Private Sub RundeMesslänge()
 
     Dim krit As String = "MessLaenge=", found As Boolean
 
     For Each file As String In _
         IO.Directory.GetFiles("C:\daten", "*.txt")
 
         Dim lines As New List(Of String)(IO.File.ReadAllLines(file))
 
         found = False
 
         For i As Integer = 0 To lines.Count - 1
            With lines(i)
               If .StartsWith(krit) Then
                  lines(i) = krit & _
                  Math.Round(CDec(.Substring(krit.Length)), 2).ToString()
                  found = true 
               End If
            End With
         Next i
 
         If found Then
            IO.File.WriteAllLines(file, lines.ToArray)
         End If
     Next file
 End Sub