45 Scenario-Based Interview Questions on .NET Core Design Patterns to Ace Your Next Interview

When preparing for a .NET Core developer interview, it’s essential to be well-versed in design patterns. Design patterns help solve common software design problems, and they are a key part of building scalable, maintainable, and efficient applications. This article explores 45 scenario-based interview questions on .NET Core design patterns, which can help you hone your skills and get ready for your next big interview.

1. Singleton Pattern

Question: Imagine you’re building a logging service for your .NET Core application. How would you implement it using the Singleton pattern?

Answer: The Singleton pattern ensures that a class has only one instance and provides a global access point. In the case of a logging service, the class would instantiate once, and subsequent calls would return the same instance.

2. Factory Method Pattern

Question: How would you implement the Factory Method pattern to create different types of database connection classes in a .NET Core application?

Answer: The Factory Method pattern defines an interface for creating objects, but allows subclasses to alter the type of objects that will be created. This pattern is perfect when we want to create different types of database connections (e.g., SQL, NoSQL) based on user input or configuration.

3. Abstract Factory Pattern

Question: In a .NET Core e-commerce application, how would you implement the Abstract Factory pattern to create different types of payment processors (e.g., PayPal, Stripe)?

Answer: The Abstract Factory pattern allows the creation of related objects without specifying their concrete classes. We can create a PaymentFactory interface, and then implement concrete classes for each payment processor (PayPal, Stripe) that create related objects such as PaymentGateway, Invoice, etc.

4. Builder Pattern

Question: If you’re developing a complex HTTP request builder for an API client, how could the Builder pattern help?

Answer: The Builder pattern allows us to construct a complex object step-by-step. We can create an API client builder that sets HTTP headers, request body, authentication, etc., before constructing the final request object.

5. Prototype Pattern

Question: How would you use the Prototype pattern to clone a complex Order object in a .NET Core application?

Answer: The Prototype pattern allows objects to be cloned to create new instances. In the case of an Order object, we could implement a Clone() method to copy the object’s state and return a new instance, allowing us to create copies of existing orders.

6. Observer Pattern

Question: How would you implement the Observer pattern to notify users when a new product is added to the catalog in a .NET Core application?

Answer: In the Observer pattern, one object (the subject) maintains a list of observers. When a new product is added, it notifies all subscribed observers. In this case, we could create a ProductCatalog class that notifies users through email or UI updates when new products are added.

7. Strategy Pattern

Question: In a payment processing system, how would you use the Strategy pattern to allow different payment methods?

Answer: The Strategy pattern allows you to define a family of algorithms and make them interchangeable. In the payment processing system, each payment method (e.g., CreditCard, PayPal) could be a strategy that gets executed at runtime depending on user choice.

8. Command Pattern

Question: In a file system management tool, how would you implement the Command pattern to handle different file operations like copy, move, and delete?

Answer: The Command pattern allows you to encapsulate requests as objects. We could create a FileCommand interface and implement concrete commands for each operation (CopyCommand, DeleteCommand, etc.). The invoker would execute the commands based on user actions.

9. Adapter Pattern

Question: How would you use the Adapter pattern to integrate a third-party payment gateway API in a .NET Core application?

Answer: The Adapter pattern allows incompatible interfaces to work together. We could create an IPaymentGateway interface and then use an adapter class to transform the third-party API’s interface into one that our application understands.

10. Facade Pattern

Question: How would you use the Facade pattern to simplify a complex subsystem of multiple services in a .NET Core application?

Answer: The Facade pattern provides a simplified interface to a complex subsystem. We could create a ShoppingFacade class that hides the complexity of interacting with various subsystems like inventory, payment, and shipping.

11. Chain of Responsibility Pattern

Question: In a .NET Core web application, how would you implement the Chain of Responsibility pattern to process user authentication?

Answer: The Chain of Responsibility pattern allows a request to pass through a series of handlers. For authentication, we could have multiple handlers like UsernameValidation, PasswordValidation, TwoFactorAuthentication, and each handler processes the request or passes it along the chain.

