title: Count number of element in List<List<T>>
body: <p>I have a <code>List&lt;List&lt;T&gt;&gt;</code>.
  How can I count all the elements in this as if it was a single <code>List&lt;T&gt;</code> in the fastest way?</p>
  
  <p>So far I have used </p>
  
  <pre><code>List&lt;int&gt; result = listOfLists
    .SelectMany(list =&gt; list)
    .Distinct()
    .ToList().Count;
  </code></pre>
  
  <p>but this actually creates a list and then counts the element which is not a very good idea.</p>