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Data Transfer Object (DTO)


A Data Transfer Object (DTO) is an object that is used to encapsulate data and carry data between independent parts of the system or even between different systems (for example between Controllers adn client applications). DTOs are simple objects that do not contain any business logic or behaviour and are only used for storage, retrieval, serialization and deserialization of its own data. DTOs can reduce the number of method calls required between processes.

DTOs are very common in other programming languages:

  • TypeScript -> represented as a type
  • Go -> represented as a type struct
  • Java -> represented as a JavaBean or POJO


Declaring a DTO

Declaring a DTO is fairly simple:

DTO <identifier name with a DTO suffix> { parameters list separated with semicolon } .

Parameters in DTOs are written in the following format:

[optional] parameterType parameterName;

Below you can see examples of DTOs in action.

DTO HelloWorldRequestDTO {
string name;
optional string email;
DTO HelloWorldResponseDTO {
string message;

All parameters are automatically required and appropriate errors will be thrown in the controllers if required parameters are missing.
To make a parameter optional you place the word optional in front of it.

Expressing a DTO

The expression of a declared DTO happens is written as follows:
<DTOName> ( { parameters evaluation list separated with semicolon } ) ; .

Parameters evaluation in DTO are written in the following format:

parameterName: parameterValue;

For example, for the DTOs that have been declared above, the expression could be:

For HelloWorldRequestDTO:

name: 'John Doe',
email: '',

As the parameter email is optional, it could also be:

name: 'John Doe',

For HelloWorldResponseDTO:

message: 'OK',

If a parameter is missing and it is not optional, appropriate errors will be thrown in the controllers.

Finally, a DTO can have nested Structs in it as follows:

DTO PersonDTO {
string firstName;
string lastName;
Address address;

PersonDTO ({
firstName: "Ada",
lastName: "Lovelace",
address: Address({ street: "Bond Street", streetNumber: StreetNumber({number: 28, numberPostfix: "B"}), postCode: "ABC 2F"}),

Further reading