25 Scenario-Based .NET SOLID Principles Interview Questions

When interviewing for a .NET developer position, one of the key areas you’re likely to encounter is object-oriented design, particularly the SOLID principles. SOLID is a set of five design principles that aim to make software designs more understandable, flexible, and maintainable. As these principles are foundational to writing clean, efficient code, knowing how to apply them in real-world scenarios is critical.

In this blog, we’ll dive into 25 scenario-based interview questions focused on .NET’s SOLID principles, helping you prepare for your next interview and sharpening your understanding of object-oriented design.

1. Single Responsibility Principle (SRP)

Question 1:

You’re working on an e-commerce application. You have a UserService class that handles user registration, email sending, and payment processing. How would you refactor the class to adhere to SRP?

Answer:

The UserService class has multiple responsibilities: user registration, email sending, and payment processing. To follow SRP, you should refactor it by creating separate classes such as UserRegistrationService, EmailService, and PaymentService. Each class should focus on a single responsibility.

2. Open/Closed Principle (OCP)

Question 2:

You are working on a reporting feature in a .NET application. The current ReportGenerator class generates various types of reports. How would you modify this class to comply with OCP?

Answer:

To follow OCP, you should make ReportGenerator class open for extension but closed for modification. Instead of modifying the class every time a new report type is added, you can use inheritance or interfaces to extend the class. For example, create different classes for each report type that implement a common IReport interface.

3. Liskov Substitution Principle (LSP)

Question 3:

You are designing an application where the Rectangle class is subclassed into Square. What potential issue could arise in violating Liskov Substitution Principle here, and how would you fix it?

Answer:

If Rectangle has methods like SetHeight() and SetWidth(), and Square overrides those methods, it could break the behavior expected from a Rectangle. A square is a specific case of a rectangle, but the two classes should not share a parent-child relationship in this case. Instead, Shape can be the common base class for both Rectangle and Square.

4. Interface Segregation Principle (ISP)

Question 4:

You have a class Printer that implements both IPrint and IFax interfaces. The Printer class is being used by several clients, but some clients only need printing functionality, and others only need faxing. How would you modify this setup to comply with ISP?

Answer:

You should break the IPrint and IFax interfaces into smaller, more specific interfaces, such as IPrintOnly and IFaxOnly. This way, classes only implement the interfaces they actually need, adhering to ISP.

5. Dependency Inversion Principle (DIP)

Question 5:

Your current class, OrderProcessor, depends directly on the SqlDatabase for saving order information. How can you refactor this code to adhere to DIP?

Answer:

To adhere to DIP, you should introduce an abstraction, such as an IDatabase interface, and make OrderProcessor depend on this interface instead of directly on the SqlDatabase class. Then, the specific database implementation (SqlDatabase) can be injected into OrderProcessor via constructor injection, enabling flexibility in the underlying database used.

6. Single Responsibility Principle (SRP)

Question 6:

You have a StudentManager class that handles student data processing, student reporting, and student database updates. How would you refactor this to follow SRP?

Answer:

To follow SRP, refactor the StudentManager class by separating responsibilities into distinct classes, like StudentDataProcessor, StudentReportGenerator, and StudentDatabaseUpdater. Each class should handle only one responsibility.

7. Open/Closed Principle (OCP)

Question 7:

You are building a payment system with different payment methods (Credit Card, PayPal, etc.). How can you modify the code to comply with OCP?

Answer:

You can implement an IPaymentMethod interface with a ProcessPayment method. Then, create separate classes for each payment method, like CreditCardPayment, PaypalPayment, etc. This way, you can add new payment methods without changing the existing codebase.

8. Liskov Substitution Principle (LSP)

Question 8:

You’re designing a system with a Bird class and two subclasses: Duck and Penguin. The Fly() method is defined in Bird. Would it violate LSP, and how would you address this issue?

Answer:

Since penguins cannot fly, having a Fly() method in the Bird class would violate LSP. Instead, you could introduce an interface like IFlyable and implement it in only those bird classes that can fly (e.g., Duck).

9. Interface Segregation Principle (ISP)

Question 9:

You have an interface IEmployee that includes methods for CalculateSalary, SubmitLeave, and ApproveLeave. Some classes only need CalculateSalary, while others only need SubmitLeave. How would you refactor this to follow ISP?

Answer:

You can split the IEmployee interface into smaller interfaces, such as ILeaveManagement and IPayrollManagement. This way, classes will only implement the methods they actually need.

10. Dependency Inversion Principle (DIP)

Question 10:

You have a ShoppingCart class that directly instantiates the InventoryService. How would you refactor this to follow DIP?

Answer:

You should introduce an IInventoryService interface and inject it into the ShoppingCart class via constructor injection. This way, ShoppingCart depends on the abstraction, not the concrete InventoryService class.

11. Single Responsibility Principle (SRP)

Question 11:

You are working on a large application where the FileHandler class both reads files and logs actions to a database. How would you refactor this to follow SRP?

Answer:

To follow SRP, you should create separate classes like FileReader for handling file operations and ActionLogger for logging to the database. This way, each class handles only one responsibility.

12. Open/Closed Principle (OCP)

Question 12:

You are building a content management system with various content types (e.g., Articles, Blogs, News). How would you refactor the content management system to comply with OCP?

