Something to keep in mind is that you can make your C# surprisingly functional using LINQ. It's inspired by Haskell and functional programming. It makes the transition from "imperative with functional features" to "functional" very simple.

And you can use Reactive Extensions in C#, which is the most accessible framework for FRP (functional reactive programming) out there and very useful for UI and asynchronous tasks. I've just started exploring it, but it's pretty fun so far.