StopWatch花時間。下面是我的結果：

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已經趕上NullReferenceException異常，這是比慢ResolveUsing與貓王操作?.

Answer 5

雖然在很多情況下，都可以採用，基於official documentation是有區別的，當它來LINQ預測。詳細的解釋可以在here找到。

長話短說：儘可能使用MapFrom。