private readonly Key[] numeric = new Key[] {Key.Back, Key.NumPad0, Key.NumPad1, Key.NumPad2, Key.NumPad3, Key.NumPad4,
                                    Key.NumPad5, Key.NumPad6, Key.NumPad7, Key.NumPad8, Key.NumPad9 };
        private void SimpleTbx_KeyDown(object sender, KeyEventArgs e)
        {
            // handles non numeric
            if (Array.IndexOf(numeric, e.Key) == -1)
            {
                e.Handled = true;
            }
        }