title: Linq Sum(a+b) different than Sum(a) + Sum(b)
body: <p>When we use a <em>nullable</em> type like <code>int?</code> or <code>decimal?</code>, The <code>Sum(a+b)</code> sometimes differs from <code>Sum(a)+Sum(b)</code></p>
  
  <p><strong>Working example</strong>:</p>
  
  <pre><code>class SomeClass
  {
      public int? One { get; set; }
      public int? Two { get; set; }
  }
  
  static void Main(string[] args)
  {
      var list = new List&lt;SomeClass&gt;()
                     {
                         new SomeClass { One = 1, Two = 2 },
                         new SomeClass { One = 1, Two = 2 },
                         new SomeClass { One = 1, Two = 2 },
                         new SomeClass { One = 1, Two = 2 },
                         new SomeClass { Two = 2 }
                     };
      Console.WriteLine(list.Sum(_ =&gt; _.One + _.Two)); // 12 // wrong
      Console.WriteLine(list.Sum(_ =&gt; _.One) + list.Sum(_ =&gt; _.Two)); // 14 
      Console.ReadLine();
  }
  </code></pre>
  
  <p>One would expect both sums (printed on the <code>Console</code>) to write out <code>14</code>. But it seems the first one ignores some elements.</p>