Plain Old CLR Object vs Data Transfer Object

Question

A POCO follows the rules of OOP. It should (but doesn't have to) have state and behavior. POCO comes from POJO, coined by Martin Fowler [anecdote here]. He used the term POJO as a way to make it more sexy to reject the framework heavy EJB implementations. POCO should be used in the same context in .Net. Don't let frameworks dictate your object's design.

A DTO's only purpose is to transfer state, and should have no behavior. See Martin Fowler's explanation of a DTO for an example of the use of this pattern.

Here's the difference: POCO describes an approach to programming (good old fashioned object oriented programming), where DTO is a pattern that is used to "transfer data" using objects.

While you can treat POCOs like DTOs, you run the risk of creating an anemic domain model if you do so. Additionally, there's a mismatch in structure, since DTOs should be designed to transfer data, not to represent the true structure of the business domain. The result of this is that DTOs tend to be more flat than your actual domain.

In a domain of any reasonable complexity, you're almost always better off creating separate domain POCOs and translating them to DTOs. DDD (domain driven design) defines the anti-corruption layer (another link here, but best thing to do is buy the book), which is a good structure that makes the segregation clear.

It's probably redundant for me to contribute since I already stated my position in my blog article, but the final paragraph of that article kind of sums things up:

So, in conclusion, learn to love the POCO, and make sure you don’t spread any misinformation about it being the same thing as a DTO. DTOs are simple data containers used for moving data between the layers of an application. POCOs are full fledged business objects with the one requirement that they are Persistence Ignorant (no get or save methods). Lastly, if you haven’t checked out Jimmy Nilsson’s book yet, pick it up from your local university stacks. It has examples in C# and it’s a great read.

BTW, Patrick I read the POCO as a Lifestyle article, and I completely agree, that is a fantastic article. It's actually a section from the Jimmy Nilsson book that I recommended. I had no idea that it was available online. His book really is the best source of information I've found on POCO / DTO / Repository / and other DDD development practices.

POCO is simply an object that does not take a dependency on an external framework. It is PLAIN.

Whether a POCO has behaviour or not it's immaterial.

A DTO may be POCO as may a domain object (which would typically be rich in behaviour).

Typically DTOs are more likely to take dependencies on external frameworks (eg. attributes) for serialisation purposes as typically they exit at the boundary of a system.

In typical Onion style architectures (often used within a broadly DDD approach) the domain layer is placed at the centre and so its objects should not, at this point, have dependencies outside of that layer.

I wrote an article for that topic: DTO vs Value Object vs POCO.

In short:

• DTO != Value Object
• DTO ⊂ POCO
• Value Object ⊂ POCO

I think a DTO can be a POCO. DTO is more about the usage of the object while POCO is more of the style of the object (decoupled from architectural concepts).

One example where a POCO is something different than DTO is when you're talking about POCO's inside your domain model/business logic model, which is a nice OO representation of your problem domain. You could use the POCO's throughout the whole application, but this could have some undesirable side effect such a knowledge leaks. DTO's are for instance used from the Service Layer which the UI communicates with, the DTO's are flat representation of the data, and are only used for providing the UI with data, and communicating changes back to the service layer. The service layer is in charge of mapping the DTO's both ways to the POCO domain objects.

Update Martin Fowler said that this approach is a heavy road to take, and should only be taken if there is a significant mismatch between the domain layer and the user interface.

TL;DR:

A DTO describes the pattern of state transfer. A POCO doesn't describe much of anything except that there is nothing special about it. It's another way of saying "object" in OOP. It comes from POJO (Java), coined by Martin Fowler who literally just describes it as a fancier name for 'object' because 'object' isn't very sexy and people were avoiding it as such.

Expanding...

Okay to explain this in a far more high-brow way that I ever thought would be needed, beginning with your original question about DTOs:

A DTO is an object pattern used to transfer state between layers of concern. They can have behavior (i.e. can technically be a poco) so long as that behavior doesn't mutate the state. For example, it may have a method that serializes itself.

