men i love using lambda ;)

I've left Stackoverflow link there - check this response https://stackoverflow.com/a/32882220
There are some explanations

how does the is_prime function work??

Array doesn't need any references and Contains is from System.Linq

there are methods Array and Contains

frrr

wOw, looks pretty

You are totally right. Thanks for this comment. But a lot of time has passed since I wrote this. And it was just like "hmmm, can I write it in one line in a couple of minutes?". Now I'm working as a middle .Net developer and I would never do that in my production code. My Upper solution is extremely slow.

You're doing the ToLower inside the loop, so you're doing the same thing (probably the most time consuming step in the whole function) over & over 26 times.

Best solution, imo. I was straining to figure out how to do this and couldn't reason it through at 11pm..

Nice work!