F# Convert String Array to String

With C#, I can use string.Join("", lines) to convert string array to string. What can I do to do the same thing with F#?

ADDED

I need to read lines from a file, do some operation, and then concatenate all the lines into a single line.

When I run this code

open System.IO
open String

let lines = 
  let re = System.Text.RegularExpressions.Regex(@"#(\d+)")
  [|for line in File.ReadAllLines("tclscript.do") ->
      re.Replace(line.Replace("{", "{{").Replace("}", "}}").Trim(), "$1", 1)|]

let concatenatedLine = String.Join("", lines)

File.WriteAllLines("tclscript.txt", concatenatedLine)

I got this error

error FS0039: The value or constructor 'Join' is not defined

I tried this code let concatenatedLine = lines |> String.concat "" to get this error

error FS0001: This expression was expected to have type
    string []    
but here has type
    string

Solution

open System.IO
open System 

let lines = 
  let re = System.Text.RegularExpressions.Regex(@"#(\d+)")
  [|for line in File.ReadAllLines("tclscript.do") ->
      re.Replace(line.Replace("{", "{{").Replace("}", "}}"), "$1", 1) + @"\n"|]

let concatenatedLine = String.Join("", lines)
File.WriteAllText("tclscript.txt", concatenatedLine)

and this one also works.

let concatenatedLine = lines |> String.concat ""