What are the SOLID principles?

Question

Accepted Answer

**SOLID** — five design principles for maintainable code.

**S — Single Responsibility:**
```csharp
// Each class has one reason to change
public class UserService { public void CreateUser(User u) { } }
public class EmailService { public void SendEmail(User u) { } }
```

**O — Open/Closed:**
```csharp
// Open for extension, closed for modification
public abstract class Shape { public abstract double Area(); }
public class Circle : Shape { public override double Area() => ...; }
```

**L — Liskov Substitution:**
```csharp
// Subtypes must be substitutable for base types
// If S is subtype of T, then objects of T can be
// replaced with objects of S without breaking behavior
```

**I — Interface Segregation:**
```csharp
// Many specific interfaces > one general interface
public interface IReadable { string Read(); }
public interface IWritable { void Write(string data); }
```

**D — Dependency Inversion:**
```csharp
// Depend on abstractions, not concretions
public class OrderService
{
    private readonly IOrderRepository _repo;  // Abstraction
    public OrderService(IOrderRepository repo) => _repo = repo;
}
```

C#: What are the SOLID principles?