title: c# LINQ Distinct() on multi level list
body: <p>I have a <code>List&lt;List&lt;int&gt;&gt;</code> with the following values:</p>
  
  <pre><code>[1] = 1,2
  [2] = 4,5,6
  [3] = 1,3
  </code></pre>
  
  <p>What I want to do is to retrieve simple List with unique values => 1,2,3,4,5,6 (number one was duplicated in original list).</p>
  
  <p>If it was 1D List, I would use </p>
  
  <pre><code>variable.Select(n=&gt;n).Distinct();
  </code></pre>
  
  <p>however when I tried to use</p>
  
  <pre><code>variable.Select(n=&gt;n.Select(x=&gt;x)).Distinct();
  </code></pre>
  
  <p>I got 2D list (with unique values I guess). How can I solve this?
  Thanks</p>