public static ProductBuilder <T> WithPrice <T>(this ProductBuilder <T> builder, string service)
     where T : ServiceProduct
 {
     builder.Configure(product => product.Service = service);
     return(builder);
 }
 public static ProductBuilder <T> WithPrice <T>(this ProductBuilder <T> builder, decimal price)
     where T : Product
 {
     builder.Configure(product => product.Price = price);
     return(builder);
 }