Add a refactoring between F# and .NET-compatible optional parameters

[xperiandri]

Offers Ctrl+. to switch between ?x: T with defaultArg x c in the body and [<Optional; DefaultParameterValue(c)>] x: T, which C# and VB callers can omit too. Converting back offers ?x with defaultArg and, from F# 10, [<Struct>] ?x with defaultValueArg. F# call sites compile against both forms, so only the member is edited.

Not offered unless every use is defaultArg x c with one constant of the parameter's type, or for overrides, abstract slots, constructors, or a file with a paired .fsi.

🤖 Generated with Claude Code

[github-actions]

✅ Release notes checked

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md

[github-actions]

🔍 Tooling Safety Check — Affects-Design-Time
Affects-Design-Time: Modifies Visual Studio code executed while projects are open.

Generated by PR Tooling Safety Check · gpt56 1.8M · ◷

[xperiandri]

Covered cases

Converts

• Shadowing default, adding the missing open: ?greeting: string with let greeting = defaultArg greeting "Hello" → [<Optional; DefaultParameterValue("Hello")>] greeting: string, the let line removed, open System.Runtime.InteropServices inserted; calls Greet("Ada") and greeting = "Hi" stay as they are
• Shadowing default round trip, both ways, when the open already exists: ?step: int + let step = defaultArg step 1 ⟷ [<Optional; DefaultParameterValue(1)>] step: int; the same for a second parameter ?step: float with 0.5
• Inline default: value + defaultArg step 1 → [<Optional; DefaultParameterValue(1)>] step: int) = value + step
• Back with a body on the member line: step: int) = value + step → ?step: int) =, then let step = defaultArg step 1 and value + step on their own lines
• Parameter with no default use: ?flag: bool (unused) → [<Optional>] flag: bool
• Back with no DefaultParameterValue: [<Optional>] step: int → ?step: int + let step = defaultArg step Unchecked.defaultof<_>
• Back to a struct optional parameter: [<Optional; DefaultParameterValue(1)>] step: int → [<Struct>] ?step: int + let step = defaultValueArg step 1; [<Optional>] step: int → … defaultValueArg step Unchecked.defaultof<_>
• Action titles: the F# form offers only Use [<Optional; DefaultParameterValue>] for optional parameter; the .NET form offers Use F# '?' optional parameter, then Use F# '[<Struct>] ?' optional parameter
• Language version gate: before F# 10 (--langversion:9.0), the .NET form offers only Use F# '?' optional parameter
• Each converted document type-checks without errors

Not offered

• Different defaults: defaultArg x 0 + defaultArg x 1
• A use other than defaultArg: static member M(?x: int) = x.IsSome
• Default that is not a constant: ?x: obj with defaultArg x (box 1)
• Constant whose type does not match the parameter: ?x: int with defaultArg x 1.0
• Parameter with no type annotation: static member M(?x) = defaultArg x 0
• Struct optional parameter: [<Struct>] ?x: int with defaultValueArg x 0
• Extra attribute in the list: [<Optional; In>] x: int
• Override of an abstract slot: abstract M: ?x: int -> int with default _.M(?x: int) = defaultArg x 0
• Ordinary parameter: static member M(x: int) = x
• File that has a signature file: static member M: ?x: int -> int

[T-Gro]

🤖 🕵️ AI review — verify independently.

[T-Gro]

🤖 🕵️ [P2] Converting x to F# ?x changes C.M() from true to false — an omitted obj receives null instead of Missing.Value.

open System.Runtime.InteropServices

type C() =
    static member M([<Optional>] x: obj) =
        obj.ReferenceEquals(x, System.Reflection.Missing.Value)

printfn "%b" (C.M())

[T-Gro]

🤖 🕵️ [P2] Converting ?x to .NET optional changes C.M() from 99 to 0 — matching defaultArg by spelling removes a call to a different function.

let defaultArg (_: int option) (_: int) = 99

type C() =
    static member M(?x: int) = defaultArg x 0

printfn "%d" (C.M())

[T-Gro]

🤖 🕵️ [P2] Converting only ?y to .NET optional produces FS1212 — the remaining F# optional ?x precedes the converted parameter.

type C() =
    static member M(?x: int, ?y: int) =
        defaultArg x 1 + defaultArg y 2

[T-Gro]

🤖 🕵️ [P2] Converting x to [<Struct>] ?x leaves a formerly valid caller failing with FS0001 — Some 1 supplies option, but the parameter now requires voption.

open System.Runtime.InteropServices

type C =
    static member M([<Optional; DefaultParameterValue(0)>] x: int) = x

let result = C.M(?x = Some 1)

[T-Gro]

🤖 🕵️ [P2] Converting ?x to .NET optional deletes let mutable x, leaving x <- x + 1 failing with FS0027 because the parameter is immutable.

type C() =
    static member M(?x: int) =
        let mutable x = defaultArg x 0
        x <- x + 1
        x

[T-Gro]

🤖 🕵️ [P2] Converting ?x to .NET optional produces FS0039 for both attributes — an open in another module suppresses the import needed at C.M.

module Other =
    open System.Runtime.InteropServices
    let x = 1

type C() =
    static member M(?x: int) = defaultArg x 0