Private Sub TextBox_Validating(ByVal sender As Object, ByVal e As _
      System.ComponentModel.CancelEventArgs) Handles TextBox1.Validating, _
      TextBox2.Validating, TextBox3.Validating
 
        Dim tb As TextBox = TryCast(sender, TextBox)
        If Not IsNumeric(tb.Text) Then
            MessageBox.Show("Ungültiger Wert")
        End If
 
    End Sub