A POCO is a plain object, but what is meant by 'plain' is that it is not special and does not have any specific requirements or conventions. It just means it's a CLR object with no implied pattern to it. A generic term. I've also heard it extended to describe the fact that it also isn't made to work with some other framework. So if your POCO has [JsonProperty] or EF decorations all over it's properties, for example, then it I'd argue that it isn't a POCO. POCOs are free and unencumbered like the objects you learn to create in school

Here some examples of different kinds of object patterns to compare:

• View Model: used to model data for a view. Usually has data annotations to assist binding and validation for particular view (i.e. generally NOT a shared object), or in this day and age, a particular view component (e.g. React). In MVVM, it also acts as a controller. It's more than a DTO; it's not transferring state, it's presenting it or more specifically, forming that state in a way that is useful to a UI.
• Value Object: used to represent values, should be immutable
• Aggregate Root: used to manage state and invariants. should not allow references to internal entities other than by ID
• Handlers: used to respond to an event/message.
• Attributes: used as decorations to deal with cross-cutting concerns. May only be allowed to be used on certain objects levels (e.g. property but not class, method but not property, etc.)
• Service: used to perform complex tasks. Typically some form of facade.
• Controller: used to control flow of requests and responses. Typically restricted to a particular protocol or acts as some sort of mediator; it has a particular responsibility.
• Factory: used to configure and/or assemble complex objects for use when a constructor isn't good enough. Also used to make decisions on which objects need to be created at runtime.
• Repository/DAO: used to access data. Typically exposes CRUD operations or is an object that represents the database schema; may be marked up with implementation specific attributes. In fact, one of these schema DAO objects is actually another kind of DTO...
• API Contracts: Likely to be marked up with serialization attributes. Typically needs to have public getters and setters and should be lightweight (not an overly complex graph); methods unrelated to serialization are not typical and discouraged.

These can be seen as just objects, but notice that most of them are generally tied to a pattern or have implied restrictions. So you could call them "objects" or you could be more specific about its intent and call it by what it is. This is also why we have design patterns; to describe complex concepts in a few words. DTO is a pattern. Aggregate root is a pattern, View Model is a pattern (e.g. MVC & MVVM).

A POCO doesn't describe a pattern. It is just a different way of referring to classes/objects in OOP which could be anything. Think of it as an abstract concept; they can be referring to anything. IMO, there's a one-way relationship though because once an object reaches the point where it can only serve one purpose cleanly, it is no longer a POCO. For example, once you mark up your class with decorations to make it work with some framework (i.e. 'instrumenting' it), it is no longer a POCO. Therefore I think there are some logical relationships like:

• A DTO is a POCO (until it is instrumented)
• A POCO might not be a DTO
• A View Model is a POCO (until it is instrumented)
• A POCO may not be View Model

The point in making a distinction between the two is about keeping patterns clear and consistent in effort to not cross concerns and lead to tight coupling. For example if you have a business object that has methods to mutate state, but is also decorated to hell with EF decorations for saving to SQL Server AND JsonProperty so that it can be sent back over an API endpoint. That object would be intolerant to change, and would likely be littered with variants of properties (e.g. UserId, UserPk, UserKey, UserGuid, where some of them are marked up to not be saved to the DB and others marked up to not be serialized to JSON at the API endpoint).

So if you were to tell me something was a DTO, then I'd probably make sure it was never used for anything other than moving state around. If you told me something was a view model, then I'd probably make sure it wasn't getting saved to a database. If you told me something was a Domain Model, then I'd probably make sure it had no dependencies on anything outside of the domain. But if you told me something was a POCO, you wouldn't really be telling me much at all other than it is not and should not be instrumented.

History

Paraphrased from Fowler's explanation: In a world where objects were fancy (e.g. followed a particular pattern, had instrumentation etc.), it somehow encouraged people to avoid using not-fancy objects to capture business logic. So they gave it a fancy name POJO. If you want an example, the one he refers to is an "Entity Bean" which is one of those kinds of objects that have very specific conventions and requirements, etc.. If you don't know what that is --> Java Beans.

In contrast, a POJO/POCO is just the regular ole object that you'd learn out to create in school.