Question

In this msdn example:

The .NET Standard Query Operators

We see this example of a LEFT JOIN:

var custTotalOrders =
 from c in customers
 join o in orders on c.CustomerID equals o.CustomerID into co
 from o in co.DefaultIfEmpty(emptyOrder)
 select new { c.Name, o.OrderDate, o.Total };

And it says that:

where emptyOrder is an Order instance used to represent a missing order.

So, where does emptyOrder come from?
How can I use it in my code?

Answer 1

There are two overloads for DefaultIfEmpty

DefaultIfEmpty()

DefaultIfEmpty(defaultValue)

The first will return default(T) where T is the type contained in the enumerable when the enumerable is empty.

The second will return the value given in the defaultValue parameter when the enumerable is empty.

Assuming you declared emptyOrder like so:

Order emtpyOrder = new Order() { Total=100 };

Then in the query:

var custTotalOrders =
from c in customers
join o in orders on c.CustomerID equals o.CustomerID into co
from o in co.DefaultIfEmpty(emptyOrder)
select new { c.Name, o.OrderDate, o.Total };

When a customer did not have a matching order, the Total property in the projected anonymous object would have the value 100.