For i As Integer = 0 To dt.Rows.Count - 1
     Dim row As DataRow = dt.Rows(i)
     If Not IsDBNull(row("angelegtam")) Then
        row("Ablageordner") = _
            "BM-" & CDate(row("angelegtam")).ToString("yyyy")
     Else
         row("Ablageordner") = String.Empty
     End If
  Next i