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

I have following code that populates a System.Collections.Generic.List I don't like it so I was wonde开发者_运维问答ring if there is a better way to do this.

let getDirectories = 
        Directory.GetDirectories(_baseFolder)
let languagesList = new System.Collections.Generic.List<string>()
Seq.cast getDirectories 
|> Seq.map(fun dir -> (new DirectoryInfo(dir)).Name) 
|> fun range -> languagesList.AddRange(range)

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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.

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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

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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.

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Use ResizeArray that equals System.Collections.Generic.List

Example:

let languagesList = ResizeArray<string>()
languagesList.Add("1");
languagesList.Add("2");