Best Practices For Mapping DTO to Domain Object?

Keeping the mapping logic inside of your entity means that your Domain Object is now aware of an "implementation detail" that it doesn't need to know about. Generally, a DTO is your gateway to the outside world (either from an incoming request or via a read from an external service/database). Since the entity is part of your Business Logic, it's probably best to keep those details outside of the entity.

Keeping the mapping somewhere else would be the only alternative - but where should it go? I've tried introducing mapping objects/services but it seemed like overengineering (and probably was) after all was said and done. I've had some success using Automapper and such for smaller projects but tools like Automapper come with their own pitfalls. I've had some pretty hard-to-find issues related to mappings because Automapper's mappings are implicit and completely decoupled from the rest of your code (not like "separation of concerns" but more like a "where does the godforsaken mapping live") so they can sometimes be hard to track down. Not to say that Automapper doesn't have its uses, because it does. I just think mapping should be something that is as obvious and transparent as possible to avoid issues.

Instead of creating a mapping service layer, I've had a lot of success keeping my mappings inside of my DTOs. Since DTOs always sit at the boundary of the application, they can be made aware of the Business Object and figure out how to map from/to them. Even when the number of mappings scales to a large amount it works cleanly. All the mappings are in one place and you don't have to manage a bunch of mapping services inside your Data Layer, Anticorruption Layer, or Presentation Layer. Instead, the mapping is just an implementation detail delegated to the DTO involved with the request/response. Since serializers generally only serialize properties and fields when sending it across the wire, you shouldn't run into any issues. Personally, I've found this the cleanest option and I can say, in my experience, it scales nicely on a large code base.

If the amount of mappings scales to an unreasonable amount (which has yet to happen to me in over 10 years) then you can always create a mapping class that lives close to your DTOs.

You could use an automapper such as the one written by Jimmy Bogard which has no connection between the objects and relies on naming conventions being adhered to.

A benefit of having a mapper that sits between your domain and your DTO is not as appearent when you are only supporting a single mapping, but as the number of mappings increases, having that code isolated from the domain helps keep the domain simpler and leaner. You won't be cluttering your domain with a lot of extra weight.

Personally, I try and keep the mapping out of my domain entities and put the responsibility in what I call "Manager / Service layer". This is a layer that sits between the application and the respository(ies), and provides business logic such as workflow coordination (If you modify A, you might have to also modify B so service A will work with Service B).

If I had a lot of possible ending formats, I might look at creating a plugable formatter that could use the Visitor pattern, for example to transform my entities, but I've not found a need yet for anything this complex.