title: how to get a SUM in Linq?
body: <p>I need to do the following, I have a <code>List</code> with a class which contains 2 integer id and count</p>
  
  <p>Now I want to do the following linq query:</p>
  
  <pre><code>get the sum of the count for each id
  </code></pre>
  
  <p>but there can be items with the same id, so it should be summerized e.g.:</p>
  
  <pre><code>id=1, count=12
  id=2, count=1
  id=1, count=2
  </code></pre>
  
  <p>sould be:</p>
  
  <pre><code>id=1 -&gt; sum 14
  id=2 -&gt; sum 1
  </code></pre>
  
  <p>how to do this?</p>