null-traverse.fs

open Microsoft.FSharp.Quotations.Patterns
open System.Reflection

let nullTraverse<'a> (expr: Quotations.Expr<'a>) (originObj : obj) : 'a option =
    let inline tryGet (p : PropertyInfo) o = 
        p.GetMethod.Invoke(o,null) 
        |> Option.ofObj

    let rec innerTraverse (nullableObj : obj) = function
        | PropertyGet(None, props, _) -> tryGet props obj

        | PropertyGet(Some expr, props, _) ->
            innerTraverse nullableObj expr
            |> Option.bind (tryGet props)
            
        | _ -> failwithf "only get properties are allowed in null traverse"

    innerTraverse originObj expr
    |> Option.map (fun x -> (x :?> 'a))

let (|NullTraverse|_|) = nullTraverse

type C = { C : obj }
type B = { B : string }
type A = { A : B }

let a = {A = { B = null}}

match a with
| NullTraverse <@a.A.B@> x -> printfn "not null: %A" x
| _ -> printfn "null"