AutoMapper is a great little library every .NET project is using (well, lots of them). I used it for the first time in 2010 and wrote a blog post about it.

Since that time I observed few things:

  • Almost every project I worked on, that needed some kind of object mapping, was using this lib. In rare cases there was some pet library or manual mapping in place.
  • Almost every project had some abstraction over the library like if it was going to be replaced or like different implementation for mapping would be needed.
  • Basic API of the library didn’t change at all. CreateMap and Map are still there and work the same. At the same time performance, testability, exception handling, and feature richness got improved significantly. Last one, in my opinion, is not such a good thing as it leads to the next point.
  • In many of those projects AutoMapper was simply misused as code placed in AfterMap or in different kinds of resolvers would simply start containg crazy things. In worst of those cases actual business logic was written in resolvers.

I have always been of an opinion:

Less Code – Less Bugs; Simple Code – Good Code.

Having seen this trend with the library, I would like to suggest simplifying its usage by limiting ourselves. Simply:

  • Use AutoMapper only for simple mapping. Basically, one property to one property. Preferably, majority of property mapping is done by the same name. If you find yourself in situation when over half of your mappings are specified explicitly in ForMember method it may be the case for doing it manually (at least for the specific type) – it will be cleaner and less confusing.
  • If you have some logic to add to you mapping, do not add it via AutoMapper. Write a separate interface/class and use it (via DI) where your logic has to be applied. You will also be able to test it nicely in this way.
  • Do not abstract AutoMapper behind interfaces/implementations. I’ve seen abstracting this in a way that you need to create a class (empty in many cases) for each mapping type pair and somewhere there would be custom reflection code that initializes all of the mappings. Instead, use built-in AutoMapper Profile class and Mapper.Initialize method. If you still want to have at least some abstraction to avoid referencing AutoMapper everywhere make it simple.

Here is how I’m using AutoMapper these days:

Somewhere in CommonAssembly a very-very simple abstraction (optional):

Somewhere in BusinessLogicAssembly and any other where you want to define mappings (can be split in as many profiles as needed):

Somewhere in startup code in BootstrappingAssembly (Global.asax etc):

And here is the usage:

That’s it. I do not understand why some simple things are made complex.

There is also another advantage of keeping it minimalistic – maintainability. I’m working on a relatively new project that was created from a company’s template, as a result it had older version of AutoMapper abstracted. To upgrade it and keep all old interfaces would mean some work as abstraction used some of the APIs that did change. Instead I threw away all of these abstractions and upgraded the lib. Next time upgrading there simply will be way less code to worry about.

Please let me know if you share the same opinion.

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