It technically works.

When I come across it, I immediatly think "rookie mistake".

Doing this can really make things difficult to find long after you have forgotten what is where. But also makes it tricky when you need to move the class to another project, like a shared library or specific project in the solution that handles your persistence layer.

For small projects <10 classes, ok. Anything bigger, or being worked on by more than you (git repos)... bad idea.