The Real Definition of Legacy Code (After 10 Years in the Trenches)
26 Nov 2025 #codingWhen you hear “legacy code,” what’s the first thing that comes to mind?
If you’re a .NET developer, maybe that’s an old ASP.NET WebForms application written in VB.NET and powered by stored procedures.
And if you’re familiar with Working Effectively with Legacy Code, you would say “Code without tests.”
But after 10 years with complex codebases, I challenge those assumptions.
What legacy code really means
Legacy code is simply code we don’t understand and want to stay away from.
A codebase doesn’t need dead languages or outdated frameworks to fit that definition.
Of course we don’t understand a codebase when we find it for the first time. But the real issue is the cognitive load we need to work with it.
With that definition, code with tests can still be legacy code.
I’ve seen it! At a past job, I worked in a hotel management solution. There were two teams working on different parts of the application. One team used the shiniest and brightest: ASP.NET Core, Entity Framework Core, you name it. But still nobody else touched their code. It was too messy.
Did it have tests? Sure! And 100% coverage to make upper management happy. But those tests gave no confidence to refactor. They only exercised mocks.
Legacy code boils down to context and understanding.
That’s why it’s so tempting to rewrite legacy code. We have to rebuild all the shared knowledge when writing the new version.
As a junior coder, I thought I’d only work on shiny projects with the latest tools. In reality, every job has involved legacy system. And each one taught me valuable skills that I included in my book, Street-Smart Coding: 30 Ways to Get Better at Coding.
Grab your copy of Street-Smart Coding here. It’s the roadmap I wish I had moving from junior to senior, and the one I hope guides you too.