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What is a Data Transfer Object?
In MVC are the model classes DTO, and if not what are the differences and do we need both?
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In the field of programming a data transfer object (DTO) is an object that carries data between processes. The motivation for its use is that communication between processes is usually done resorting to remote interfaces (e.g., web services), where each call is an expensive operation.
DTO, which stands for Data Transfer Object, is a design pattern conceived to reduce the number of calls when working with remote interfaces. As Martin Fowler defines in his blog, the main reason for using a Data Transfer Object is to batch up what would be multiple remote calls into a single one.
A Data Transfer Object (commonly known as a DTO) is usually an instance of a POCO (plain old CLR object) class used as a container to encapsulate data and pass it from one layer of the application to another. You would typically find DTOs being used in the service layer to return data back to the presentation layer.
A DTO is helpful whenever you need to group values in ad hoc structures for passing data around. From a pure design perspective, DTOs are a solution really close to perfection. DTOs help to further decouple presentation from the service layer and the domain model.
A Data Transfer Object is an object that is used to encapsulate data, and send it from one subsystem of an application to another.
DTOs are most commonly used by the Services layer in an N-Tier application to transfer data between itself and the UI layer. The main benefit here is that it reduces the amount of data that needs to be sent across the wire in distributed applications. They also make great models in the MVC pattern.
Another use for DTOs can be to encapsulate parameters for method calls. This can be useful if a method takes more than four or five parameters.
When using the DTO pattern, you would also make use of DTO assemblers. The assemblers are used to create DTOs from Domain Objects, and vice versa.
The conversion from Domain Object to DTO and back again can be a costly process. If you're not creating a distributed application, you probably won't see any great benefits from the pattern, as Martin Fowler explains here.
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