title: LINQ sum some column when 2 other column values are the same
body: <p>i've the following table</p>
  
  <pre><code>+-------+--------+----------+
  | catid |  name  | quantity |
  +-------+--------+----------+
  |     1 | table  |       10 |
  |     2 | chair  |        5 |
  |     2 | chair  |       10 |
  |     1 | table  |       10 |
  |     2 | chair  |       15 |
  |     3 | pencil |       20 |
  +-------+--------+----------+
  </code></pre>
  
  <p>using LINQ i want to sum the quanitiy for the rows that have the same name  catid and save this to the same datatable</p>
  
  <p>so the result would be this table</p>
  
  <pre><code>+-------+--------+----------+
  | catid |  name  | quantity |
  +-------+--------+----------+
  |     1 | table  |       20 |
  |     2 | chair  |       30 |
  |     3 | pencil |       20 |
  +-------+--------+----------+
  </code></pre>