title: linq how to GroupBy correctly
body: <p>I have following class:</p>
  
  <pre><code>class AssembledDTO
  {
      int pid,
      int blockId,
      List&lt;string&gt; references
  }
  </code></pre>
  
  <p>I am trying to group by everything in following way:</p>
  
  <pre><code>AssembledParts.GroupBy(entry =&gt; new
                          {
                              entry.PID,
                              entry.BlockId
                          }).
                          Select( (key , val)=&gt; new AssembledDTO
                              {
                                  BlockId = key.Key.BlockId,
                                  PID = key.Key.PID,
                                  References = val.
                              }) 
  </code></pre>
  
  <p>In  <code>References</code> I want to get list of all <code>references</code> added together of each group that I grouped by. </p>
  
  <p>How I can do so ?what I miss here ?</p>