My first time developing a C# project with a friend. He decided it would be a good idea to put 15 classes into a single .cs file. Ever had a person do that to you?

Coincidentally, I am working with a friend on a C# project and he decided to use a single multi tabbed window for all the screens. Now he has a 9000+ lines .cs files with all classes for all tabs in there and even some SQL functions there .-.