F# - How to populate an System.Collections.Generic.List from array

Have you tried:

let list = new System.Collections.Generic.List<string>(arr)

List<'T> has a constructor that takes an IEnumerable<'T> so it happily takes any seq<'T> you pass to it.

The F# alias for System.Collections.Generic.List<_> is ResizeArray<_> as kvb noted. The F# PowerPack includes a ResizeArray module for working with BCL Lists in an idiomatic F# fashion similar to the Seq and List modules.

However, for some strange reason this module seems to include ofArray and ofList and toSeq but not ofSeq.

In addition to Mehrdad's answer

I find it helpful to define helper modules for many standard collections and .Net types to make them more F# friendly. Here I would define the following

module BclListUtil =
  let ofArray (arr: 'T array) = new System.Collections.Generic.List<'T>(arr)
  let ofSeq (arr: 'T seq) = new System.Collections.Generic.List<'T>(arr)

Then you could change your original code to the following

let getDirectories = 
        Directory.GetDirectories(_baseFolder)
let languagesList = 
      getDirectiories
      |> Seq.map (fun dir -> (new DirectoryInfo(dir)).Name)
      |> BclListUtil.ofSeq