tryGet and exists

#27: unstructured objects, check o?myProp without eating a RuntimeBinderException.

These ship on Dynamitey 3.0.3. They do not need Dynamitey 4.0.0.

Dyn.tryGet

open System.Dynamic
open FSharp.Interop.Dynamic

let o = ExpandoObject()
o?myProp <- "hi"

let present: string option = o |> Dyn.tryGet "myProp"   // Some "hi"
let missing: string option = o |> Dyn.tryGet "nope"     // None

Lookup is Dynamic.InvokeGet only. Then the existing conversion / callable wrapping runs outside the catch.

Situation	Result
Member missing	None
Member present, value converts to 'T	Some value
Member present, value is null	Some null (for a reference 'T)
Member present, value cannot convert to 'T	throws (InvalidCastException or a binder exception) — not None
Member missing, 'T is a function type	None immediately (does not return a lazy callable)
Getter throws something else	that exception propagates

Dyn.exists

o |> Dyn.exists "myProp"   // true
o |> Dyn.exists "nope"     // false

True when InvokeGet succeeds. A present null is still present. A throwing getter is not reported as missing.

exists does not convert the value. That is the difference from tryGet >> Option.isSome when conversion would fail: exists can be true while tryGet as int throws.

Do not use GetMemberNames for this

Dynamitey exposes Dynamic.GetMemberNames. It lists names; it is not the same test as o?myProp. COM, some IDynamicMetaObjectProvider implementations, and binder tricks can succeed on a get whose name is not in that list. tryGet / exists use the same InvokeGet as ?.