ExceptionHandling

ExceptionHandling.fs

open System


let (|Is|_|) (x: 'b) : 'a option =
    match box x with
    | :? 'a as a -> Some a
    | _ -> None

let (|Inner|_|) (b: exn): 'a option when 'a :> Exception =
    match box b.InnerException with
    | :? 'a as a -> Some a
    | _ -> None

try
    ()
with
| :? AggregateException as a when (a.InnerException :? NullReferenceException) ->
    let b = a.InnerException :?> NullReferenceException
    printfn "%s" b.Message 

try
    let a = new AggregateException(new NullReferenceException("MOCK"))
    raise a
with
| Is (a: AggregateException) & Inner (b: NullReferenceException) ->
    printfn "%s" b.Message // b is NullReferenceException here, no need to cast