AggregateDemo.cs

private static void AggregateDemo(List<Order> orders)
{
    const int initialQuantity = 0;
    var totalQuantities = orders.Aggregate(initialQuantity, (sum, order) => sum + order.Quantity);
    // Same as Order simply use a convinient `Sum()` method.
    // var totalQuantities = orders.Sum(order => order.Quantity);
    System.Console.WriteLine($"Total Quantities: {totalQuantities}");
}