title: Quickest way to compare two generic lists for differences
body: <p>What is the quickest (and least resource intensive) to compare two massive (>50.000 items) and as a result have two lists like the ones below:</p>
  
  <ol>
  <li>items that show up in the first list but not in the second</li>
  <li>items that show up in the second list but not in the first</li>
  </ol>
  
  <p>Currently I'm working with the List or IReadOnlyCollection and solve this issue in a linq query:</p>
  
  <pre><code>var list1 = list.Where(i =&gt; !list2.Contains(i)).ToList();
  var list2 = list2.Where(i =&gt; !list.Contains(i)).ToList();
  </code></pre>
  
  <p>But this doesn't perform as good as i would like.
  Any idea of making this quicker and less resource intensive as i need to process a lot of lists?</p>