Refactoring Legacy Code with GitHub Copilot - Visual Studio 2026 | Random Number Generator
Refactoring legacy .NET code using GitHub Copilot in Visual Studio 2026.
A real-world C# legacy code rewrite showing how AI performs when dealing with poorly written, undocumented applications.
This video focuses on a legacy code rewrite rather than simple refactoring, highlighting architectural, behavioral, and performance changes that occur when modernizing old C#/.NET code.
👉 Watch the complete series in order:
• Refactoring Legacy Code with GitHub Copilo...
🔍 PROBLEM THIS VIDEO SOLVES:
How do you safely rewrite legacy C#/.NET code when documentation is missing, the original developer is gone, and AI tools provide incomplete or misleading explanations?
In this real-world case study, I use GitHub Copilot inside Visual Studio 2026 to analyze and rewrite a poorly-written Random Number Generator class from an old application. The goal is not just to make the code cleaner, but to understand what it was originally trying to achieve before introducing architectural changes.
This episode demonstrates where AI-assisted coding helps — and where deep developer knowledge is still essential. I compare explanations from different AI models (ChatGPT 5 mini vs Grok) to show how model choice dramatically affects code understanding when dealing with technical debt.
Topics covered include:
• Legacy code rewrite vs refactoring
• Understanding old C#/.NET code written years ago
• Trimming unused methods safely
• Async vs sync decisions for CPU-bound work
• Thread safety concerns when refactoring collections
• When a domain class becomes a service
• Moving logic into proper Domain, Infrastructure, and Services layers
• AI-generated integration tests (without mocks)
• Common pitfalls when prompting AI coding tools
⏱️ TIMESTAMPS:
00:00 – Introduction to Legacy Code Rewrite
01:17 – Analyzing the Random Numbers Generator
03:00 – Rewriting the Class with GitHub Copilot
06:07 – AI Explains My Old Code (ChatGPT vs Grok)
09:28 – New Implementation & Architecture
10:47 – AI Generated Integration Tests
12:35 – Key Lessons About AI Coding Tools
💡 KEY TAKEAWAYS:
GitHub Copilot and AI coding assistants are powerful productivity tools, but they cannot replace understanding the problem domain. Different AI models provide different levels of useful insight, and blindly trusting AI explanations can lead to incorrect architectural decisions during a rewrite.
This video covers a real-world scenario many developers face: taking over a legacy system when the original developers have left and documentation is incomplete or nonexistent.
🎯 WHO THIS VIDEO IS FOR:
• Developers maintaining legacy C# or .NET codebases
• Engineers modernizing older applications
• Teams dealing with technical debt
• GitHub Copilot users seeking real-world, non-trivial examples
• Software engineers exploring AI-assisted development workflows
📚 ALL VIDEOS IN THIS SERIES:
📹 Episode 1: GitHub Copilot AI Code Review – Can AI Understand Legacy C# .NET Code?
• GitHub Copilot AI Code Review – Can AI Und...
📹 Episode 2: Creating .NET Projects and Solution Structure (Code Rewrite)
• GitHub Copilot Visual Studio 2026 | Creati...
➡️ Episode 3: Legacy Code Rewrite – Random Number Generator (THIS VIDEO)
• Refactoring Legacy Code with GitHub Copilo...
📹 Episode 4: Coming Soon
⭐ Binge the entire series in order:
• Code Rewrite (Lottron2000) - GitHub Copilo...
🔗 QUICK NAVIGATION:
⬅️ Previous Video: • GitHub Copilot Visual Studio 2026 | Creati...
➡️ Next Video: Coming Soon
🏠 Start from Episode 1: • GitHub Copilot AI Code Review – Can AI Und...
⚙️ TOOLS & TECHNOLOGIES FEATURED:
GitHub Copilot • Visual Studio 2026 • .NET 10 • C# Programming • ChatGPT 5 mini • Grok AI • xUnit Integration Testing
#aicoding #githubcopilot #visualstudio
💬 Have you rewritten legacy code using AI? Share your experience in the comments.
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🔗 CONNECT WITH ME
If you'd like to see more of my work or get in touch:
💼 Hire Me on Upwork:
https://www.upwork.com/freelancers/~0...
💻 GitHub (Code & Projects):
https://github.com/mpholoane-bapela
▶️ Second YouTube Channel:
/ @codelessdeveloper
🔗 LinkedIn:
/ mpholoane-bapela-3349146a
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