Public Class frmCSV
 
    Dim ofd As New OpenFileDialog
 
    Dim FileName_In As String = ""
 
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) _
      Handles Button1.Click
        'Eingabedatei abfragen
        With ofd
            .Filter = "CSV|*csv"
            .CheckFileExists = True
            .Multiselect = False
            If .ShowDialog = Windows.Forms.DialogResult.OK Then FileName_In = _
              .FileName
 
            Const linesperfile As Integer = 300 'max. Zeilen pro Ausgabedatei
            Dim filename As String = SaveFileDialog1.FileName 'Ausgabedatei(en)
            Const Searchstring As String = "in stock"
 
            Dim filecounter As Integer = 0
            Dim linecounter As Integer = 0
 
            Dim sr As IO.StreamWriter = Nothing
 
            If IO.File.Exists(FileName_In) Then
                For Each zeile As String In IO.File.ReadAllLines(FileName_In)
                    Dim fields As New List(Of String)(zeile.Split(";"c))
                    If fields.Contains(Searchstring) Then
 
                        If linecounter = 0 Then
                            filecounter += 1
                            sr = New IO.StreamWriter("00" & _
                              filecounter.ToString & SaveFileDialog1.FileName)
                        End If
 
                        If fields.Count >= 5 Then
                            RichTextBox1.Text = (fields(9))
                            RichTextBox2.Text = (fields(11))
                            linecounter += 1
 
                            If linecounter = linesperfile Then
                                sr.Close()
                                linecounter = 0
                            End If
                        End If
                    End If
                Next zeile
                sr.Close()
            End If
 
        End With
    End Sub
 
    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) _
      Handles Button2.Click
 
        SaveFileDialog1.Filter = "CSV Files (*.csv)|*.csv"
        If SaveFileDialog1.ShowDialog = Windows.Forms.DialogResult.OK Then
 
            'Statt Konstanten könnten Dialogeingaben verwendet werden 
 
            Const linesperfile As Integer = 300 'max. Zeilen pro Ausgabedatei
            Dim filename As String = SaveFileDialog1.FileName 'Ausgabedatei(en)
            Const Searchstring As String = "in stock"
 
            Dim filecounter As Integer = 0
            Dim linecounter As Integer = 0
 
            Dim sr As IO.StreamWriter = Nothing
 
 
            If IO.File.Exists(FileName_In) Then
                For Each zeile As String In IO.File.ReadAllLines(FileName_In)
                    Dim fields As New List(Of String)(zeile.Split(";"c))
                    If fields.Contains(Searchstring) Then
 
                        If linecounter = 0 Then
                            filecounter += 1
                            sr = New IO.StreamWriter("00" & _
                              filecounter.ToString & SaveFileDialog1.FileName)
                        End If
 
                        If fields.Count >= 5 Then
                            sr.WriteLine(fields(9) & ";" & fields(11))
                            linecounter += 1
 
                            If linecounter = linesperfile Then
                                sr.Close()
                                linecounter = 0
                            End If
                        End If
                    End If
                Next zeile
                sr.Close()
            End If
        End If
    End Sub
 
 
 
End Class

sr = New IO.StreamWriter("00" & filecounter.ToString & _
  SaveFileDialog1.FileName) Das angegebene Pfadformat wird nicht unterstützt.