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Deep Thought

Data Transfer Objects

Data Transfer Objects (DTO)

What

Data transfer objects - as the name implies - are objects that are used to transfer data.

Very often this means sending data over a network boundary - for example via a REST-API.

Data transfer objects solely hold data values. While classes or records that are used to implement DTOs can have additional functionality as methods, the main purpose is to (de-)serialize the data fields of DTOs so they can be transfered to its destination.

Usually DTOs don’t offer functionality that goes beyond storage, retrieval and (de-)serialization of their own data.

Implementation

Commonly DTOs are implemented as simple classes with only properties (POCOs) or as record types.

public class Person {
  public string Name { get; set; }
}
// or
public record Person(string Name);

DTOs for REST API Design

DTOs can be very useful when designing REST APIs. Consider this data model:

ErDiagram.png

Entities in APIs

When using entities (for accessing and storing data) you might and up with something like this:

public class Student {
  [Key]
  public int Id { get; set; }
  public string Name { get; set; }
  public string Email { get; set; }


  // Navigation Property
  public Collection<Grade> Grades { get; set; } = [];
}

When using the entity above in your REST API you might end up with a POST Endpoint that would require you to send data in this form when adding a new student:

{
  "id": 1,
  "name": "Hugo",
  "email": "hugo@provider.com",
  "grades": []
}

This raises at least 2 questions:

  • Why do I need to specify an ID for the student? Shouldn’t the server take care of that?
  • Why do I need to specify a list of grades for that student when I just want to add a student without any grades yet.

Creating DTOs for different endpoints

TODO