I can't speak for all .net shops, but in my experience you roll your own. You sit down, discover your problem, and write a solution for whatever you need from scratch specifically tailored to your need and performance requirements. Afterward, you understand the code (and the intent behind it) very very well.
I don't lament the lack of third party frameworks. I take them as an inspiration when I find interesting ones, and I know my toolset well enough to re-work their solutions and make them my own in pretty good time.
For me, as long as whatever it is is meant to run on windows (and doesn't require native performance), C#'s sweet spot is whatever I decide it needs to be.
Having a C#-like language in which I can write code with performance comparable to native will put some icing on my cake.