Sum a collection of objects which contain numeric properties only with LINQ

Question

I have an object model like this:

public class Quantity
{
    public decimal Weight { get; set; }
    public decimal Volume { get; set; }
    // perhaps more decimals...

    public static Quantity operator +(Quantity quantity1, Quantity quantity2)
    {
        return new Quantity()
        {
            Weight = quantity1.Weight + quantity2.Weight,
            Volume = quantity1.Volume + quantity2.Volume
        };
    }
}

public class OrderDetail
{
    public Quantity Quantity { get; set; }
}

public class Order
{
    public IEnumerable<OrderDetail> OrderDetails { get; set; }
}

Now I want to introduce a readonly property TotalQuantity on the Order class which should sum up the quantities of all OrderDetails.

I am wondering if there is better "LINQ way" than this:

public class Order
{
    // ...
    public Quantity TotalQuantity
    {
        get
        {
            Quantity totalQuantity = new Quantity();
            if (OrderDetails != null)
            {
                totalQuantity.Weight =
                    OrderDetails.Sum(o => o.Quantity.Weight);
                totalQuantity.Volume =
                    OrderDetails.Sum(o => o.Quantity.Volume);
            }
            return totalQuantity;
        }
    }
}

It's not a nice solution as it iterates twice through the OrderDetails. And something like this is not supported (even though a + operator is provided in the Quantity class):

Quantity totalQuantity = OrderDetails.Sum(o => o.Quantity); // doesn't compile

Is there a better way to build the total sum in LINQ?

(Just for theoretical interest, a simple foreach loop would also do its job well of course.)

Thanks for feedback!

Answer 1

Try:

OrderDetails.Select(o => o.Quantity).Aggregate((x, y) => x + y)

If you'd prefer not to have the overhead of the new Quantity object for each addition (RE comment), you could use something like:

new Quantity {
    Weight = OrderDetails.Select(o => o.Quantity.Weight).Sum(),
    Volume = OrderDetails.Select(o => o.Quantity.Volume).Sum()
};

Not as nice as the first one, and slightly more awkward (and slower?) than just the plain foreach loop.

You can find more about the Aggregate() method on MSDN.