Tutorials Design Patterns in C#

How Senior Developers Use Design Patterns — Complete Guide

How Senior Developers Use Design Patterns — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of Design Patterns in C# on Toolliyo Academy.

8 min read Updated 8/19/2026

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Design Patterns in C# · Lesson 65 of 69

How Senior Developers Use Design Patterns

GoF Core ✓ → Enterprise ✓ → Cloud & Craft

Cloud & Craft · 3 — Microservices & interviews · ~10 min · Module 8: Interview & System Design

What is this?

Seniors introduce patterns to remove duplication or risk — not to decorate resumes. They start simple and refactor toward a pattern when pain appears.

Why should you care?

ShopNest codebases stay maintainable when patterns earn their keep.

See it live — copy this example

Paste into a C# console or class library project and run dotnet run.

Console.WriteLine("smell: switch on shipping in 6 places");
Console.WriteLine("refactor: IShippingStrategy + DI");
Console.WriteLine("stop: don't add Visitor for one report");

What happened?

Smell-driven adoption beats pattern-first design.
Prefer language features and libraries.
Document why in an ADR when the pattern is non-obvious.

Practice next

Find a ShopNest switch smell.
Refactor to Strategy only if duplication is real.
Write a 5-line ADR.
Remove a useless interface.
Replace hand-rolled retry with Polly.

Remember

Patterns follow pain. Keep it pragmatic. ADRs for non-obvious choices.

ShopNest pragmatic refactors

Team extracts Strategy after third shipping courier.

Outcome: Abstraction arrives exactly when needed.

Interview prep for this lesson

Practice these questions aloud after reading—each links to a full structured answer.

Mid PDF Detailed

Abstract Factory (LoggerFactory):?

Short answer: The LoggerFactory class defines a factory method CreateLogger that returns an ILogger object. This is a generic interface for creating various logger types without specifying the concrete class directly. Re…

Mid PDF Detailed

Redo Functionality:?

Short answer: You can extend the system by adding redo functionality. After an undo operation, you could store the undone command in a separate stack and allow users to redo the previous undo operation. Real-world exampl…

Mid PDF Detailed

Abstract Expression (IExpression):?

Short answer: The IExpression interface declares an Interpret method, which is the core of the interpreter pattern. This method is used to interpret and evaluate expressions. public interface IExpression Example code { i…

Mid PDF Detailed

Mediator (ChatMediator):?

Short answer: The mediator manages communication between the users. It maintains a list of all users and broadcasts messages to all other users when one user sends a message. This keeps the users from directly knowing ab…

Mid PDF Detailed

Component (ICoffee):?

Short answer: This is the base interface that defines the common methods for the Cost() and Description() that every coffee component will implement. public interface ICoffee Example code { double Cost(); string Descript…

All course Q&A Practice exams

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