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20 .NET Core Design Patterns Every Developer Must Know | Mock Interview 2026 | Ep. 20
The DotNet Way
Overview
This video serves as a masterclass on 20 essential design patterns for .NET Core developers, presented through a mock interview format. It explains why design patterns are crucial for building reusable, testable, and maintainable software. The discussion covers a wide range of patterns, from fundamental ones like Repository and Factory to more advanced concepts like Mediator, Circuit Breaker, and Saga, illustrating their practical application in real-world enterprise .NET Core projects and how they solve common architectural challenges.
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Chapters
- Design patterns offer standardized, proven solutions to recurring software design problems.
- They enhance code reusability, making it easier to build new features.
- Patterns improve code testability, allowing for more robust unit and integration tests.
- Adopting patterns leads to more maintainable codebases, reducing long-term development costs.
Understanding design patterns is fundamental for senior developers as they provide a common language and proven strategies for building robust, scalable, and maintainable applications.
Instead of reinventing solutions for common problems, developers apply battle-tested patterns, saving time and reducing the risk of introducing bugs.
- The Repository Pattern abstracts data access logic, decoupling services from specific ORMs like Entity Framework.
- Using an interface for repositories (e.g., IProductRepository) simplifies mocking for unit tests and allows for easy switching of data persistence implementations.
- The Unit of Work pattern coordinates transactions across multiple repositories, ensuring atomicity for complex operations.
- Unit of Work's SaveChangesAsync() guarantees that all changes across different tables are committed in a single, safe database transaction.
These patterns are critical for managing data interactions efficiently and reliably, ensuring data integrity and simplifying testing in applications that interact with databases.
Implementing IUnitOfWork.SaveChangesAsync() to commit changes across multiple tables in a single, atomic database transaction.
- The Singleton Pattern ensures a class has only one instance throughout the application's lifetime, often managed via Dependency Injection's AddSingleton.
- The Factory Pattern creates objects without exposing complex instantiation logic to the client, using conditional logic (like switch expressions) to select the correct object.
- The Strategy Pattern encapsulates interchangeable algorithms, allowing different behaviors (like discount rules) to be swapped easily.
- Strategy promotes the Open/Closed Principle by allowing new behaviors to be added without modifying existing code.
These patterns help manage object creation and behavior flexibility, leading to cleaner, more adaptable code that is easier to extend and modify.
A Factory using a switch expression to instantiate either a PayPal or Stripe processor based on input, keeping the controller unaware of the specifics.
- The Mediator Pattern centralizes communication between components, reducing direct dependencies.
- In CQRS with MediatR, commands and queries are routed to specific handlers, decoupling the API from business logic.
- The Decorator Pattern dynamically adds new behaviors to an object without altering its original code, often used for cross-cutting concerns like logging or retries.
- The Adapter Pattern acts as a bridge between incompatible interfaces, translating requests from one format to another.
- The Null Object Pattern replaces null returns with a 'no-operation' object, preventing NullReferenceExceptions and simplifying control flow.
These patterns improve system communication, add functionality flexibly, and handle edge cases gracefully, contributing to more robust and decoupled architectures.
Using Scrutor's services.Decorate to wrap an EmailService with a LoggingEmailService, adding logging without changing the original email logic.
- The Builder Pattern separates the construction of complex objects from their representation, enabling readable, step-by-step object creation.
- Fluent APIs in .NET Core often utilize the Builder pattern, allowing method chaining for configuration.
- The Chain of Responsibility Pattern processes requests through a pipeline of handlers, where each handler decides whether to process the request or pass it to the next.
- ASP.NET Core Middleware is a prime example of the Chain of Responsibility pattern.
- The Options Pattern provides strongly-typed access to configuration settings (like appsettings.json), enhancing type safety and validation.
These patterns streamline the creation of complex objects and the handling of requests, leading to more organized, readable, and type-safe code.
Chaining methods like `builder.SetPropertyA().SetPropertyB()` to construct an object, as seen in .NET Core's Fluent APIs.
