title: LINQ selecting distinct from 4 IEnumerable lists
body: <p>Imagine four lists, all at least have this Id string property, but may have other properties:</p>
  
  <pre><code>public class A //or B, C, D
  {
      public string Id { get; set; }
      //..other properties
  }
  
  //so:  
  List&lt;A&gt; a1 = new List&lt;A&gt;();
  List&lt;B&gt; a2 = new List&lt;B&gt;();
  List&lt;C&gt; a3 = new List&lt;C&gt;();
  List&lt;D&gt; a4 = new List&lt;D&gt;();
  </code></pre>
  
  <p>I want to select all DISTINCT ids in:
  a1, combined with a2, a3 and a4</p>
  
  <p>I thought LINQ syntax would be ideal but how do you combine these to an IEnumerable result with a single string property, for example something with a definition of class A.</p>