Interview Q&A

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.

Builder separates complex object construction from its representation. For example, building

n HttpRequest with optional headers, query params, and body:

public class HttpRequestBuilder
{
private HttpRequestMessage _request = new HttpRequestMessage();
public HttpRequestBuilder SetMethod(HttpMethod method)
{
_request.Method = method;
return this;
}
public HttpRequestBuilder AddHeader(string key, string value)
{

_request.Headers.Add(key, value);

return this;
}
public HttpRequestMessage Build() => _request;
}

llows building requests step-by-step fluently.

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.