According to the .NET documentation, HashSet<T> does not maintain order when enumerated so I implemented my own using F#.

Can anyone help review if this is threadsafe and correct?

open System open System.Collections open System.Collections.Generic  type LinkedHashSet<'a when 'a : equality> () =     let mutable index = 0     let indexed = SortedDictionary<int, 'a> ()     let hash = HashSet<'a> ()     let theLock = obj ()      interface ICollection<'a> with         member __.Count = lock theLock (fun () -> hash.Count)         member __.IsReadOnly = false         member __.Add e =             lock theLock (fun () ->                  if hash.Add e                 then                     indexed.[index] <- e                     index <- index + 1             )                     member __.Clear () =             lock theLock (fun () ->                 index <- 0                 indexed.Clear ()                 hash.Clear ()             )                     member __.Contains e =             lock theLock (fun () -> hash.Contains e)         member __.CopyTo (array, arrayIndex) =             lock theLock (fun () ->                 let kvps =                      Seq.toArray indexed                 let kvpLength = Array.length kvps                             for i in arrayIndex..kvpLength - 1 do                     array.[i - arrayIndex] <- kvps.[i].Value             )                     member __.GetEnumerator () =             let values = lock theLock (fun () ->                 indexed                 |> Seq.map (fun kvp -> kvp.Value)             )             values.GetEnumerator ()                    member this.GetEnumerator () : IEnumerator =             (this :> IEnumerable).GetEnumerator ()         member __.Remove e =             let matcher (kvp: KeyValuePair<int, 'a>) = kvp.Value = e             lock theLock (fun () ->                 if hash.Remove e                 then                     let kvps = indexed :> seq<KeyValuePair<int, 'a>>                     match Seq.tryFind matcher kvps with                     | Some x -> indexed.Remove x.Key                     | None -> false                 else false             ) 

Usage (distinctChar takes a string aabbb and returns ab):

let distinctChars (s: string) =     s.ToCharArray ()     |> Array.fold (fun (set: ICollection<char>) e -> set.Add e; set) (LinkedHashSet () :> ICollection<char>)     |> Seq.toArray     |> String