Embrace, Extend Extinguish
An anti-competitive strategy by Microsoft to gain marketshare and push their products. A number of internal memos were leaked which specifically use this name (though I imagine a few other companies did similar).
So in this case it was against Java. Microsoft made J++ directly in response to Sun Microsystem's Java, got told off by US legal so made J# (as a half-way point) and finally CSharp.NET.
The damaging part was mainly J++. If they were not stopped in time, they were slowly extending the API with their own incompatible "improvements".
J# was damage control and CSharp.NET is really just a standard competitor.
Some more info (including the Java one I outlined):
(It is important people remember this (or learn about it) because Microsoft never truly changes and they will be doing the same with Linux and GitHub shortly. Already Microsoft is disabling standard password functionality (for dumb oauth). I'm not sure SSH keys will be an alternative for too long. WSL2 is already starting to be used as a "Linux + drivers + DirectX" kind of product).
It is fair that you may not agree with what I have wrote (or experienced it) but this is the main reason many guys don't engage with companies like Microsoft. Its up to you to decide how scummy the company is. Just stay safe and don't let them bite you in the butt ;)