Implementing Domain-Driven Design

Using Law of Demeter and Tell, Don’t Ask

Both Law of Demeter [Appleton, LoD] and Tell, Don’t Ask [PragProg, TDA] are design principles that can be used when implementing Aggregates, both of which stress information hiding. Consider the high-level guiding principles to see how we can benefit:

• Law of Demeter: This guideline emphasizes the principle of least knowledge. Think of a client object and another object the client object uses to execute some system behavior; refer to the second object as a server. When the client object uses the server object, it should know as little as possible about the server’s structure. The server’s attributes and properties—its shape—should remain completely unknown to the client. The client can ask the server ...