12. State Pattern

Question: How would you use the State pattern to manage an OrderState in an online store system, where the order can be in states like Pending, Shipped, and Delivered?

Answer: The State pattern allows an object to alter its behavior when its internal state changes. We could create an Order class that holds a reference to the current state (e.g., PendingState, ShippedState), and the behavior of the order would change based on the current state.

13. Proxy Pattern

Question: How would you implement the Proxy pattern to handle lazy loading of a resource (e.g., large file) in a .NET Core application?

Answer: The Proxy pattern provides a surrogate or placeholder for another object. In the case of lazy loading, we could create a FileProxy class that loads the file only when it’s needed, improving performance.

14. Decorator Pattern

Question: In a .NET Core application, how would you use the Decorator pattern to add additional behavior to a logging service, like writing logs to a file and sending them over email?

Answer: The Decorator pattern allows behavior to be added to an object dynamically. We could create a base ILogger interface and then create decorators like FileLoggerDecorator and EmailLoggerDecorator that add additional functionality to the base logging behavior.

15. Composite Pattern

Question: How would you use the Composite pattern to manage a collection of UI components (e.g., buttons, text fields, containers) in a .NET Core MVC application?

Answer: The Composite pattern allows individual objects and composites of objects to be treated uniformly. We could create a UIComponent interface and then implement concrete components (e.g., Button, TextField) and composite components (e.g., Panel, Form) that contain other UI components.

16. Interpreter Pattern

Question: How would you implement the Interpreter pattern to parse and execute simple expressions (like “A + B”) in a .NET Core application?

Answer: The Interpreter pattern allows the definition of a grammar for interpreting expressions. We could define an abstract expression interface and implement classes for each terminal and non-terminal symbol, such as AdditionExpression or LiteralExpression.

17. Memento Pattern

Question: In a .NET Core application, how would you use the Memento pattern to implement undo functionality in a text editor?

Answer: The Memento pattern allows you to capture and externalize an object’s state without violating encapsulation. For undo functionality, we could create a TextMemento object that stores the current text and then use a TextEditor class to revert to a previous state.

18. Flyweight Pattern

Question: How would you use the Flyweight pattern to manage a large number of text strings in a .NET Core application, minimizing memory usage?

Answer: The Flyweight pattern reduces memory usage by sharing common object states. In the case of text strings, we could create a shared StringFlyweightFactory that stores only one instance of each unique string, which is reused across multiple clients.

19. Bridge Pattern

Question: How would you use the Bridge pattern to separate a UI abstraction from its implementation in a .NET Core application?

Answer: The Bridge pattern decouples abstraction from implementation. In this case, we could define an abstract UIControl class and create different implementations for rendering the control on different platforms (e.g., Web, Desktop). The UIControl uses the implementation through a bridge interface.

20. Template Method Pattern

Question: How would you implement the Template Method pattern in a .NET Core application to define a skeleton of an algorithm in a base class, leaving some steps to be implemented by subclasses?

Answer: The Template Method pattern defines the structure of an algorithm, allowing subclasses to implement specific steps. We could create a base DataProcessor class with a template method for processing data and leave certain steps (e.g., ReadData(), WriteData()) to be implemented by subclasses.

21. Visitor Pattern

Question: How would you implement the Visitor pattern to perform various operations (e.g., calculating total cost, applying discounts) on different types of products in a shopping cart in a .NET Core application?

Answer: The Visitor pattern allows you to define new operations on elements of an object structure without changing the classes of the elements. We could create a ProductVisitor interface with methods for different operations, and each product type implements an Accept method to accept a visitor.

22. Mediator Pattern

Question: In a .NET Core messaging system, how would you implement the Mediator pattern to manage communication between different modules (e.g., messaging, notification, and user service)?

Answer: The Mediator pattern centralizes communication between objects, reducing direct dependencies. We could create a IMediator interface and implement a MessageMediator class to handle interactions between the modules.

