Enrico Buonanno in his excellent free online book Functional Programming in C#, teaches you to apply functional thinking to real-world problems using the C# language.
Employing functional programming concepts can improve the quality of your code. Though using functional-programming-inspired features [in C#] is not the same as programming in a pure functional language, like Haskell. Or even a hybrid language but with stronger emphasis on the functional side, like F#.
Ed Charbeneau wrote an excellent guide which explains writing functional programming in C# through an example use case.