title: Counting average on list<T> field
body: <p>I have list of A, and I want to count average on it's field a.<br>
  What's the best way to do it? </p>
  
  <pre><code>class A
  {
      int a;
      int b;
  }
  void f()
  {
      var L = new List&lt;A&gt;();
      for (int i=0; i&lt;3; i++)
      {
          L.Add(new A(){a = i});
      }
  }
  </code></pre>