Question

I have the following query:

 DateTime cutoffDate = new DateTime(1997, 1, 1); 

 var orders = 
     from c in customers 
     where c.Region == "WA" 
     from o in c.Orders 
     where o.OrderDate >= cutoffDate 
     select new { c.CustomerID, o.OrderID };

How can this be written in Linq Lambda?
BTW, is this known as a SelectMany query?

Also this can be done with a join, what is the pros and cons with doing it as shown above.

Answer 1

Yes, this is a SelectMany. You use SelectMany to ‘flatten’ a nested or tiered collection (in this case, the orders are nested under customers) into a simple single-tier collection.

customers.Where(c => c.Region == "WA")
    .SelectMany(c => c.Orders)
    .Where(o => o.Orderdate >= cutoffDate)
    .Select(x => new { x.OrderID, x.Customer.CustomerID });

If the Orders are a property of the Customer then there is no need to use a join.