Bad code is an infection that spreads

I've worked on my current project for many years now. When I first started I was appalled at the quality of the code. But I jumped in thinking that I could clean things up and make it better.

While I have made it better over the years, I am disappointed that the code is still no where near the quality level that I would like.

My question is why after nearly 10 years of working on this code base can't I make more progress in cleaning it up. I know my team is capable of writing excellent code and when I look at the new code they write it's a pleasure to work with.

However, when I look at code they and I write that interacts with bad code I find that the bad code doesn't get better. It seems to grow and infect the new code around it. It's a lot like a fungus, like athletes foot. If you don't entirely eliminate the bad code it will come back.

We are often faced with the tradeoff of delivering a new product vs. cleaning up a bad design. If necessary make the tradeoff of buttoning things up and delivering the product. But, when you have to go back into the bad code to fix bugs or to extend functionality, clean it up first. Then fix you bug.

My nephew taught me that when building a house spend a lot of time getting the foundation spot on. If the foundation is off level by a half inch then you have to adapt the framing to correct for that. This domino effect runs all the way up the house.

So clean up that bad code before you extend the functionality or it will infect your new code.