- The Facade Pattern provides a simplified, unified interface to a complex subsystem, hiding internal complexities.
- The Proxy Pattern acts as a placeholder or surrogate for another object, often used for performance optimizations like caching.
- A Caching Proxy intercepts calls, checks a cache (e.g., Redis) first, and only accesses the underlying resource if data is not cached.
- The Observer Pattern establishes a one-to-many dependency where state changes in one object notify multiple dependent objects.
- In distributed systems, Pub/Sub message buses often replace traditional C# events for implementing the Observer pattern.
These patterns help manage complexity by simplifying interfaces and improve performance through techniques like caching and asynchronous event handling.
A Caching Proxy checking Redis before querying the actual database to retrieve data, thereby saving processing time.
- The Circuit Breaker Pattern prevents an application from repeatedly attempting operations that are likely to fail, improving system resilience.
- Using libraries like Polly, the Circuit Breaker can temporarily block traffic to a failing service, allowing it time to recover and preventing cascading failures.
- The State Pattern allows an object to alter its behavior based on its internal state, encapsulating state-specific logic into separate classes.
- The Template Method Pattern defines a skeleton of an algorithm in a base class, allowing subclasses to override specific steps without changing the overall algorithm structure.
These patterns are crucial for building fault-tolerant and adaptable systems that can gracefully handle failures and manage complex, state-dependent behavior.
Polly library's Circuit Breaker breaking the connection to a failing API after three consecutive errors to prevent further issues.
- The Saga Pattern is used in microservices architectures to manage distributed transactions across multiple services.
- Since ACID transactions are not feasible across services, Sagas coordinate a sequence of local transactions.
- If a step in a Saga fails, compensating transactions are triggered to undo previous steps, ensuring eventual consistency.
- The Saga Pattern is vital for maintaining data consistency in distributed, cloud-native applications where services manage their own data.
The Saga pattern is essential for managing complex workflows and ensuring data consistency in microservices architectures where traditional distributed transactions are not practical.
An Order Service creating an order, a Payment Service processing payment, and if the Inventory Service fails, the Saga triggers a refund and order cancellation.
Key takeaways
- Design patterns provide a shared vocabulary and proven solutions for common software design challenges, leading to more robust and maintainable code.
- Abstracting data access with the Repository pattern and managing transactions with Unit of Work are fundamental for reliable data operations.
- Patterns like Factory and Strategy enable flexible object creation and interchangeable algorithms, making code easier to modify and extend.
- Mediator, Decorator, and Adapter patterns help manage complexity by centralizing communication, adding behavior dynamically, and bridging incompatible interfaces.
- Builder and Chain of Responsibility patterns improve code readability and streamline request processing, with ASP.NET Core Middleware being a key example.
- Patterns like Facade and Proxy simplify complex subsystems and enhance performance through caching, while Observer handles event-driven architectures.
- Circuit Breaker and State patterns are crucial for building resilient applications that can handle failures and manage complex object behaviors.
- The Saga pattern is indispensable for managing distributed transactions and ensuring eventual consistency in microservices architectures.
Key terms
Design PatternRepository PatternUnit of Work PatternSingleton PatternFactory PatternStrategy PatternMediator PatternDecorator PatternAdapter PatternNull Object PatternBuilder PatternChain of Responsibility PatternOptions PatternFacade PatternProxy PatternObserver PatternCircuit Breaker PatternState PatternTemplate Method PatternSaga PatternCQRSDependency Injection
Test your understanding
- Why are design patterns considered essential for senior .NET Core developers?
- How does the Repository Pattern improve code testability and maintainability compared to direct data access?
- What is the primary difference between the Repository and Unit of Work patterns, and when would you use them together?
- Explain how the Factory Pattern simplifies object creation and decouples clients from complex instantiation logic.
- How can the Strategy Pattern help in adhering to the Open/Closed Principle when implementing varying business rules?
- What problem does the Circuit Breaker pattern solve in distributed or cloud-native applications, and how does it work?
- Describe a scenario where the Saga Pattern would be necessary in a microservices architecture, and what are its key components?