F# vs C#

F# and C# both target the .NET runtime, compile to the same Common Intermediate Language (CIL), and share the base class library and NuGet ecosystem, yet they differ sharply in default paradigm. F# is functional-first: values are immutable by default, type inference removes most explicit annotations, and code reads as a series of expressions. C# is object-oriented and imperative-first: variables are mutable by default and classes are the primary organizing unit, though modern C# has borrowed functional features such as LINQ, records, and pattern matching.

F# has a strong, inferred type system, and values of ordinary F# types cannot be null by default; instead, the idiomatic way to represent an optional value is Option<'T>, which forces callers to pattern-match on Some x or None before using the value. C# historically allowed null on any reference type, a common source of NullReferenceException at runtime. C# 8 introduced nullable reference types as an opt-in compiler feature, but the checks are advisory warnings rather than a runtime guarantee, so a null can still slip through if warnings are ignored or suppressed.

Because F# and C# compile to the same CIL, projects of one language can reference projects of the other directly within a single .NET solution, with no interop shims required. Many teams write core business logic, domain models, and data pipelines in F# for correctness and testability, while using C# for ASP.NET Core controllers, EF Core migrations, or UI-heavy code where imperative style and a larger hiring pool matter. F# Interactive (FSI), a REPL bundled with the F# tooling, also enables rapid prototyping and data exploration that has no direct equivalent in typical C# workflows.