Dim path As String = "C:\Testordner\"
      Dim oldExt As String = ".txt"
      Dim newExt As String = ".tmp"
 
      Try
         Dim di As New IO.DirectoryInfo(path)
 
         For Each file As IO.FileInfo In di.GetFiles(oldExt)
            IO.File.Move(file.FullName, IO.Path.GetFileNameWithoutExtension( _
              file.FullName) & newExt)
         Next
      Catch ex As Exception
         Debug.WriteLine("Fehler: " & ex.Message)
      End Try