Interview Q&A

Short answer: Register DbContext with DI in Startup.cs: services.AddDbContext<AppDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConne ction"))); Implement Repositories for entities injecting AppDbContext. Implement Unit of Work which holds multiple repositories and calls SaveChanges() on the DbContext: public interface IUnitOfWork : IDisposable

Example code

{ IProductRepository Products { get; } int Complete();
}
public class UnitOfWork : IUnitOfWork
{
private readonly AppDbContext _context;
public IProductRepository Products { get; private set; }
public UnitOfWork(AppDbContext context)
{
_context = context;
Products = new ProductRepository(_context);
}
public int Complete() => _context.SaveChanges();
public void Dispose() => _context.Dispose();
} Register UnitOfWork in DI container as Scoped.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use mocking libraries like Moq, NSubstitute, or FakeItEasy. Create mocks of interfaces and inject them into the class under test: var mockRepo = new Mock<IProductRepository>(); mockRepo.Setup(repo => repo.GetAll()).Returns(new List<Product> { ... }); var service = new ProductService(mockRepo.Object); // Act & Assert This allows testing in isolation without hitting real databases or external services.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Chain of Responsibility: Middleware components form a pipeline where each decides to pass control or handle the request. Decorator: Middleware wraps around the next component, adding behavior before or after. Factory: Middleware components can be created via factories for configurable pipeline setup.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Define a caching interface: public interface ICacheStrategy { void Cache(string key, object value); object Retrieve(string key); } Implement strategies like MemoryCacheStrategy, DistributedCacheStrategy. Use DI or factory to inject the chosen strategy at runtime: public class CacheContext

Example code

{
private readonly ICacheStrategy _cacheStrategy;
public CacheContext(ICacheStrategy cacheStrategy)
{
_cacheStrategy = cacheStrategy;
}
public void Cache(string key, object value) => _cacheStrategy.Cache(key, value); } This enables switching caching mechanisms without code changes.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Abstract Factory: Provides an interface for creating families of related objects without specifying concrete classes.

Example code

Creating UI components for different OS (Windows, Mac). Builder: Focuses on step-by-step construction of a complex object, allowing different representations. Example: Building a complex House with various parts (walls, doors, roof). Summary: Abstract Factory is about families of products, Builder is about complex construction process.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Apply Single Responsibility Principle (SRP) by splitting responsibilities into smaller classes. Use composition instead of inheritance to delegate behavior. Extract business logic into services or helpers. Introduce abstractions to isolate concerns. Continuously refactor large classes and add unit tests.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Resharper: Provides code analysis and refactoring hints. SonarQube / SonarCloud: Analyzes code quality and reports SOLID violations. FxCop / Roslyn analyzers: Provide static analysis with custom rules. NDepend: Deep architecture and dependency analysis tool. StyleCop: Enforces coding style which indirectly helps maintain SOLID code.

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Mediator decouples components by centralizing communication, supporting SRP and DIP by reducing direct dependencies. It fits DI because the mediator itself can be injected where needed. It supports OCP by allowing new communication routes or handlers without modifying existing components. Helps avoid tight coupling in complex workflows or CQRS patterns. Behavioral / Conceptual Questions

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: In a recent project, we had a monolithic service class handling multiple responsibilities, making it hard to maintain and extend.

Explain a bit more

By applying Single Responsibility Principle (SRP), we split the class into focused services, each with a clear purpose. This drastically improved readability, reduced bugs, and made it easier to add new features without risking regressions. The project became more testable because each small class could be unit tested independently.

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Applications? Yes. In one project, a singleton was used without thread safety, causing race conditions when accessed concurrently. This led to inconsistent state and application crashes. We resolved it by implementing thread-safe lazy initialization using Lazy<T> in .NET, ensuring the singleton instance was created safely once, even under heavy… parallel…

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Yes. In one project, a singleton was used without thread safety, causing race conditions when accessed concurrently. This led to inconsistent state and application crashes. We resolved it by implementing thread-safe lazy initialization using Lazy<T> in .NET, ensuring the singleton instance was created safely once, even under heavy parallel access.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: I prioritize YAGNI (You Aren’t Gonna Need It) to avoid over-engineering.

Explain a bit more

SOLID principles guide design for flexibility and maintainability, but I apply them pragmatically: start with simple solutions and refactor as requirements evolve. Writing tests early helps identify pain points justifying additional abstractions. Communication with the team ensures we don’t add complexity prematurely but keep the codebase adaptable.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Challenges include: Legacy code with tight coupling, making decomposition hard. Risk of introducing bugs while splitting responsibilities or introducing abstractions. Managing dependencies and lifetimes correctly when injecting dependencies. Convincing stakeholders that refactoring time is valuable. Balancing between adhering strictly to SOLID vs. keeping code understandable and performant.

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: I use a combination of: Simple examples showing before/after code refactoring.

Explain a bit more

Pair programming sessions to explain thought processes. Encouraging reading and discussing classic books like “Clean Code” and “Design Patterns”. Practical coding exercises and code reviews focused on SOLID principles. Showing real project scenarios where principles improved code quality and maintainability. Promoting a culture of continuous learning and curiosity. Bonus / Miscellaneous

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Inheritance creates an “is-a” relationship, where a subclass inherits behavior and properties from a parent class. It can lead to tight coupling and fragile hierarchies if overused. Composition creates a “has-a” relationship, where a class contains instances of other classes and delegates behavior to them. It’s more flexible and promotes loose coupling.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Decorator adds additional responsibilities to objects dynamically without altering their interface. It wraps the original object to extend behavior. Proxy controls access to an object, possibly adding lazy initialization, access control, or logging, without changing its interface.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Adapter converts the interface of a class into another interface clients expect, allowing incompatible interfaces to work together. It promotes Open/Closed Principle (OCP) by enabling new integrations without modifying existing code.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use the Proxy or Virtual Proxy pattern where a placeholder object controls access to the real object and defers its creation until needed. In .NET, Lazy<T> provides built-in lazy loading.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Service Locator anti-pattern: Hides dependencies instead of injecting them explicitly. Overusing Singleton: Leads to hidden global state and testing difficulties. Improper Singleton thread safety: Causes race conditions. Injecting concrete implementations: Violates DIP.

Real-world example (ShopNest)

ShopNest payment fees use Strategy: IFeeCalculator with UPI/Card implementations chosen at runtime.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Template Method defines the skeleton of an algorithm in a base class, deferring some steps to subclasses. It allows subclasses to redefine parts of the algorithm without changing its structure. It supports OCP by enabling extensions through inheritance.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: SOLID promotes small, single-responsibility services (SRP), clear interfaces (ISP), loose coupling (DIP), and extendable design (OCP). This aligns well with microservices by encouraging modular, maintainable, and testable service boundaries.

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: The Command Pattern encapsulates requests as objects, allowing operations to be stored, undone, or redone by maintaining command history.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Use interfaces and abstractions (DIP). Apply Dependency Injection. Modularize code into bounded contexts or separate projects. Use events or messaging for decoupled communication. Avoid static state and global variables.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Encapsulates a request as an object with methods to execute and possibly undo the operation. The invoker calls commands without knowing the action details, supporting decoupling and flexible request handling.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Cohesion: Degree to which elements of a module belong together. High cohesion means focused, well-defined responsibilities. Coupling: Degree of interdependence between modules. Low coupling means modules are independent and changes in one don’t affect others.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.