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# SOLID Principles in C#: A Practical Guide with Examples
SOLID is a group of five design principles that can make object-oriented C# applications easier to change, test, and maintain. They are guidelines rather than rigid rules.
## 1. Single Responsibility Principle
A class should have one clear responsibility. If a class validates orders, saves them, sends emails, and creates reports, it has several reasons to change.
Split unrelated responsibilities into focused services:
```csharp
public class OrderValidator { }
public class OrderRepository { }
public class OrderNotificationService { }
```
## 2. Open/Closed Principle
Software should generally be open for extension but closed for unnecessary modification. Interfaces and polymorphism can help add behavior without repeatedly changing a large conditional block.
```csharp
public interface IPaymentProcessor
{
Task PayAsync(decimal amount);
}
```
Different payment implementations can then provide the behavior independently.
## 3. Liskov Substitution Principle
A derived implementation should be usable wherever its abstraction is expected without breaking the caller's assumptions.
If a subtype cannot correctly support the contract of its base abstraction, the abstraction may need redesigning.
## 4. Interface Segregation Principle
Prefer small, focused interfaces over large interfaces that force every implementation to support unrelated methods.
For example, separate read and write responsibilities when consumers only need one of them.
## 5. Dependency Inversion Principle
High-level application logic should depend on abstractions rather than concrete infrastructure details.
```csharp
public class OrderService
{
private readonly IOrderRepository _repository;
public OrderService(IOrderRepository repository)
{
_repository = repository;
}
}
```
Dependency injection can then provide the concrete repository.
## Do not over-engineer
SOLID does not mean creating an interface, factory, and service for every class. Introduce abstractions where they solve a real maintenance, testing, or extensibility problem.
## Summary
The five principles are **Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion**. Applied thoughtfully, they help keep C# code modular without turning simple applications into unnecessary layers of abstraction.
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