title: find common items across multiple lists in C#
body: <p>I have two generic list :</p>
  
  <pre><code>List&lt;string&gt; TestList1 = new List&lt;string&gt;();
  List&lt;string&gt; TestList2 = new List&lt;string&gt;();
  TestList1.Add("1");
  TestList1.Add("2");
  TestList1.Add("3");
  TestList2.Add("3");
  TestList2.Add("4");
  TestList2.Add("5");
  </code></pre>
  
  <p>What is the fastest way to find common items across these lists?</p>