C# Interview Question #100

How would you design clean and maintainable C# code for a production application?

SOLID, DI, Testing & Practical C# Mid-Level Intermediate

Quick Interview Answer

Clean production C# code should have clear responsibilities, meaningful names, appropriate abstractions, predictable error handling, and strong testability.

Detailed Explanation

Clean production C# code should have clear responsibilities, meaningful names, appropriate abstractions, predictable error handling, and strong testability.

A typical ASP.NET Core application separates HTTP concerns, business logic, data access, and infrastructure rather than placing everything inside controllers. Dependency injection is used to provide services, and interfaces are introduced where they provide a useful abstraction boundary.

Database queries should be efficient and asynchronous where appropriate. DTOs should prevent unnecessary exposure of persistence entities. Input should be validated, exceptions should be handled centrally where practical, logging should contain useful structured context, and secrets should remain outside source code.

SOLID principles, automated tests, code reviews, nullable reference types, analyzers, consistent formatting, and observability all contribute to maintainability. Architecture should remain as simple as the application's requirements allow; unnecessary layers and abstractions can be as harmful as insufficient structure.

This completes the C# interview question series from Question 1 through Question 100.

Code Example

// Typical dependency direction:

Controller
    -> Application/Business Service
        -> Repository or DbContext abstraction where appropriate
        -> External service abstractions

// Example controller dependency:
public ProductsController(IProductService productService)
{
    _productService = productService;
}