As is often the case in CS, there are no definitions for plenty of concepts. FP is simply “programming with functions”. Most languages are multi-paradigm, and in my book any language that has lambdas is already functional - but it is a spectrum. Java has ways to express functional programs, but it is of course much less functional than, Haskell - which itself is also not 100% pure FP.
No, C# has plenty of features making functional programming possible, so it is definitely on the spectrum of FP languages.
Let’s go at it from the other direction - is Haskell functional? Because it has unsafe pointer access and unsafe IO, so it can pretty much express every C# program as well, right?
