Rules for F# code that C# calls, and C# code that calls F#, in morphir-dotnet. Covers parsed domain types, nullness metadata, TryParse/Create members, Match members for unions, collections, option and null, equality, ToString and AOT safety, and the .fsproj settings. Use when you add or change a public F# type, member or module that a C# project uses; when C# code calls F# types (unions, records, Result, option, list); when you create an F# project that C# references; or when you review a diff that crosses the F#/C# boundary. Triggers include "F# interop", "C# interop", "call F# from C#", "F# union in C#", "MaybeNullWhen", "NotNullWhen", "CompiledName", "F# nullness", "Nullable enable fsproj", "oblivious", "TryParse", "parse don't validate".