C# nullable reference types should be enabled, as nullable values are automatically bridged to Swift Optionals. If null is passed in any other context, an exception is raised. To take advantage of this automatic bridging, the C# compiler must not be configured to remove nullable metadata for private members (https://github.com/dotnet/roslyn/blob/master/docs/features/nullable-metadata.md#private-members).
F# Option and ValueOption are also automatically bridged as Swift Optional. For automatic bridging of F# ValueOption, you must be using a version of FSharp.Core newer than 4.5.2.