C# Web Services SOAP – Multiple DTOs vs Single DTO vs Primitives

The "verb" and "noun" terminology is somewhat unfortunate here. As you already mentioned, you can easily create object for function. All object oriented languages except Java have that transformation built-in and in Java you end up doing it all the time anyway ending up with lots of objects with single method and often one called "invoke", "execute", "apply" or somesuch (so it's the programming languages where the "verb"/"noun" distinction actually doesn't make sense).

The "verbs" of REST is more like classifying your methods to getters, setters (deleters; can be considered kind of setters) and other. And trying to do everything with getters and setters. The reason for this is:

1. Easier semantics in face of communication failure, since both getters and setters are idempotent. Getting the resource twice has no additional effect and nor does setting it to the value it already has.
2. Defining some semantics that can be used by possibly caching proxy that does not understand the specific interface. Getters are cached and setters are known to invalidate the cache.

HTTP was designed from the beginning with both caches and fault-tolerance in mind, so these two points lead to it's four basic methods:

• GET is a getter. It's assumed not to modify server state and return the same value each time with possibility to specify expiration and revalidation policy.
• PUT and DELETE are the setter and deleter (= setter with nil). They are not normally used in context of normal web, but make sense for REST.
• POST is a generic "invoke" kitchen sink for which caches can assume nothing.

REST is a design pattern describing how to use raw HTTP or similar network protocols to implement interface that allows easy handling of failures by simple retrying and works nicely with caching proxies.

It doesn't correspond easily to regular object-oriented programming API. I think it is actually a good thing. The challenges of interfacing over network, which is inherently unreliable and where round-trips are much slower than transferring even moderate amount of data call for different design approach than in-process API, so when it looks different, people don't try to apply invalid experience from the other domain that much (that's the bane of SOAP, XML-RPC and such; it looks like procedure calls, but doesn't work like it and ends up being pain to work with).

The key is "are never going to be used outside of the class"

Structure doesn't matter when the implementation remains private. You're free to change/modify it however you want because nobody will depend on the structure.

Public members/interfaces OTOH, do matter. When you expose a member publically/internally, you should have an explicit reason for doing so because somebody may/will depend on it in the future. That's the dividing line between application and API development. In APIs access control becomes very important because any member/interface that people depend on is one that can't be changed/removed without breaking somebody's code.

In essence:

your private parts are nobody's business but your own