Microsoft·Software Engineer·Onsite - System Design / Architecture·Intermediate

Intermediate

Jun 2026

Summary

Microsoft software engineer interview with a system design round focused on object-oriented modeling. The prompt was basically a business logic description and you had to build the class hierarchy from scratch, which sounds manageable until you're actually in it.

Questions Asked (1)

Given a description of business logic similar to what the team builds, design and implement the relevant classes in an OO style. Identify entities, responsibilities, relationships like composition vs inheritance, interfaces or abstract methods, and key invariants. Then walk through how your implementation maps to the requirements and discuss how you'd extend it.

System DesignTechnical Trade-offsData Modeling

Author's notes

I spent way too long trying to nail down the perfect entity breakdown before writing any code.

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Suggested Approach

Start by clarifying the business requirements and identifying the core entities, their responsibilities, and relationships. Then design the class structure using appropriate OO principles like composition over inheritance, interfaces for flexibility, and abstract classes for shared behavior. Finally, implement the classes, ensuring key invariants are enforced, and discuss how the design maps to requirements and can be extended.

Pro tip: Focus on explaining the 'why' behind your design choices, especially trade-offs between inheritance and composition, and how your design adheres to SOLID principles. This demonstrates deeper understanding and maturity.

1. Clarify Requirements and Identify Entities

Ask clarifying questions to understand the business logic, constraints, and expected behaviors. Identify the main entities (nouns) and their responsibilities (verbs) from the description.

2. Define Relationships and Interfaces

Determine relationships between entities: composition, inheritance, or association. Decide where to use interfaces or abstract classes to promote loose coupling and extensibility.

3. Design Classes and Invariants

For each entity, define its class with attributes and methods. Specify key invariants that must hold true (e.g., balance >= 0) and how they will be enforced (e.g., through constructors, setters, or validation).

4. Implement and Map to Requirements

Write the code for the classes, ensuring it aligns with the identified responsibilities and relationships. Walk through how each requirement is satisfied by the implementation.

5. Discuss Extensibility and Trade-offs

Explain how the design can be extended for future requirements (e.g., adding new types via interfaces). Discuss trade-offs made (e.g., inheritance vs composition) and why they are appropriate.

Key Points to Mention

Composition over inheritance: prefer composing objects for flexibility and to avoid fragile base class problems.
Interfaces and abstract classes: use interfaces to define contracts and abstract classes for shared implementation.
SOLID principles: especially Single Responsibility and Open/Closed principles to ensure maintainable and extensible code.
Key invariants: identify and enforce business rules (e.g., unique IDs, valid state transitions) within the domain model.
Design patterns: mention relevant patterns like Factory, Strategy, or Observer if applicable to the business logic.
Extensibility: discuss how new requirements can be accommodated with minimal changes, e.g., through dependency injection or polymorphism.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.