This is blatantly false. Just on my current project I'm using :

* OpenTK - opensource API around OpenGL

* ReactiveUI - opensource MVVM framework using RX for databinding

* Command Line Parser Library - opensource, does what it says

* NLog - logging library

* sqlite-net - micro ORM for sqlite

I excluded :

* RX - opensource but comes from Microsoft so not a community, but also a nice counterexample of porting from .NET to JVM as RX was innovated on .NET and later ported to other platforms

* system.data.sqlite.org - opensource .NET sqlite implementation, arguably a port from a bigger project

And that's just the references I have in my current project, there are a bunch of open source .NET projects that do cool things, for eg. MonoGame, bunch of MVVM frameworks, etc.