Public Function JoinCsv(ListFields As List(Of String)) As String
 For i As Integer = 0 To ListFields.Count - 1
  Dim s As String = ListFields.Item(i)
  s = s.Replace(""""c, """""")
  If s.Contains(""""c) OrElse s.Contains(","c) OrElse s <> s.Trim OrElse ContainsSpecial(s) Then
   ListFields.Item(i) = """" & s & """"
  End If
 Next
 Return String.Join(","c, ListFields)
End Function

Private Function ContainsSpecial(s As String) As Boolean
 If s.Contains(""""c) OrElse s.Contains("'"c) OrElse s.Contains(","c) Then Return True
 Dim regex1 As New System.Text.RegularExpressions.Regex("[^\x20-\x7E]")
 Dim match1 As System.Text.RegularExpressions.Match = regex1.Match(s)
 Return match1.Success
End Function
  

Caution - if you try to read a CSV file as lines of text you will experience problems caused by "linefeed" characters embedded in fields. You must ensure that any CSV records sent to function are complete. (i.e. not split over two or more lines.)
 
Reading CSV files Record at a Time using CSV reader (described next) is usually offers a better solution.

Public Function SplitCSV(ByVal entireRecord As String, ByVal parts As Integer) As List(Of String)
 entireRecord = entireRecord.Replace("""""", """")
 Dim ret As New List(Of String)
 Dim cPos, qPos As Integer
 entireRecord = Trim(entireRecord)
 While entireRecord <> ""
  If Mid(entireRecord, 1, 1) = """" Then
   entireRecord = Mid(entireRecord, 2, Len(entireRecord) - 1)
   qPos = InStr(entireRecord, """")
   If qPos < 1 Then entireRecord &= """" : qPos = Len(entireRecord)
   ret.Add(Mid(entireRecord, 1, qPos - 1).Replace("""", """"))
   Mid(entireRecord, 1, qPos) = Space(qPos)
   entireRecord = Trim(entireRecord)
   ' skip any white space before next comma
   If entireRecord <> String.Empty Then
    cPos = InStr(entireRecord, ",")
    If cPos < 1 Then
     entireRecord = String.Empty
    Else
     Mid(entireRecord, 1, cPos) = Space(cPos)
    End If
    entireRecord = Trim(entireRecord)
   End If
  Else
   cPos = InStr(entireRecord, ",")
   If cPos < 1 Then entireRecord &= "," : cPos = Len(entireRecord)
   ret.Add(Trim(Mid(entireRecord, 1, cPos - 1).Replace("""", """""")))
   Mid(entireRecord, 1, cPos) = Space(cPos)
   entireRecord = Trim(entireRecord)
  End If
  If sections >= parts Then Exit While
 End While
 Return ret
End Function
  

Trying to read CSV files line by line can cause a lot of problems e.g. How do you handle embedded line feeds? Wheneber possible, use of the built-in CSVreader

Private Shared ReadOnly delimiters As String() = {","}
Public Sub SplitCSV(filename As String)
    Dim ListFields As List(Of String)
    Using csvParser As New TextFieldParser(filename)
        csvParser.CommentTokens = {}
        csvParser.SetDelimiters(delimiters)
        csvParser.HasFieldsEnclosedInQuotes = True
        While Not csvParser.EndOfData
            ListFields = csvParser.ReadFields().ToList


' Your logic for handling fields in a record goes here
' the fields of the current record are in ListFields


        End While
    End Using
End Sub
  

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