private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key != Key.Back && e.Key < Key.D0 || e.Key > Key.D9 && e.Key < Key.NumPad0 || e.Key > Key.NumPad9)
            {
                e.Handled = true;
            }
        }


private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if ((e.KeyChar < 48 || e.KeyChar > 57) && e.KeyChar != 8)
            {
                e.Handled = true;
            }
        }

  private void textBox1_KeyDown(object sender, KeyPressEventArgs e)
        {
            if (!char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar))
            {
                e.Handled = true;
            }
        }