Design Patterns in C#
Lesson 69/69

Tutorials Design Patterns in C#

Refactoring Legacy Code Using Design Patterns — Complete Guide

Refactoring Legacy Code Using 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 8/20/2026
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Design Patterns in C# · Lesson 69 of 69

Refactoring Legacy Code Using Design Patterns

GoF Core ✓ → Enterprise ✓ → Cloud & Craft

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

What is this?

Refactor legacy by characterizing tests first, then introducing patterns at seams (Adapter, Strategy, Strangler) in small steps.

Why should you care?

ShopNest monolith improves safely when each PR is reversible and tested.

See it live — copy this example

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

Console.WriteLine("1) characterization tests around Checkout.Do()");
Console.WriteLine("2) Extract IShippingStrategy from switch");
Console.WriteLine("3) Adapter around legacy SOAP");
Console.WriteLine("4) Strangle route to new service");

What happened?

  • Do not pattern-bomb untested code.
  • Find seams, add tests, extract interfaces, migrate routes.
  • Celebrate deletions.

Practice next

  1. Add tests around a ShopNest legacy method.
  2. Extract one Strategy.
  3. Wrap one external SDK with Adapter.
  4. Measure coverage on the legacy module.
  5. Delete dead branches after extraction.

Remember

Tests before reshapes. One seam at a time. Patterns as tools, not goals.

ShopNest legacy checkout tame

Team extracts shipping Strategy behind tests.

Outcome: New courier ships without fear.

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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Design Patterns in C#
Course syllabus

Design Patterns in C# Tutorial

Module 1: Creational Design Patterns
Module 2: Structural Design Patterns
Module 3: Behavioral Design Patterns
Module 4: Enterprise Design Patterns
Module 5: Modern Enterprise Patterns
Module 6: Microservices & Cloud Patterns
Module 7: ASP.NET Core Architecture Patterns
Module 8: Interview & System Design
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