title: Linq Grouping by Date (month)
body: <p>I would like to know how I could group some data by month and sum a field.
  For example I have a list of <code>MyObjects(DateTimeField, AmountField)</code>. 
  I want to group by <code>DateTimeField</code>, but not entire date, just by month, and then sum the <code>AmountField</code> for each month.</p>
  
  <p>I don't know why the <code>objectgrouped</code> is null?</p>
  
  <pre><code>    IEnumerable&lt;MyObject&gt; objectList= GetMyObject();
  
      var ObjectGrouped = objectList.GroupBy(l =&gt; l.ObjectDate.GetValueOrDefault().Month)
                            .Select(lg =&gt;
                                  new
                                  {
                                      CurrentMonth = lg.Key,
                                      Total = lg.Sum(w =&gt; w.ObjectAmount)
                                  });
  
  ObjectDate      ObjectAmount
  
  1/1/2013              3.3
  3/1/2013              6.9
  13/2/2013            5
  3/4/2013              3.6
  13/4/2013            15.2
  </code></pre>