23. Null Object Pattern

Question: How would you use the Null Object pattern to handle situations where a service or object might be absent in a .NET Core application?

Answer: The Null Object pattern provides an object that behaves like a real object but does nothing. For a service that might be null, we could return a NullLoggingService that silently ignores log messages instead of throwing an exception.

24. Object Pool Pattern

Question: How would you use the Object Pool pattern to manage the reuse of database connections in a .NET Core application?

Answer: The Object Pool pattern maintains a pool of reusable objects. We could create a DatabaseConnectionPool class that manages a collection of database connections, allowing connections to be reused rather than created and destroyed repeatedly.

25. Dependency Injection Pattern

Question: How would you implement the Dependency Injection pattern to inject a service (e.g., a logging service) into a controller in a .NET Core MVC application?

Answer: The Dependency Injection pattern allows you to inject services into classes instead of creating them internally. In .NET Core, you can register the logging service in the Startup.ConfigureServices method, and it will be automatically injected into the controller’s constructor.

26. Singleton Pattern (Revisited)

Question: How would you implement the Singleton pattern in .NET Core to ensure that a configuration service only has one instance across the entire application lifecycle?

Answer: The Singleton pattern ensures a class has only one instance and provides a global access point. In .NET Core, you could register the service as a singleton in the Startup.ConfigureServices method, ensuring that the same instance is used throughout the application’s lifecycle.

27. Repository Pattern

Question: How would you implement the Repository pattern to abstract database operations in a .NET Core application?

Answer: The Repository pattern abstracts data access logic. We can create an interface (e.g., IProductRepository) with methods like GetAll, Find, and Save, and implement it in a concrete repository class to interact with the database.

28. Unit of Work Pattern

Question: How would you use the Unit of Work pattern to manage transactions in a .NET Core application?

Answer: The Unit of Work pattern manages multiple database operations within a single transaction. We could create a IUnitOfWork interface that coordinates commits and rollbacks for multiple repository operations.

29. Data Transfer Object (DTO) Pattern

Question: How would you use the DTO pattern to transfer data between layers in a .NET Core application?

Answer: The Data Transfer Object (DTO) pattern helps to transfer data between layers. We could create a ProductDTO class to transfer product data between the service and presentation layers, reducing the need for direct database entities.

30. Service Locator Pattern

Question: How would you use the Service Locator pattern in a .NET Core application to locate services dynamically at runtime?

Answer: The Service Locator pattern provides a centralized point to access services. While it’s often discouraged due to hidden dependencies, it can be useful in specific scenarios. You would register services in the container and then retrieve them using a service locator.

31. Strategy Pattern (Revisited)

Question: How would you use the Strategy pattern to implement different sorting algorithms in a .NET Core application?

Answer: The Strategy pattern allows you to define a family of algorithms and make them interchangeable. For sorting, we could create a SortStrategy interface with methods like Sort(), and implement different sorting algorithms (e.g., BubbleSort, QuickSort) that can be chosen dynamically at runtime.

32. Factory Method Pattern (Revisited)

Question: How would you implement the Factory Method pattern to create different types of user notifications (e.g., email, SMS, push notifications) in a .NET Core application?

Answer: The Factory Method pattern allows a class to delegate object creation to subclasses. We could create a NotificationFactory class with a method that creates different types of INotification objects, such as EmailNotification, SMSNotification, and PushNotification.

33. Adapter Pattern (Revisited)

Question: How would you use the Adapter pattern to integrate a legacy XML-based payment gateway in a modern .NET Core application?

Answer: The Adapter pattern helps adapt incompatible interfaces. We could create an IPaymentGateway interface and implement an XmlPaymentGatewayAdapter that converts XML responses into a format that the modern application understands.

34. Command Pattern (Revisited)

Question: How would you use the Command pattern to implement a queue of background tasks (e.g., sending emails) in a .NET Core application?

Answer: The Command pattern allows us to encapsulate requests as objects. We could create a ICommand interface with a method like Execute(), and then implement concrete commands such as SendEmailCommand that get executed by a task queue or background worker.

