Option Explicit

Private Sub cmdBerechnen_Click()
  Dim varValue As Variant

  txtNFakultaet.Text = ""

  varValue = txtN.Text
  If Not IsNumeric(varValue) Then
    txtNFakultaet.Text = "Fehler!"
  Else
    txtNFakultaet.Text = Fakultaet(CInt(varValue))
  End If
End Sub

Private Function Fakultaet(ByVal intN As Integer) As String
  Dim dblFakultaet As Double
  Dim lngZaehler As Long

  On Error Resume Next
  dblFakultaet = 1

  For lngZaehler = 1 To intN
    dblFakultaet = dblFakultaet * lngZaehler
  Next lngZaehler

  If Err = False Then
    Fakultaet = Format$(dblFakultaet, "Scientific")
  Else
    Fakultaet = "Fehler!"
    Err.Clear
  End If

  On Error GoTo 0
End Function