How to implement The NOT IN clause in LINQ?

First way:

var query =

from c in dc.Customers

where dc.Orders.FirstOrDefault(o=>o.CustomerID == c.CustomerID) ==  null

select c;

Second way:

var query =

from c in dc.Customers

where !(from o in dc.Orders

select o.CustomerID)

.Contains(c.CustomerID)

select c;

http://introducinglinq.com/blogs/marcorusso/archive/2008/01/14/the-not-in-clause-in-linq-to-sql.aspx