title: Linq Sub-Select
body: <p>How do I write a sub-select in LINQ.</p>
  
  <p>If I have a list of customers and a list of orders I want all the customers that have no orders.</p>
  
  <p>This is my pseudo code attempt:</p>
  
  <pre><code>    var  res = from c in customers 
  where c.CustomerID ! in (from o in orders select o.CustomerID) 
  select c
  </code></pre>