Public Class Form1
 
    Private Filename_in As String
 
    Private Sub btn_Click(sender As System.Object, e As System.EventArgs) _
      Handles btn.Click
 
 
        Const linesperfile As Integer = 300 'max. Zeilen pro Ausgabedatei
        Dim filecounter As Integer = 0
        Dim linecounter As Integer = 0
        'Dim filename As String = "C:\Users\xxx\Documents\test" 'Ausgabedatei( _
          en)
 
        Dim file_open As New OpenFileDialog
        file_open.Filter = "csv-dateien|*.csv"
        file_open.AddExtension = True
 
        Dim sr As IO.StreamWriter = Nothing
 
        If file_open.ShowDialog = Windows.Forms.DialogResult.OK Then
            Dim auswahlListe As New List(Of String)
 
            For Each zeile As String In IO.File.ReadAllLines(file_open.FileName)
                Dim fields As New List(Of String)(zeile.Split(";"c))
                If fields.Contains("in stock") Then
 
                    If linecounter = 0 Then
                        filecounter += 1
                        sr = New IO.StreamWriter(Filename_in & "00" & _
                          filecounter.ToString & ".csv")
                    End If
                    sr.WriteLine(fields(2) & ";" & fields(7))
                    linecounter += 1
 
                    If linecounter = linesperfile Then
                        sr.Close()
                        linecounter = 0
                    End If
                End If
            Next zeile
            sr.Close()
        End If
    End Sub
 
    Private Sub save_Click(sender As System.Object, e As System.EventArgs) _
      Handles save.Click
        IO.File.ReadAllLines(Filename_in)
    End Sub
End Class