I built an ECS from scratch back in 2019 using C#. It was my first programming project in earnest and I was 19 at the time. Fortunately, I committed to version control and sometimes I still look at it: https://github.com/randyselimi/ECSRogue

I came up with everything on my own and I don’t know how I was so inspired then. Most of the ECS code is in a folder tilted Partis (that was the name of the ECS system I wanted to develop)

Probably my favorite piece of code is in ECSRogue/Partis/Entities/EntityIndexManager.cs. I’d love if someone with more experience could critique this code and my implementation. Keep in mind, I just transferred from community college and hadn’t even taken a single programming/data structure course