title: Linq get sum of data group by date
body: <p>my data look like:</p>
  
  <pre><code> read_date |  T1  |  T2  |
  15.02.2000 |   2  |   3  |
  16.02.2000 |   4  |   5  |
  15.03.2000 |   2  |   3  |
  16.03.2000 |   5  |   4  |
  </code></pre>
  
  <p>I want to get mountly sum of T1 and T2, like this:</p>
  
  <pre><code> read_date |  T1  |  T2  |
     02.2000 |   6  |   8  |
     03.2000 |   7  |   7  |   
  </code></pre>
  
  <p>I try to write something like this:</p>
  
  <pre><code>var result = from s in meter_readings.Take(10)
               group s by new { s.read_date} into g
               select new
               {
                   read_date = g.Key.read_date,
                   T1 = g.Sum(x =&gt; x.T1),
                   T2 = g.Sum(x =&gt; x.T2)
               };
  </code></pre>
  
  <p>but this does not give expected data.Is there any example to give data hourly sums, daily sums, etc.</p>
  
  <p>Thanks</p>