public Compra(int id, int acidez, float pres, int cant)
 {
     d         = new Disolvente(acidez, pres);
     clave     = Conexion.getUltimaCompra() + 1;
     idCliente = id;
     cantidad  = cant;
     totalC    = d.calcularPrecio(cantidad);
 }
        private void btCalcular_Click(object sender, RoutedEventArgs e)
        {
            float present = Conexion.getPresentacion(cbAcidez.SelectedIndex);

            MessageBox.Show("" + present);
            Disolvente d        = new Disolvente(cbAcidez.SelectedIndex + 1, present);
            int        cantidad = Int16.Parse(txtCantidad.Text);
            int        total    = d.calcularPrecio(cantidad);

            if (total > 0)
            {
                txtTotal.Text = total + ".0 $";
            }
            else
            {
                MessageBox.Show("error ");
            }
        }