Purpose of Class Diagram vs real world code

In the class diagram, we have entities. We define attributes, methods on entities and then we define relationship between entities.
But in the real world application, when we code, we usually use the entities as either POCO or DTO (data transfer objects), and the business logic usually goes into the services part.
Why there is this mismatch ? What is the need of class diagrams then ? How to represent these services in the class diagrams ?

Hi @ANKIT_BAJPAI , Class diagrams are a way to model the structure of an application, including its objects and their relationships. They are a valuable tool for visualizing and documenting a system’s design and can help ensure that it is consistent, complete, and correct.

There is a mismatch between the entities in class diagrams and the entities used in code because class diagrams are abstract system representations. In contrast, code is a concrete implementation of that system. In a class diagram, you model the entities in the system and their relationships without considering the implementation details, such as the actual code that implements the entities and their behavior.

In the real world, entities are often implemented as POCO (Plain Old CLR Object) or DTO (Data Transfer Object) classes, which are used to store data, or as services that contain the system’s business logic.

In summary, class diagrams are a valuable tool for modeling the structure of a system, but they do not necessarily represent the concrete implementation of that system in code. To fully understand the structure and behavior of a system, both class diagrams and code implementation must be considered.

Hope it helps!