Question

What do I need to do to be able to call .ToArray () for a HashSet? I have tried the following but it did not work:

let a = System.Collections.Generic.HashSet ()
a.Add 5 |> ignore
a.ToArray () // Not possible
let b = a :> System.Collections.IEnumerable
b.ToArray () // Not possible

Here, this is stated:

ToArray : Creates an array from a IEnumerable. (Defined by Enumerable.)

so there should be a way.

Answer 1

IEnumerable.ToArray() is defined as an extension method under System.Linq.
Alternatively, use Seq.toArray or Array.ofSeq.

open System.Collections.Generic
open System.Linq

let hs = HashSet()
hs.Add 4
hs.Add 2

let arr = hs.ToArray()       // needs System.Linq
let arr2 = hs |> Seq.toArray // provided by F#