SOLID is an acronym that stands for five fundamental principles of object-oriented programming and design. These principles were first introduced by Robert C. Martin (also known as Uncle Bob) and have since become a cornerstone of software development best practices. Each letter in SOLID represents a principle that, when applied correctly, leads to more maintainable and modular code.


Let’s dive into each of the SOLID principles and understand how they contribute to building high-quality software systems:


Single Responsibility Principle (SRP):

The SRP states that a class should have only one reason to change. In other words, a class should have a single responsibility or a single job. This principle encourages developers to break down complex systems into smaller, cohesive modules. By ensuring that each class has a focused responsibility, it becomes easier to understand, test, and modify the code without affecting other parts of the system.


Open-Closed Principle (OCP):

The OCP promotes the idea that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This principle emphasizes the importance of designing systems that can be easily extended with new functionality without modifying existing code. By relying on abstractions, interfaces, and inheritance, developers can add new features by writing new code rather than changing the existing one. This approach reduces the risk of introducing bugs or unintended side effects.


Liskov Substitution Principle (LSP):

The LSP states that objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program. In simpler terms, if a class is a subtype of another class, it should be able to be used interchangeably with its parent class without causing any issues. This principle ensures that inheritance hierarchies are well-designed, avoiding situations where subclass behavior contradicts or breaks the functionality defined by the superclass. Adhering to the LSP leads to more flexible and reusable code.


Interface Segregation Principle (ISP):

The ISP advises developers to design interfaces that are specific to the needs of the clients that use them. It suggests that clients should not be forced to depend on interfaces they don’t use. By creating small and focused interfaces, rather than large and monolithic ones, the ISP enables clients to be decoupled from unnecessary dependencies. This principle enhances modularity, testability, and maintainability, as changes to one part of the system are less likely to impact other parts.


Dependency Inversion Principle (DIP):

The DIP encourages high-level modules to depend on abstractions rather than concrete implementations. It states that high-level modules should not depend on low-level modules; both should depend on abstractions. This principle promotes loose coupling between components, making it easier to substitute or modify dependencies without affecting the overall system. By relying on interfaces or abstract classes, the DIP facilitates extensibility, testability, and the ability to adapt to changing requirements.


By applying the SOLID principles, software engineers can create codebases that are modular, flexible, and easy to maintain. These principles provide a roadmap for designing systems that are resilient to change, promote code reusability, and improve collaboration among development teams. SOLID principles are not strict rules but rather guidelines that help developers make informed design decisions and create high-quality software systems.


It’s worth mentioning that the SOLID principles should not be applied blindly in all situations. Context matters, and there may be scenarios where strict adherence to one principle might not be the best approach. However, understanding and incorporating these principles into the software design process can significantly improve the overall quality of the codebase.




XPO is designed with SOLID design principles in mind. Here’s how XPO applies each of the SOLID principles:


Single Responsibility Principle (SRP)


XPO uses separate classes for each major concern such as mapping, persistence, connection providers, and data access. Each class has a clearly defined purpose and responsibility.


Open-Closed Principle (OCP)


XPO is extensible and customizable, allowing you to create your own classes and derive them from the XPO base classes. XPO also provides a range of extension points and hooks to allow for customization and extension without modifying the core XPO code.


Liskov Substitution Principle (LSP)


XPO follows this principle by providing a uniform API that works with all persistent objects, regardless of their concrete implementation. This allows you to write code that works with any persistent object, without having to worry about the specific implementation details.


Interface Segregation Principle (ISP)


XPO provides a number of interfaces that define specific aspects of its behavior, allowing clients to use only the interfaces they need. This reduces the coupling between the clients and the XPO library.


Dependency Inversion Principle (DIP)


XPO was developed prior to the widespread popularity of Dependency Injection (DI) patterns and frameworks.


As a result, XPO does not incorporate DI as a built-in feature. However, this does not mean that XPO cannot be used in conjunction with DI. While XPO itself does not provide direct support for DI, you can still integrate it with popular DI containers or frameworks, such as the .NET Core DI container or any other one.


By integrating XPO with a DI container, you can leverage the benefits of DI principles in your application while utilizing XPO’s capabilities for database access and mapping. The DI container can handle the creation and management of XPO-related objects, facilitating loose coupling, improved modularity, and simplified testing.


A clear example is the XPO Extensions for ASP.NET Core DI:

using DevExpress.Xpo.DB; 
Using Microsoft.Extensions.DependencyInjection; // ...

var builder = WebApplication.CreateBuilder(args); builder.Services.AddXpoDefaultUnitOfWork(true, options => options.UseConnectionString(builder.Configuration.GetConnectionString("MSSqlServer")) .UseAutoCreationOption(AutoCreateOption.DatabaseAndSchema) .UseEntityTypes(new Type[] { typeof(User) }));

More information about XPO and DI here:

And that’s all for this post, until next time ))

We are excited to announce that we are currently in the process of writing a comprehensive book about DevExpress XPO. As we work on this project, we believe it is essential to involve our readers and gather their valuable feedback. Therefore, we have decided to share articles from the book as we complete them, giving you an opportunity to provide input and suggestions that we can consider for inclusion in the final release. Keep in mind that the content presented is subject to change. We greatly appreciate your participation in this collaborative effort.

Related Articles

What is an O.R.M (Object-Relational Mapping)

ADO The origin of data access in .NET

Relational database systems: the holy grail of data