title: Using LINQ to find all keys from one collection that are not in another?
body: <p>I'm trying to locate all the keys in one Dictionary that are not in another Dictionary.  Obviously, I can do this using a nested loop, but I'm trying to learn LINQ at the moment and I was wondering if I might use it to accomplish this task?</p>
  
  <p>Here's what I have so far:</p>
  
  <pre><code>Dictionary&lt;string, List&lt;string&gt;&gt; DBtables = this.CollectTableListings();
  var generic = from Dictionary&lt;string,List&lt;string&gt;&gt; tab
                in DBtables
                where !_tables.ContainsKey(???)
                select tab;
  </code></pre>
  
  <p>Any idea what should go in place of the question marks (or perhaps instead of the entire where clause)?</p>