Interview Q&A

Several design patterns are built around the idea of making systems extensible without

modifying core logic:

Pattern How It Helps With OCP

Strategy Allows changing behavior by swapping strategies.

Decorator Adds new responsibilities dynamically without changing original code.

Template Method Allows subclasses to override certain steps in an algorithm.

Factory Method Makes it easy to introduce new types without altering existing logic.

Observer Extends behavior in reaction to events without altering the source.

Design patterns provide structured, reusable solutions that embody SOLID principles. For

example, the Strategy pattern supports OCP by allowing behavior extension without

modifying existing code; Repository separates data access (SRP); Dependency Injection

supports DIP by decoupling high- and low-level modules. Using patterns helps keep code

clean, modular, and maintainable.

nd why?

The Mediator Pattern centralizes communication between components, preventing direct

dependencies and reducing complexity. It promotes loose coupling and simplifies

interactions. Alternatively, the Observer Pattern enables event-driven decoupling, and

Facade Pattern provides a simplified interface to complex subsystems.

The Mediator Pattern centralizes communication between components, preventing direct

dependencies and reducing complexity. It promotes loose coupling and simplifies

interactions. Alternatively, the Observer Pattern enables event-driven decoupling, and

Facade Pattern provides a simplified interface to complex subsystems.

• Identify classes violating SRP and break them down.
• Introduce abstractions and interfaces to decouple components (DIP).
• Replace conditional logic with polymorphism to respect OCP.
• Split large interfaces (ISP).
• Check inheritance hierarchies to maintain LSP.
• Inject dependencies instead of direct instantiation.
• Incrementally refactor with unit tests to ensure behavior remains consistent.

Use AOP (Aspect-Oriented Programming) techniques or design patterns like Decorator

to separate cross-cutting concerns from business logic. In .NET, middleware, filters, or

interceptors can manage concerns like logging or authorization, keeping SRP intact.

• Define plugin contracts with interfaces (DIP).
• Load plugins dynamically using reflection or MEF.
• Use DI to inject dependencies into plugins.
• Ensure plugins follow SRP with focused responsibilities.
• Use Factory or Strategy patterns to instantiate plugins.
• Keep core system closed for modification but open for extension (OCP).
• Separate cross-cutting concerns externally.

Practical .NET Questions

• 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.

• 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.

• 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

In a recent project, we had a monolithic service class handling multiple responsibilities,

making it hard to maintain and extend. 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.

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.

I use a combination of:

• Simple examples showing before/after code refactoring.
• 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

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.

What interviewers expect

• A clear definition tied to SOLID in Design Patterns & SOLID projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Design Patterns & SOLID application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Design Patterns & SOLID architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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.

What interviewers expect

• A clear definition tied to SOLID in Design Patterns & SOLID projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Design Patterns & SOLID application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Design Patterns & SOLID architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

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.

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

What interviewers expect

• A clear definition tied to SOLID in Design Patterns & SOLID projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Design Patterns & SOLID application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Design Patterns & SOLID architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Use API contracts and versioning. Employ service discovery and load balancing. Prefer asynchronous messaging/event-driven architecture to reduce tight coupling. Apply circuit breakers and retries for fault tolerance.

What interviewers expect

• A clear definition tied to SOLID in Design Patterns & SOLID projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Design Patterns & SOLID application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Design Patterns & SOLID architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Interfaces define contracts with no implementation, supporting ISP and DIP by llowing flexible implementations. Abstract classes provide a base implementation and shared code, useful for Template Method pattern and partial abstraction.

What interviewers expect

• A clear definition tied to SOLID in Design Patterns & SOLID projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Design Patterns & SOLID application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Design Patterns & SOLID architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Observer supports SRP by separating notification logic from core business logic and DIP by depending on abstractions (observers). It fits DI because observers can be injected, allowing flexible runtime subscriptions and loose coupling.

What interviewers expect

• A clear definition tied to SOLID in Design Patterns & SOLID projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Design Patterns & SOLID application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Design Patterns & SOLID architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.