DTO objects are used to deserialize data into objects from different sources. Those objects are NOT your Model (POCO) objects. You need to transform those objects into your Model (POCO) objects. The transformation is mostly a copy operation. You can fill those POCO objects directly from the source if its an internal source, but its not adviceable if its an external source. External sources have API's with descriptions of the Schema they use. Its much easier then to load the request data in an DTO and after that transform those in your POCO's. Yes its an extra step, but with a reason. The rule is to load the data from your source in an object. It can be JSON, XML whatever. When loaded then transform that data in what you need in your model. So most of times the DTO is an object image of the external source. Sometimes you even get the Schema's of the source providers then you can deserialize even easier, XML works like that with XSD's.

TL; diana:

DTO描述状态传输的模式。POCO并没有描述太多东西，除了它没有什么特别之处。这是OOP中“对象”的另一种说法。它来自POJO (Java)，由Martin Fowler创造，他只是把它描述为“对象”的一个更花哨的名字，因为“对象”不是很性感，人们都在回避它。

扩大……

好吧，用一种我认为需要的更高尚的方式来解释这个问题，从你最初关于dto的问题开始:

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. For it to be a proper DTO, it needs to be a simple property bag; it needs to be clear that this object is not a strong model, it has no implied semantic meaning, and it doesn't enforce any form of business rule or invariant. It literally only exists to move data around.

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 a bunch of EF decorations all over it's properties, for example, then it I'd argue that it isn't a simple POCO and that it's more in the realm of DAO, which I would describe as a combination of DTO and additional database concerns (e.g. mapping, etc.). POCOs are free and unencumbered like the objects you learn to create in school

这里有一些不同类型的对象模式的例子来比较:

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.

可以将它们仅仅视为对象，但请注意，它们中的大多数通常绑定到一个模式或具有隐含的限制。所以你可以称它们为"对象"或者你可以更具体地描述它的意图，用它是什么来称呼它。这也是为什么我们有设计模式;用几句话描述复杂的概念。DTO是一种模式。聚合根是一个模式，视图模型是一个模式(例如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:

DTO是POCO(直到它被插入) POCO可能不是DTO 视图模型是一个POCO(直到它被仪器化) POCO可能不是视图模型

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, and I'd know that it's ok to put 'hacky' things in there to make sure the data is usable by a UI. 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 and certainly no dependencies on any technical implementation details (databases, services etc.), only abstractions. 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.

历史

转述Fowler的解释:在一个对象很花哨的世界里(例如，遵循特定的模式，有仪器等)，它在某种程度上鼓励人们避免使用不花哨的对象来捕获业务逻辑。所以他们给它起了个花哨的名字POJO。如果你想要一个例子，他提到的是“实体Bean”，这是一种具有非常特定的约定和需求的对象。如果你不知道那是什么——> Java Beans。

相比之下，POJO/POCO只是您在学校学习创建的常规ole对象。

甚至不要称它们为dto。他们被称为模型....句号。模特从来没有行为。我不知道是谁想出了这个愚蠢的术语DTO，但它一定是一个。net的东西，这是我所能想到的。想想MVC中的视图模型，同样的东西，模型是用来在层与层之间传递状态的服务器端或线上传输，它们都是模型。属性与数据。这些是你传递的模型。模特，模特。就是这样。

我希望DTO这个愚蠢的术语能从我们的词汇表中消失。

我已经在我的博客文章中阐述了我的立场，所以我在这里发表评论可能是多余的，但那篇文章的最后一段似乎总结了一些事情:

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.

顺便说一句，帕特里克，我把POCO当作一篇生活方式的文章来读，我完全同意，这是一篇很棒的文章。这实际上是我推荐的吉米·尼尔森书中的一个章节。我不知道网上也有。他的书确实是我在POCO / DTO / Repository /和其他DDD开发实践方面找到的最好的信息来源。

我认为DTO可以是POCO。DTO更多地是关于对象的使用，而POCO更多地是关于对象的风格(与体系结构概念分离)。

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.

Martin Fowler说这种方法是一条艰难的道路，只有在域层和用户界面之间存在严重不匹配的情况下才应该采用。

POCO遵循面向对象的规则。它应该(但不必)具有状态和行为。POCO来自于POJO，由Martin Fowler创造。他使用术语POJO作为一种方式，使其更性感地拒绝框架繁重的EJB实现。在. net中，POCO应该在相同的上下文中使用。不要让框架支配你的对象设计。

DTO的唯一目的是传输状态，不应该有任何行为。请参阅Martin Fowler对DTO的解释，以获得使用此模式的示例。

区别在于:POCO描述了一种编程方法(老式的面向对象编程)，而DTO是一种使用对象来“传输数据”的模式。

虽然您可以像对待dto一样对待poco，但是如果您这样做，就会有创建贫血域模型的风险。此外，还存在结构上的不匹配，因为dto应该被设计为传输数据，而不是代表业务领域的真实结构。这样做的结果是dto往往比您的实际域更平坦。

在任何合理复杂的域中，最好创建单独的域poco并将它们转换为dto。DDD(领域驱动设计)定义了反腐败层(这里有另一个链接，但最好的方法是购买这本书)，这是一个很好的结构，使隔离清晰。