Answer:

You could define a base Content class or an interface and then create subclasses like ArticleContent, BlogContent, and NewsContent. This would allow you to add new content types without modifying the existing codebase.

13. Liskov Substitution Principle (LSP)

Question 13:

You have a class Employee with methods like GetSalary() and GetWorkHours(). You subclass it to create a Manager class, but managers don’t track work hours. How would you modify the design to adhere to LSP?

Answer:

To adhere to LSP, you can create a separate `WorkHoursTracker` class or interface, which can be implemented by employees who need to track hours. The `Manager` class would not need to track hours and could omit that feature.

14. Interface Segregation Principle (ISP)

Question 14:

You are building a notification system. There is a single interface INotification that includes methods for sending email, SMS, and push notifications. How would you refactor this to comply with ISP?

Answer:

To follow ISP, you should break the INotification interface into smaller, more specific interfaces like IEmailNotification, ISmsNotification, and IPushNotification. This way, classes only implement the interfaces they actually need.

15. Dependency Inversion Principle (DIP)

Question 15:

You are designing an application where a `LoggingService` class is instantiated inside the `UserService` class. How would you refactor this to adhere to DIP?

Answer:

You should introduce an interface like ILogger and inject the dependency via constructor injection. The UserService class would then depend on the abstraction, not the concrete implementation of the LoggingService.

16. Single Responsibility Principle (SRP)

Question 16:

You have a class OrderProcessor that handles both order validation and payment processing. How would you refactor this to follow SRP?

Answer:

To follow SRP, you should split the responsibilities by creating separate classes like OrderValidator and PaymentProcessor. This way, each class focuses on only one responsibility.

17. Open/Closed Principle (OCP)

Question 17:

You are building a reporting system with different report formats (e.g., PDF, Excel). How can you modify the code to comply with OCP?

Answer:

Use an interface like IReport with a method GenerateReport. Implement this interface in separate classes for each format, such as PdfReport and ExcelReport. This allows you to add new formats without modifying the existing code.

18. Liskov Substitution Principle (LSP)

Question 18:

You are building a shipping system with a base class ShippingMethod and subclasses StandardShipping and ExpressShipping. How can you ensure LSP compliance when adding new shipping methods?

Answer:

To comply with LSP, ensure that subclasses of ShippingMethod adhere to the same behavior expectations. If a subclass overrides a method, ensure it maintains the original behavior so that clients of ShippingMethod can use subclasses interchangeably without issues.

19. Interface Segregation Principle (ISP)

Question 19:

Your codebase includes an IMultiFunctionPrinter interface that has methods for printing, scanning, and faxing. Some printers only support printing. How would you refactor this to follow ISP?

Answer:

Break IMultiFunctionPrinter into smaller interfaces such as IPrinter, IScanner, and IFax. This allows clients to implement only the functionality they need.

20. Dependency Inversion Principle (DIP)

Question 20:

Your code directly references the HttpClient class for making API calls. How would you refactor this to follow DIP?

Answer:

Introduce an interface IHttpClient and make the code depend on this abstraction. Then, use dependency injection to provide the concrete HttpClient implementation. This improves testability and flexibility.

21. Single Responsibility Principle (SRP)

Question 21:

You have a class InvoiceGenerator that handles invoice creation, tax calculation, and emailing the invoice to the customer. How would you refactor this to follow SRP?

Answer:

To follow SRP, you should split the responsibilities into separate classes: InvoiceCreator for creating invoices, TaxCalculator for calculating taxes, and EmailService for emailing invoices. Each class should handle only one responsibility.

22. Open/Closed Principle (OCP)

Question 22:

You are working on a notification system that currently supports email notifications. You now need to add support for SMS notifications. How would you modify the system to comply with OCP?

Answer:

To comply with OCP, you should define an INotification interface with a SendNotification method. Then, implement separate classes for each notification type, such as EmailNotification and SmsNotification. You can easily add new types of notifications by extending the system without modifying the existing code.

23. Liskov Substitution Principle (LSP)

Question 23:

You have a base class Shape with a method CalculateArea(), and two subclasses: Rectangle and Circle. How would you ensure LSP compliance?

Answer:

Ensure that both subclasses override the CalculateArea() method and return the expected results. The behavior of subclasses should match the base class expectations, so that you can substitute a Rectangle or Circle anywhere a Shape is used, without breaking the system.

24. Interface Segregation Principle (ISP)

Question 24:

You are working with a system where the interface IVehicle includes methods for Drive, Fly, and Swim. You have a class Car and a class Boat. How would you refactor this system to follow ISP?

Answer:

To comply with ISP, split IVehicle into multiple interfaces, such as IDriveable, IFlyable, and ISwimmable. This way, Car will implement only IDriveable, while Boat will implement ISwimmable. This ensures that classes only implement the methods relevant to their functionality.

25. Dependency Inversion Principle (DIP)

Question 25:

You have a service OrderService that directly instantiates a PaymentGateway class. How would you refactor this to follow DIP?

Answer:

To follow DIP, introduce an interface like IPaymentGateway and inject it into the OrderService class via constructor injection. This allows OrderService to depend on the abstraction rather than the concrete PaymentGateway class. It also makes it easier to swap out different payment gateway implementations in the future.