Is there a Functional Programming library for .NET?

LanguageExt looks very promising for making functional style programming in C# easier.

https://github.com/louthy/language-ext

have you looked into F#?

Also a neat blog post would be here that talks about how to use the new generic / lambda expressions built into c# 3.0.

If you just add using System.Linq to the top of your source file there are a LOT of nice new functions added to working with collections such as folding / filtering / etc.

Assuming you can't use F# for whatever reason, and just want to use functional paradigms and idioms in your C# code to improve quality & reliability:

Functional style pattern matching for C#

Monad library for C#/.Net

There is also 'elevate' which has some functional things like option types (maybes) etc.