title: Linq get elements for the first N groups
body: <p>I would like to get the first N groups elements with Linq of a ObservableCollection, an example trying to get the first 2 groups:</p>
  
  <p>Data example:</p>
  
  <pre><code>GROUP1 item1
  GROUP1 item2
  GROUP1 item3
  GROUP2 item1
  GROUP2 item2
  GROUP3 item1
  GROUP3 item2
  GROUP3 item3
  GROUP4 item1
  GROUP4 item2
  GROUP4 item3
  </code></pre>
  
  <p>Result wanted:</p>
  
  <pre><code>GROUP1 item1
  GROUP1 item2
  GROUP1 item3
  GROUP2 item1
  GROUP2 item2
  </code></pre>
  
  <p>Hope someone can help me with this. Any ideas?</p>
  
  <p>Can you provide a VB.Net sample?</p>
  
  <p>Many thanks.</p>