MoreLinq

Provides extra methods to LINQ to Objects.

Optuple

.NET Standard Library for giving (bool, T) Option-like semantics in a non-obtrusive way; this is, there is no new option type dependency for a library or its users.

JFlepp.Maybe

A Maybe type for C#, aimed as an idiomatic port of the option type in F# to C#

Optional

A robust option type for C#

language-ext

This library uses and abuses the features of C# 6+ to provide a functional 'Base class library', that, if you squint, can look like extensions to the language itself. It also includes an 'Erlang like' process system (actors) that can optionally persist messages and state to Redis (note you can use it without Redis for in-app messaging). The process system additionally supports Rx streams of messages and state allowing for a complete system of reactive events and message dispatch.

Curryfy

Provides strongly typed extensions methods for C# delegates to take advantages of functional programming techniques, like currying and partial application.