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23. SOLID Principles

3 min read

SOLID Principles

SOLID is an acronym representing five key principles of object-oriented design:

1. Single Responsibility Principle (SRP)

A class should have only one reason to change.

  • Promotes focused and maintainable classes.

2. Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

  • Encourages adding new features without changing existing code.

3. Liskov Substitution Principle (LSP)

Objects of a derived class should be substitutable for objects of the base class without affecting the correctness of the program.

  • Ensures that inheritance relationships are used appropriately.

4. Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

  • Promotes smaller, more focused interfaces.

5. Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Abstractions should not depend on details. Details should depend on abstractions.

  • Encourages loose coupling and flexibility.

Benefits of SOLID Principles

  • Maintainability: Makes your code easier to understand, modify, and extend.
  • Testability: Promotes writing unit tests by encouraging loose coupling and dependency injection.
  • Flexibility: Makes your code adaptable to changes in requirements.
  • Reusability: Encourages the creation of reusable components.

Example Code

SRP (Single Responsibility Principle)

public class ProductService
{
    // Handles product-related logic, like adding or retrieving products.
}
 
public class OrderService
{
    // Handles order-related logic, like creating or processing orders.
}

OCP (Open/Closed Principle)

public interface IPaymentProcessor
{
    void ProcessPayment(PaymentDetails details);
}
 
public class CreditCardPaymentProcessor : IPaymentProcessor { /* ... */ }
public class PayPalPaymentProcessor : IPaymentProcessor { /* ... */ }

LSP (Liskov Substitution Principle)

public class Rectangle
{
    public virtual int Width { get; set; }
    public virtual int Height { get; set; }
}
 
public class Square : Rectangle // Violates LSP
{
    public override int Width
    {
        get => base.Width;
        set
        {
            base.Width = value;
            base.Height = value; // Setting width also sets height
        }
    }
 
    public override int Height
    {
        get => base.Height;
        set
        {
            base.Height = value;
            base.Width = value; // Setting height also sets width
        }
    }
}

ISP (Interface Segregation Principle)

public interface IPrinter
{
    void Print();
}
 
public interface IScanner
{
    void Scan();
}
 
public class PrintScanMachine : IPrinter, IScanner { /* ... */ }

DIP (Dependency Inversion Principle)

public class OrderProcessor
{
    private readonly IPaymentProcessor _paymentProcessor;
 
    public OrderProcessor(IPaymentProcessor paymentProcessor)
    {
        _paymentProcessor = paymentProcessor;
    }
 
    // ...
}

Interview Tips

  • Understanding: Be able to explain each principle clearly and concisely.
  • Examples: Provide real-world or code examples that demonstrate how to apply each principle.
  • Benefits: Articulate the advantages of adhering to SOLID principles.
  • Trade-offs: Acknowledge that there might be trade-offs in applying these principles in certain situations.
  • Practical Application: Discuss how you have used or would use SOLID principles in your own projects.

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