35. Composite Pattern (Revisited)

Question: How would you use the Composite pattern to represent a hierarchical file system structure in a .NET Core application?

Answer: The Composite pattern allows us to treat individual objects and groups of objects uniformly. We could create an abstract FileSystemComponent class, with concrete implementations like File and Directory, where directories can contain files or other directories.

36. State Pattern (Revisited)

Question: How would you use the State pattern to manage user account status (e.g., Active, Suspended, Pending) in a .NET Core application?

Answer: The State pattern allows an object’s behavior to change when its internal state changes. We could create an UserAccount class with states such as ActiveState, SuspendedState, and PendingState, and transition between states based on specific conditions.

37. Observer Pattern (Revisited)

Question: How would you implement the Observer pattern to notify users of a new message in a chat application built using .NET Core?

Answer: The Observer pattern allows one object (subject) to notify multiple observers when an event occurs. In the chat app, we could have a MessageService that notifies registered users (observers) via email, push notifications, or UI updates when a new message is received.

38. Builder Pattern (Revisited)

Question: How would you use the Builder pattern to construct a complex product order in a .NET Core e-commerce application?

Answer: The Builder pattern allows us to construct a complex object step by step. For an order, we could create an OrderBuilder class that assembles the product, quantity, shipping details, and payment method before constructing the final Order object.

39. Proxy Pattern (Revisited)

Question: How would you use the Proxy pattern to implement access control to sensitive data in a .NET Core application?

Answer: The Proxy pattern provides a surrogate for another object. We could create a DataProxy class that checks if the user has the appropriate permissions before delegating access to the actual sensitive data object.

40. Mediator Pattern (Revisited)

Question: How would you use the Mediator pattern to manage communication between various components (e.g., payment processing, inventory, and shipping) in a .NET Core e-commerce application?

Answer: The Mediator pattern centralizes communication between objects, reducing direct dependencies. We could create a TransactionMediator that handles the communication between the payment, inventory, and shipping services to ensure the order is processed seamlessly.

41. Template Method Pattern (Revisited)

Question: How would you use the Template Method pattern to implement a workflow for processing customer orders in a .NET Core application?

Answer: The Template Method pattern defines a skeleton of an algorithm. We could create an abstract OrderProcessor class that provides a template method for processing orders, with customizable steps like ValidateOrder(), ProcessPayment(), and ShipOrder().

42. Chain of Responsibility Pattern (Revisited)

Question: How would you implement the Chain of Responsibility pattern to process different levels of logging (e.g., Debug, Info, Error) in a .NET Core application?

Answer: The Chain of Responsibility pattern allows multiple handlers to process a request. We could have a chain of loggers, each responsible for handling a specific log level (e.g., DebugLogger, InfoLogger, ErrorLogger), and each logger passes the request to the next handler in the chain.

43. Abstract Factory Pattern (Revisited)

Question: How would you use the Abstract Factory pattern to implement a cross-platform GUI (e.g., Windows, Linux, MacOS) in a .NET Core application?

Answer: The Abstract Factory pattern provides an interface for creating families of related objects. We could create abstract factory classes for each platform (e.g., WindowsFactory, LinuxFactory) that produce platform-specific UI components (e.g., Button, TextBox).

44. Singleton Pattern (Revisited)

Question: How would you implement the Singleton pattern to manage configuration settings that are shared across multiple modules in a .NET Core application?

Answer: The Singleton pattern ensures only one instance of a class exists. We could implement a ConfigurationManager class that loads configuration settings once and provides global access to the configuration throughout the application.

45. Data Access Object (DAO) Pattern

Question: How would you implement the Data Access Object (DAO) pattern to interact with a database in a .NET Core application?

Answer: The DAO pattern abstracts the data access logic. We could create a ProductDAO class that handles database operations like GetAll, Save, and Delete, and use this class in the service layer to decouple the database logic from the business logic.

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