Brief description

Defines an object to transport modifications to a target collection.

What does it solve?

When working in a multi-layered architecture, an application layer may need to modify the model (or, in DDD, a domain model or a domain object). Some models/domain objects may contain collection associations:

// Sample Customer class in C#
public class Customer
	public IList<string> EmailAddresses { get; set; }

Usually the application layer modifies the model/domain using data-transfer objects (DTOs):

public class CustomerDto 
	// Sample Customer DTO
	public IList<string> EmailAddresses { get; set; }

And a big problem arises: a simple collection-to-collection mapping from the DTO to the model wouldn’t be enough, because otherwise it would mean that the DTO should transport the entire source collection to avoid losing data.

For example, let’s say that an already persisted Customer has email addresses ["", "", ""]. When the whole Customer is sent to the application layer, it comes with the all EmailAddresses, and they’re bound to the user interface. The user removes an email and saves the Customer. How does the model/domain know what email address was removed?

  • A valid approach would be computing an intersection of DTO’s EmailAddresses and target model/domain object EmailAddresses counterpart.
  • Another valid approach would be just mapping the DTO’s EmailAddresses collection to the model/domain object one..

Both approaches have a big drawback: client must send the entire EmailAddresses collection as part of Customer DTO, while it would be nice that the DTO itself could express just drop this email address instead of re-sending the rest of email addresses that they are already persisted.


A valid approach to solve the problem described before could be a conventional DTO that splits a single collection into three collections:

  • A collection of items to be created.
  • A collection of items to be updated.
  • A collection of items to be deleted.

That is, the DTO can express which items should be created, updated or deleted and the model/domain will map proposed changes to persist requested changes.