That is most certainly wrong. I checked with a lawyer when I was there, in 2008, and their conclusion was that MS owns everything I invent during my tenure. The key "trick" is that they own "what's relevant to the business of Microsoft", which of course any type of software is!

You should check with a lawyer as well.

This wasn't the case when I worked there 10 years ago. Check your paperwork.

It changed with the launch of Windows Phone. They opened this up to enable folks to write apps on their spare time and publish them.