Linq Sub-Select

Question

How do I write a sub-select in LINQ.

If I have a list of customers and a list of orders I want all the customers that have no orders.

This is my pseudo code attempt:

    var  res = from c in customers 
where c.CustomerID ! in (from o in orders select o.CustomerID) 
select c

Answer 1

If this is database-backed, try using navigation properties (if you have them defined):

var res = from c in customers
          where !c.Orders.Any()
          select c;

On Northwind, this generates the TSQL:

SELECT /* columns snipped */
FROM [dbo].[Customers] AS [t0]
WHERE NOT (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[Orders] AS [t1]
    WHERE [t1].[CustomerID] = [t0].[CustomerID]
    ))

Which does the job quite well.

Answer 2

var  res = (from c in orders where c.CustomerID == null
               select c.Customers).ToList();

or Make Except()

Answer 3

How about:

var res = from c in customers 
          where !orders.Select(o => o.CustomerID).Contains(c.CustomerID)
          select c;

Another option is to use:

var res = from c in customers
          join o in orders 
               on c.CustomerID equals o.customerID 
               into customerOrders
          where customerOrders.Count() == 0
          select c;

Are you using LINQ to SQL or something else, btw? Different flavours may have different "best" ways of doing it