.Net wasn't a priority due to our very limited resources. We will be expanding the number of drivers and ORMs greatly over the next while.

That being said, I tested npgsql a few days ago and it worked perfectly. Nothing too extensive, just created a db, 2 tables, inserted some data and did a join. So please give it a shot and if it doesn't work, file issues and we'll address them.

(disclosure: I work at cockroach labs)