Private Function WriteText(cText As String) As Long
    Dim n As Long
    n = FreeFile
    Open FileName For Append As #n
    WriteText = LOF(n)
    Print #n, cText & vbLf;
    Close #n
End Function

   Private Function WriteText(ByVal cText As String) As Long
 
        Dim fi As FileInfo
        fi = My.Computer.FileSystem.GetFileInfo("C:\Test\Test.pdf")
        WriteText = CLng(fi.Length)
 
        'das eigentliche schreiben per StreamWriter lasse ich hier mal weg, da 
        ' es nur um das LOF geht
 
    End Function

Dim Laenge as Long
Laenge = fs.Length

    Private Sub FileLengthDemo()
 
        Dim filename As String = "C:\daten\textdemo.txt"
 
        Dim enc As System.Text.Encoding = System.Text.Encoding.UTF8
 
        My.Computer.FileSystem.WriteAllText(filename, "Das ist ein Text", _
          False, enc)
 
        Dim fl0 As Long = New IO.FileInfo(filename).Length
 
        Dim fl1, fl2, fl3, fl4, fl5 As Long
 
        Using fs As New IO.FileStream(filename, IO.FileMode.Open, _
          IO.FileAccess.ReadWrite)
 
            fl1 = fs.Length
 
            fs.Position = fs.Length
 
            Dim anh As String = vbCrLf & "Angehängter Text"
            Dim textbytes() As Byte = enc.GetBytes(anh)
 
            fs.Write(textbytes, 0, textbytes.Length)
 
            fl2 = fs.Length  'aktuelle Länge
 
            fl3 = FileLen(filename) 'Länge beim Öffnen
 
            fs.Flush() 'alles speichern
 
            fl4 = FileLen(filename) 'aktuelle Länge
 
        End Using
 
        fl5 = FileLen(filename)
 
    End Sub