language-ext-csharp

Create an Option<T> with Some(value), None, or Optional(possiblyNull) after adding using static LanguageExt.Prelude. Consume it with Match using named arguments (Some:, None:) or transform it with Map and Bind rather than branching on IsSome.

Either<L,R> short-circuits on the first error and suits sequential workflows, while Validation<F,T> accumulates every failure and suits input parsing. Use Apply for applicative composition with Validation; LINQ from/select on Validation binds monadically and loses later errors.

Chain Aff<T> values with LINQ from/select syntax, adapting Task<T> via .ToAff() and Validation via .ToAff(Error.Many). Call await aff.Run() once at the boundary, then pattern-match the resulting Fin<T> with Succ and Fail cases.

This guidance targets LanguageExt 4.x where async effects are Aff<T> and sync effects are Eff<T>. LanguageExt 5 reworks the abstraction into K<F, A> and renames much of the surface, so the concepts carry over but the syntax does not.

ValueUnsafe bypasses the type system and returns default(T), which is null for reference types, when the Option is None. That reintroduces exactly the null bug Option exists to prevent, so use Match or IfNone for extraction instead.

Skip these patterns when the codebase does not import LanguageExt and no migration is planned, or for pure F#, Scala, or Kotlin questions since LanguageExt is a C# library. Also avoid Optional(x) on non-nullable values, which always returns Some.