Try
      With My.Application
         If .CommandLineArgs.Count > 0 Then
            Dim filename As String = .CommandLineArgs(0)
            If IO.File.Exists(filename) Then
               Dim fileext As String = IO.Path.GetExtension(filename).ToUpper
               If fileext = ".RTF" Or fileext = ".TXT" Then
                  RichTextBox1.LoadFile(filename)
               End If
            End If
         End If
      End With
    Catch
      'was auch immer  
    End Try