'variante1 
 Dim d As Double
 Dim Eingabe As String
 If Not Double.TryParse(Eingabe, Globalization.NumberStyles.Integer, New _
   System.Globalization.CultureInfo( _
   System.Globalization.CultureInfo.CurrentCulture.Name), d) Then
 
   MsgBox("fehler")
  Exit Sub
 End If
 
'variante2 
        If Not IsNumeric(Eingabe) Then
            MsgBox("fehler")
            Exit Sub
        End If